diff --git a/Cargo.lock b/Cargo.lock
index 537571ee6b542..09eb0d98efc90 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3654,10 +3654,10 @@ version = "0.0.0"
 dependencies = [
  "arrayvec",
  "bitflags 2.4.1",
+ "either",
  "elsa",
  "ena",
  "indexmap",
- "itertools",
  "jobserver",
  "libc",
  "measureme",
diff --git a/compiler/rustc_data_structures/Cargo.toml b/compiler/rustc_data_structures/Cargo.toml
index 9d598c32e6fc4..08aa68ca54a7a 100644
--- a/compiler/rustc_data_structures/Cargo.toml
+++ b/compiler/rustc_data_structures/Cargo.toml
@@ -7,10 +7,10 @@ edition = "2021"
 # tidy-alphabetical-start
 arrayvec = { version = "0.7", default-features = false }
 bitflags = "2.4.1"
+either = "1.0"
 elsa = "=1.7.1"
 ena = "0.14.2"
 indexmap = { version = "2.0.0" }
-itertools = "0.11"
 jobserver_crate = { version = "0.1.27", package = "jobserver" }
 libc = "0.2"
 measureme = "11"
diff --git a/compiler/rustc_data_structures/src/sharded.rs b/compiler/rustc_data_structures/src/sharded.rs
index 162dbd234d676..4b02b18346069 100644
--- a/compiler/rustc_data_structures/src/sharded.rs
+++ b/compiler/rustc_data_structures/src/sharded.rs
@@ -3,7 +3,7 @@ use crate::fx::{FxHashMap, FxHasher};
 use crate::sync::{is_dyn_thread_safe, CacheAligned};
 use crate::sync::{Lock, LockGuard, Mode};
 #[cfg(parallel_compiler)]
-use itertools::Either;
+use either::Either;
 use std::borrow::Borrow;
 use std::collections::hash_map::RawEntryMut;
 use std::hash::{Hash, Hasher};
diff --git a/compiler/rustc_data_structures/src/sso/map.rs b/compiler/rustc_data_structures/src/sso/map.rs
index 04e359a54708c..2ef4a2ccd8476 100644
--- a/compiler/rustc_data_structures/src/sso/map.rs
+++ b/compiler/rustc_data_structures/src/sso/map.rs
@@ -1,6 +1,6 @@
 use crate::fx::FxHashMap;
 use arrayvec::ArrayVec;
-use itertools::Either;
+use either::Either;
 use std::fmt;
 use std::hash::Hash;
 use std::ops::Index;
diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index 85d10872b3d1d..9fb318e2ae70d 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -208,6 +208,10 @@ language_item_table! {
     FnMut,                   sym::fn_mut,              fn_mut_trait,               Target::Trait,          GenericRequirement::Exact(1);
     FnOnce,                  sym::fn_once,             fn_once_trait,              Target::Trait,          GenericRequirement::Exact(1);
 
+    AsyncFn,                 sym::async_fn,            async_fn_trait,             Target::Trait,          GenericRequirement::Exact(1);
+    AsyncFnMut,              sym::async_fn_mut,        async_fn_mut_trait,         Target::Trait,          GenericRequirement::Exact(1);
+    AsyncFnOnce,             sym::async_fn_once,       async_fn_once_trait,        Target::Trait,          GenericRequirement::Exact(1);
+
     FnOnceOutput,            sym::fn_once_output,      fn_once_output,             Target::AssocTy,        GenericRequirement::None;
 
     Iterator,                sym::iterator,            iterator_trait,             Target::Trait,          GenericRequirement::Exact(0);
diff --git a/compiler/rustc_hir_analysis/src/coherence/builtin.rs b/compiler/rustc_hir_analysis/src/coherence/builtin.rs
index 8d362f74b0a26..5a3878445937b 100644
--- a/compiler/rustc_hir_analysis/src/coherence/builtin.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/builtin.rs
@@ -25,14 +25,21 @@ use rustc_trait_selection::traits::ObligationCtxt;
 use rustc_trait_selection::traits::{self, ObligationCause};
 use std::collections::BTreeMap;
 
-pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
+pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) -> Result<(), ErrorGuaranteed> {
     let lang_items = tcx.lang_items();
-    Checker { tcx, trait_def_id }
-        .check(lang_items.drop_trait(), visit_implementation_of_drop)
-        .check(lang_items.copy_trait(), visit_implementation_of_copy)
-        .check(lang_items.const_param_ty_trait(), visit_implementation_of_const_param_ty)
-        .check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
-        .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn);
+    let checker = Checker { tcx, trait_def_id };
+    let mut res = checker.check(lang_items.drop_trait(), visit_implementation_of_drop);
+    res = res.and(checker.check(lang_items.copy_trait(), visit_implementation_of_copy));
+    res = res.and(
+        checker.check(lang_items.const_param_ty_trait(), visit_implementation_of_const_param_ty),
+    );
+    res = res.and(
+        checker.check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized),
+    );
+    res.and(
+        checker
+            .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn),
+    )
 }
 
 struct Checker<'tcx> {
@@ -41,33 +48,40 @@ struct Checker<'tcx> {
 }
 
 impl<'tcx> Checker<'tcx> {
-    fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
+    fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> Result<(), ErrorGuaranteed>
     where
-        F: FnMut(TyCtxt<'tcx>, LocalDefId),
+        F: FnMut(TyCtxt<'tcx>, LocalDefId) -> Result<(), ErrorGuaranteed>,
     {
+        let mut res = Ok(());
         if Some(self.trait_def_id) == trait_def_id {
             for &impl_def_id in self.tcx.hir().trait_impls(self.trait_def_id) {
-                f(self.tcx, impl_def_id);
+                res = res.and(f(self.tcx, impl_def_id));
             }
         }
-        self
+        res
     }
 }
 
-fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
+fn visit_implementation_of_drop(
+    tcx: TyCtxt<'_>,
+    impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
     // Destructors only work on local ADT types.
     match tcx.type_of(impl_did).instantiate_identity().kind() {
-        ty::Adt(def, _) if def.did().is_local() => return,
-        ty::Error(_) => return,
+        ty::Adt(def, _) if def.did().is_local() => return Ok(()),
+        ty::Error(_) => return Ok(()),
         _ => {}
     }
 
     let impl_ = tcx.hir().expect_item(impl_did).expect_impl();
 
-    tcx.dcx().emit_err(errors::DropImplOnWrongItem { span: impl_.self_ty.span });
+    Err(tcx.dcx().emit_err(errors::DropImplOnWrongItem { span: impl_.self_ty.span }))
 }
 
-fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
+fn visit_implementation_of_copy(
+    tcx: TyCtxt<'_>,
+    impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
 
     let self_type = tcx.type_of(impl_did).instantiate_identity();
@@ -79,59 +93,68 @@ fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
 
     let span = match tcx.hir().expect_item(impl_did).expect_impl() {
-        hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. } => return,
+        hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. } => return Ok(()),
         hir::Impl { self_ty, .. } => self_ty.span,
     };
 
     let cause = traits::ObligationCause::misc(span, impl_did);
     match type_allowed_to_implement_copy(tcx, param_env, self_type, cause) {
-        Ok(()) => {}
+        Ok(()) => Ok(()),
         Err(CopyImplementationError::InfringingFields(fields)) => {
-            infringing_fields_error(tcx, fields, LangItem::Copy, impl_did, span);
+            Err(infringing_fields_error(tcx, fields, LangItem::Copy, impl_did, span))
         }
         Err(CopyImplementationError::NotAnAdt) => {
-            tcx.dcx().emit_err(errors::CopyImplOnNonAdt { span });
+            Err(tcx.dcx().emit_err(errors::CopyImplOnNonAdt { span }))
         }
         Err(CopyImplementationError::HasDestructor) => {
-            tcx.dcx().emit_err(errors::CopyImplOnTypeWithDtor { span });
+            Err(tcx.dcx().emit_err(errors::CopyImplOnTypeWithDtor { span }))
         }
     }
 }
 
-fn visit_implementation_of_const_param_ty(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
+fn visit_implementation_of_const_param_ty(
+    tcx: TyCtxt<'_>,
+    impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
     let self_type = tcx.type_of(impl_did).instantiate_identity();
     assert!(!self_type.has_escaping_bound_vars());
 
     let param_env = tcx.param_env(impl_did);
 
     let span = match tcx.hir().expect_item(impl_did).expect_impl() {
-        hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. } => return,
+        hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. } => return Ok(()),
         impl_ => impl_.self_ty.span,
     };
 
     let cause = traits::ObligationCause::misc(span, impl_did);
     match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, cause) {
-        Ok(()) => {}
+        Ok(()) => Ok(()),
         Err(ConstParamTyImplementationError::InfrigingFields(fields)) => {
-            infringing_fields_error(tcx, fields, LangItem::ConstParamTy, impl_did, span);
+            Err(infringing_fields_error(tcx, fields, LangItem::ConstParamTy, impl_did, span))
         }
         Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed) => {
-            tcx.dcx().emit_err(errors::ConstParamTyImplOnNonAdt { span });
+            Err(tcx.dcx().emit_err(errors::ConstParamTyImplOnNonAdt { span }))
         }
     }
 }
 
-fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
+fn visit_implementation_of_coerce_unsized(
+    tcx: TyCtxt<'_>,
+    impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 
     // Just compute this for the side-effects, in particular reporting
     // errors; other parts of the code may demand it for the info of
     // course.
     let span = tcx.def_span(impl_did);
-    tcx.at(span).coerce_unsized_info(impl_did);
+    tcx.at(span).ensure().coerce_unsized_info(impl_did)
 }
 
-fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
+fn visit_implementation_of_dispatch_from_dyn(
+    tcx: TyCtxt<'_>,
+    impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 
     let span = tcx.def_span(impl_did);
@@ -166,26 +189,28 @@ fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDef
     match (source.kind(), target.kind()) {
         (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
             if infcx.at(&cause, param_env).eq(DefineOpaqueTypes::No, r_a, *r_b).is_ok()
-                && mutbl_a == *mutbl_b => {}
-        (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
+                && mutbl_a == *mutbl_b =>
+        {
+            Ok(())
+        }
+        (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => Ok(()),
         (&Adt(def_a, args_a), &Adt(def_b, args_b)) if def_a.is_struct() && def_b.is_struct() => {
             if def_a != def_b {
                 let source_path = tcx.def_path_str(def_a.did());
                 let target_path = tcx.def_path_str(def_b.did());
 
-                tcx.dcx().emit_err(errors::DispatchFromDynCoercion {
+                return Err(tcx.dcx().emit_err(errors::DispatchFromDynCoercion {
                     span,
                     trait_name: "DispatchFromDyn",
                     note: true,
                     source_path,
                     target_path,
-                });
-
-                return;
+                }));
             }
 
+            let mut res = Ok(());
             if def_a.repr().c() || def_a.repr().packed() {
-                tcx.dcx().emit_err(errors::DispatchFromDynRepr { span });
+                res = Err(tcx.dcx().emit_err(errors::DispatchFromDynRepr { span }));
             }
 
             let fields = &def_a.non_enum_variant().fields;
@@ -207,11 +232,11 @@ fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDef
                         infcx.at(&cause, param_env).eq(DefineOpaqueTypes::No, ty_a, ty_b)
                     {
                         if ok.obligations.is_empty() {
-                            tcx.dcx().emit_err(errors::DispatchFromDynZST {
+                            res = Err(tcx.dcx().emit_err(errors::DispatchFromDynZST {
                                 span,
                                 name: field.name,
                                 ty: ty_a,
-                            });
+                            }));
 
                             return false;
                         }
@@ -222,13 +247,13 @@ fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDef
                 .collect::<Vec<_>>();
 
             if coerced_fields.is_empty() {
-                tcx.dcx().emit_err(errors::DispatchFromDynSingle {
+                res = Err(tcx.dcx().emit_err(errors::DispatchFromDynSingle {
                     span,
                     trait_name: "DispatchFromDyn",
                     note: true,
-                });
+                }));
             } else if coerced_fields.len() > 1 {
-                tcx.dcx().emit_err(errors::DispatchFromDynMulti {
+                res = Err(tcx.dcx().emit_err(errors::DispatchFromDynMulti {
                     span,
                     coercions_note: true,
                     number: coerced_fields.len(),
@@ -244,7 +269,7 @@ fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDef
                         })
                         .collect::<Vec<_>>()
                         .join(", "),
-                });
+                }));
             } else {
                 let ocx = ObligationCtxt::new(&infcx);
                 for field in coerced_fields {
@@ -261,21 +286,25 @@ fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDef
                 }
                 let errors = ocx.select_all_or_error();
                 if !errors.is_empty() {
-                    infcx.err_ctxt().report_fulfillment_errors(errors);
+                    res = Err(infcx.err_ctxt().report_fulfillment_errors(errors));
                 }
 
                 // Finally, resolve all regions.
                 let outlives_env = OutlivesEnvironment::new(param_env);
-                let _ = ocx.resolve_regions_and_report_errors(impl_did, &outlives_env);
+                res = res.and(ocx.resolve_regions_and_report_errors(impl_did, &outlives_env));
             }
+            res
         }
-        _ => {
-            tcx.dcx().emit_err(errors::CoerceUnsizedMay { span, trait_name: "DispatchFromDyn" });
-        }
+        _ => Err(tcx
+            .dcx()
+            .emit_err(errors::CoerceUnsizedMay { span, trait_name: "DispatchFromDyn" })),
     }
 }
 
-pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> CoerceUnsizedInfo {
+pub fn coerce_unsized_info<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    impl_did: LocalDefId,
+) -> Result<CoerceUnsizedInfo, ErrorGuaranteed> {
     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
     let span = tcx.def_span(impl_did);
 
@@ -292,8 +321,6 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
     let param_env = tcx.param_env(impl_did);
     assert!(!source.has_escaping_bound_vars());
 
-    let err_info = CoerceUnsizedInfo { custom_kind: None };
-
     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 
     let infcx = tcx.infer_ctxt().build();
@@ -337,14 +364,13 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
             if def_a != def_b {
                 let source_path = tcx.def_path_str(def_a.did());
                 let target_path = tcx.def_path_str(def_b.did());
-                tcx.dcx().emit_err(errors::DispatchFromDynSame {
+                return Err(tcx.dcx().emit_err(errors::DispatchFromDynSame {
                     span,
                     trait_name: "CoerceUnsized",
                     note: true,
                     source_path,
                     target_path,
-                });
-                return err_info;
+                }));
             }
 
             // Here we are considering a case of converting
@@ -419,12 +445,11 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
                 .collect::<Vec<_>>();
 
             if diff_fields.is_empty() {
-                tcx.dcx().emit_err(errors::CoerceUnsizedOneField {
+                return Err(tcx.dcx().emit_err(errors::CoerceUnsizedOneField {
                     span,
                     trait_name: "CoerceUnsized",
                     note: true,
-                });
-                return err_info;
+                }));
             } else if diff_fields.len() > 1 {
                 let item = tcx.hir().expect_item(impl_did);
                 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(t), .. }) = &item.kind {
@@ -433,7 +458,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
                     tcx.def_span(impl_did)
                 };
 
-                tcx.dcx().emit_err(errors::CoerceUnsizedMulti {
+                return Err(tcx.dcx().emit_err(errors::CoerceUnsizedMulti {
                     span,
                     coercions_note: true,
                     number: diff_fields.len(),
@@ -442,9 +467,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
                         .map(|&(i, a, b)| format!("`{}` (`{}` to `{}`)", fields[i].name, a, b))
                         .collect::<Vec<_>>()
                         .join(", "),
-                });
-
-                return err_info;
+                }));
             }
 
             let (i, a, b) = diff_fields[0];
@@ -453,8 +476,9 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
         }
 
         _ => {
-            tcx.dcx().emit_err(errors::DispatchFromDynStruct { span, trait_name: "CoerceUnsized" });
-            return err_info;
+            return Err(tcx
+                .dcx()
+                .emit_err(errors::DispatchFromDynStruct { span, trait_name: "CoerceUnsized" }));
         }
     };
 
@@ -477,7 +501,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
     let outlives_env = OutlivesEnvironment::new(param_env);
     let _ = ocx.resolve_regions_and_report_errors(impl_did, &outlives_env);
 
-    CoerceUnsizedInfo { custom_kind: kind }
+    Ok(CoerceUnsizedInfo { custom_kind: kind })
 }
 
 fn infringing_fields_error(
diff --git a/compiler/rustc_hir_analysis/src/coherence/mod.rs b/compiler/rustc_hir_analysis/src/coherence/mod.rs
index 561a254e89ef7..dafa899ef248f 100644
--- a/compiler/rustc_hir_analysis/src/coherence/mod.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/mod.rs
@@ -10,6 +10,7 @@ use rustc_errors::{error_code, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
+use rustc_span::ErrorGuaranteed;
 use rustc_trait_selection::traits;
 
 mod builtin;
@@ -18,7 +19,11 @@ mod inherent_impls_overlap;
 mod orphan;
 mod unsafety;
 
-fn check_impl(tcx: TyCtxt<'_>, impl_def_id: LocalDefId, trait_ref: ty::TraitRef<'_>) {
+fn check_impl(
+    tcx: TyCtxt<'_>,
+    impl_def_id: LocalDefId,
+    trait_ref: ty::TraitRef<'_>,
+) -> Result<(), ErrorGuaranteed> {
     debug!(
         "(checking implementation) adding impl for trait '{:?}', item '{}'",
         trait_ref,
@@ -28,18 +33,18 @@ fn check_impl(tcx: TyCtxt<'_>, impl_def_id: LocalDefId, trait_ref: ty::TraitRef<
     // Skip impls where one of the self type is an error type.
     // This occurs with e.g., resolve failures (#30589).
     if trait_ref.references_error() {
-        return;
+        return Ok(());
     }
 
-    enforce_trait_manually_implementable(tcx, impl_def_id, trait_ref.def_id);
-    enforce_empty_impls_for_marker_traits(tcx, impl_def_id, trait_ref.def_id);
+    enforce_trait_manually_implementable(tcx, impl_def_id, trait_ref.def_id)
+        .and(enforce_empty_impls_for_marker_traits(tcx, impl_def_id, trait_ref.def_id))
 }
 
 fn enforce_trait_manually_implementable(
     tcx: TyCtxt<'_>,
     impl_def_id: LocalDefId,
     trait_def_id: DefId,
-) {
+) -> Result<(), ErrorGuaranteed> {
     let impl_header_span = tcx.def_span(impl_def_id);
 
     // Disallow *all* explicit impls of traits marked `#[rustc_deny_explicit_impl]`
@@ -59,18 +64,17 @@ fn enforce_trait_manually_implementable(
             err.code(error_code!(E0328));
         }
 
-        err.emit();
-        return;
+        return Err(err.emit());
     }
 
     if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable =
         tcx.trait_def(trait_def_id).specialization_kind
     {
         if !tcx.features().specialization && !tcx.features().min_specialization {
-            tcx.dcx().emit_err(errors::SpecializationTrait { span: impl_header_span });
-            return;
+            return Err(tcx.dcx().emit_err(errors::SpecializationTrait { span: impl_header_span }));
         }
     }
+    Ok(())
 }
 
 /// We allow impls of marker traits to overlap, so they can't override impls
@@ -79,22 +83,22 @@ fn enforce_empty_impls_for_marker_traits(
     tcx: TyCtxt<'_>,
     impl_def_id: LocalDefId,
     trait_def_id: DefId,
-) {
+) -> Result<(), ErrorGuaranteed> {
     if !tcx.trait_def(trait_def_id).is_marker {
-        return;
+        return Ok(());
     }
 
     if tcx.associated_item_def_ids(trait_def_id).is_empty() {
-        return;
+        return Ok(());
     }
 
-    struct_span_code_err!(
+    Err(struct_span_code_err!(
         tcx.dcx(),
         tcx.def_span(impl_def_id),
         E0715,
         "impls for marker traits cannot contain items"
     )
-    .emit();
+    .emit())
 }
 
 pub fn provide(providers: &mut Providers) {
@@ -115,23 +119,23 @@ pub fn provide(providers: &mut Providers) {
     };
 }
 
-fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) {
+fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) -> Result<(), ErrorGuaranteed> {
     // Trigger building the specialization graph for the trait. This will detect and report any
     // overlap errors.
-    tcx.ensure().specialization_graph_of(def_id);
+    let mut res = tcx.ensure().specialization_graph_of(def_id);
 
     let impls = tcx.hir().trait_impls(def_id);
     for &impl_def_id in impls {
         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity();
 
-        check_impl(tcx, impl_def_id, trait_ref);
-        check_object_overlap(tcx, impl_def_id, trait_ref);
+        res = res.and(check_impl(tcx, impl_def_id, trait_ref));
+        res = res.and(check_object_overlap(tcx, impl_def_id, trait_ref));
 
-        unsafety::check_item(tcx, impl_def_id);
-        tcx.ensure().orphan_check_impl(impl_def_id);
+        res = res.and(unsafety::check_item(tcx, impl_def_id));
+        res = res.and(tcx.ensure().orphan_check_impl(impl_def_id));
     }
 
-    builtin::check_trait(tcx, def_id);
+    res.and(builtin::check_trait(tcx, def_id))
 }
 
 /// Checks whether an impl overlaps with the automatic `impl Trait for dyn Trait`.
@@ -139,12 +143,12 @@ fn check_object_overlap<'tcx>(
     tcx: TyCtxt<'tcx>,
     impl_def_id: LocalDefId,
     trait_ref: ty::TraitRef<'tcx>,
-) {
+) -> Result<(), ErrorGuaranteed> {
     let trait_def_id = trait_ref.def_id;
 
     if trait_ref.references_error() {
         debug!("coherence: skipping impl {:?} with error {:?}", impl_def_id, trait_ref);
-        return;
+        return Ok(());
     }
 
     // check for overlap with the automatic `impl Trait for dyn Trait`
@@ -173,7 +177,7 @@ fn check_object_overlap<'tcx>(
                 let mut supertrait_def_ids = traits::supertrait_def_ids(tcx, component_def_id);
                 if supertrait_def_ids.any(|d| d == trait_def_id) {
                     let span = tcx.def_span(impl_def_id);
-                    struct_span_code_err!(
+                    return Err(struct_span_code_err!(
                         tcx.dcx(),
                         span,
                         E0371,
@@ -189,9 +193,10 @@ fn check_object_overlap<'tcx>(
                             tcx.def_path_str(trait_def_id)
                         ),
                     )
-                    .emit();
+                    .emit());
                 }
             }
         }
     }
+    Ok(())
 }
diff --git a/compiler/rustc_hir_analysis/src/coherence/unsafety.rs b/compiler/rustc_hir_analysis/src/coherence/unsafety.rs
index 7b146573a1b3b..e4c407af53ff0 100644
--- a/compiler/rustc_hir_analysis/src/coherence/unsafety.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/unsafety.rs
@@ -6,8 +6,9 @@ use rustc_hir as hir;
 use rustc_hir::Unsafety;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::LocalDefId;
+use rustc_span::ErrorGuaranteed;
 
-pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
+pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), ErrorGuaranteed> {
     let item = tcx.hir().expect_item(def_id);
     let impl_ = item.expect_impl();
 
@@ -18,7 +19,7 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
             impl_.generics.params.iter().find(|p| p.pure_wrt_drop).map(|_| "may_dangle");
         match (trait_def.unsafety, unsafe_attr, impl_.unsafety, impl_.polarity) {
             (Unsafety::Normal, None, Unsafety::Unsafe, hir::ImplPolarity::Positive) => {
-                struct_span_code_err!(
+                return Err(struct_span_code_err!(
                     tcx.dcx(),
                     tcx.def_span(def_id),
                     E0199,
@@ -31,11 +32,11 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "",
                     rustc_errors::Applicability::MachineApplicable,
                 )
-                .emit();
+                .emit());
             }
 
             (Unsafety::Unsafe, _, Unsafety::Normal, hir::ImplPolarity::Positive) => {
-                struct_span_code_err!(
+                return Err(struct_span_code_err!(
                     tcx.dcx(),
                     tcx.def_span(def_id),
                     E0200,
@@ -54,11 +55,11 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "unsafe ",
                     rustc_errors::Applicability::MaybeIncorrect,
                 )
-                .emit();
+                .emit());
             }
 
             (Unsafety::Normal, Some(attr_name), Unsafety::Normal, hir::ImplPolarity::Positive) => {
-                struct_span_code_err!(
+                return Err(struct_span_code_err!(
                     tcx.dcx(),
                     tcx.def_span(def_id),
                     E0569,
@@ -77,7 +78,7 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "unsafe ",
                     rustc_errors::Applicability::MaybeIncorrect,
                 )
-                .emit();
+                .emit());
             }
 
             (_, _, Unsafety::Unsafe, hir::ImplPolarity::Negative(_)) => {
@@ -92,4 +93,5 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
             }
         }
     }
+    Ok(())
 }
diff --git a/compiler/rustc_hir_analysis/src/lib.rs b/compiler/rustc_hir_analysis/src/lib.rs
index 0a3a71ba1a28d..ad5b666467748 100644
--- a/compiler/rustc_hir_analysis/src/lib.rs
+++ b/compiler/rustc_hir_analysis/src/lib.rs
@@ -172,19 +172,15 @@ pub fn check_crate(tcx: TyCtxt<'_>) -> Result<(), ErrorGuaranteed> {
 
     tcx.sess.time("coherence_checking", || {
         // Check impls constrain their parameters
-        let res =
+        let mut res =
             tcx.hir().try_par_for_each_module(|module| tcx.ensure().check_mod_impl_wf(module));
 
-        // FIXME(matthewjasper) We shouldn't need to use `track_errors` anywhere in this function
-        // or the compiler in general.
-        res.and(tcx.sess.track_errors(|| {
-            for &trait_def_id in tcx.all_local_trait_impls(()).keys() {
-                tcx.ensure().coherent_trait(trait_def_id);
-            }
-        }))
+        for &trait_def_id in tcx.all_local_trait_impls(()).keys() {
+            res = res.and(tcx.ensure().coherent_trait(trait_def_id));
+        }
         // these queries are executed for side-effects (error reporting):
-        .and(tcx.ensure().crate_inherent_impls(()))
-        .and(tcx.ensure().crate_inherent_impls_overlap_check(()))
+        res.and(tcx.ensure().crate_inherent_impls(()))
+            .and(tcx.ensure().crate_inherent_impls_overlap_check(()))
     })?;
 
     if tcx.features().rustc_attrs {
diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index d486f069989ee..9dc365526d467 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/compiler/rustc_hir_typeck/src/callee.rs
@@ -220,6 +220,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             (self.tcx.lang_items().fn_trait(), Ident::with_dummy_span(sym::call), true),
             (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
             (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
+            (self.tcx.lang_items().async_fn_trait(), Ident::with_dummy_span(sym::async_call), true),
+            (
+                self.tcx.lang_items().async_fn_mut_trait(),
+                Ident::with_dummy_span(sym::async_call_mut),
+                true,
+            ),
+            (
+                self.tcx.lang_items().async_fn_once_trait(),
+                Ident::with_dummy_span(sym::async_call_once),
+                false,
+            ),
         ] {
             let Some(trait_def_id) = opt_trait_def_id else { continue };
 
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs b/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
index 01cd3c57925b3..21ea48c6c83da 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
@@ -294,6 +294,75 @@ impl<T> Trait<T> for X {
                             );
                         }
                     }
+                    (ty::Dynamic(t, _, ty::DynKind::Dyn), ty::Alias(ty::Opaque, alias))
+                        if let Some(def_id) = t.principal_def_id()
+                            && tcx.explicit_item_bounds(alias.def_id).skip_binder().iter().any(
+                                |(pred, _span)| match pred.kind().skip_binder() {
+                                    ty::ClauseKind::Trait(trait_predicate)
+                                        if trait_predicate.polarity
+                                            == ty::ImplPolarity::Positive =>
+                                    {
+                                        trait_predicate.def_id() == def_id
+                                    }
+                                    _ => false,
+                                },
+                            ) =>
+                    {
+                        diag.help(format!(
+                            "you can box the `{}` to coerce it to `Box<{}>`, but you'll have to \
+                             change the expected type as well",
+                            values.found, values.expected,
+                        ));
+                    }
+                    (ty::Dynamic(t, _, ty::DynKind::Dyn), _)
+                        if let Some(def_id) = t.principal_def_id() =>
+                    {
+                        let mut impl_def_ids = vec![];
+                        tcx.for_each_relevant_impl(def_id, values.found, |did| {
+                            impl_def_ids.push(did)
+                        });
+                        if let [_] = &impl_def_ids[..] {
+                            let trait_name = tcx.item_name(def_id);
+                            diag.help(format!(
+                                "`{}` implements `{trait_name}` so you could box the found value \
+                                 and coerce it to the trait object `Box<dyn {trait_name}>`, you \
+                                 will have to change the expected type as well",
+                                values.found,
+                            ));
+                        }
+                    }
+                    (_, ty::Dynamic(t, _, ty::DynKind::Dyn))
+                        if let Some(def_id) = t.principal_def_id() =>
+                    {
+                        let mut impl_def_ids = vec![];
+                        tcx.for_each_relevant_impl(def_id, values.expected, |did| {
+                            impl_def_ids.push(did)
+                        });
+                        if let [_] = &impl_def_ids[..] {
+                            let trait_name = tcx.item_name(def_id);
+                            diag.help(format!(
+                                "`{}` implements `{trait_name}` so you could change the expected \
+                                 type to `Box<dyn {trait_name}>`",
+                                values.expected,
+                            ));
+                        }
+                    }
+                    (ty::Dynamic(t, _, ty::DynKind::DynStar), _)
+                        if let Some(def_id) = t.principal_def_id() =>
+                    {
+                        let mut impl_def_ids = vec![];
+                        tcx.for_each_relevant_impl(def_id, values.found, |did| {
+                            impl_def_ids.push(did)
+                        });
+                        if let [_] = &impl_def_ids[..] {
+                            let trait_name = tcx.item_name(def_id);
+                            diag.help(format!(
+                                "`{}` implements `{trait_name}`, `#[feature(dyn_star)]` is likely \
+                                 not enabled; that feature it is currently incomplete",
+                                values.found,
+                            ));
+                        }
+                    }
                     (_, ty::Alias(ty::Opaque, opaque_ty))
                     | (ty::Alias(ty::Opaque, opaque_ty), _) => {
                         if opaque_ty.def_id.is_local()
diff --git a/compiler/rustc_macros/src/query.rs b/compiler/rustc_macros/src/query.rs
index ad1980136f3b9..25675e06e38d6 100644
--- a/compiler/rustc_macros/src/query.rs
+++ b/compiler/rustc_macros/src/query.rs
@@ -120,7 +120,8 @@ struct QueryModifiers {
 
     /// Forward the result on ensure if the query gets recomputed, and
     /// return `Ok(())` otherwise. Only applicable to queries returning
-    /// `Result<(), ErrorGuaranteed>`
+    /// `Result<T, ErrorGuaranteed>`. The `T` is not returned from `ensure`
+    /// invocations.
     ensure_forwards_result_if_red: Option<Ident>,
 }
 
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index c3d6c21c4027d..0b352a02b64c7 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -236,7 +236,14 @@ provide! { tcx, def_id, other, cdata,
     impl_polarity => { table_direct }
     defaultness => { table_direct }
     constness => { table_direct }
-    coerce_unsized_info => { table }
+    coerce_unsized_info => {
+        Ok(cdata
+            .root
+            .tables
+            .coerce_unsized_info
+            .get(cdata, def_id.index)
+            .map(|lazy| lazy.decode((cdata, tcx)))
+            .process_decoded(tcx, || panic!("{def_id:?} does not have coerce_unsized_info"))) }
     mir_const_qualif => { table }
     rendered_const => { table }
     asyncness => { table_direct }
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 2d4e49e27d9b9..542caf8622385 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1994,7 +1994,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 // if this is an impl of `CoerceUnsized`, create its
                 // "unsized info", else just store None
                 if Some(trait_ref.def_id) == tcx.lang_items().coerce_unsized_trait() {
-                    let coerce_unsized_info = tcx.coerce_unsized_info(def_id);
+                    let coerce_unsized_info = tcx.coerce_unsized_info(def_id).unwrap();
                     record!(self.tables.coerce_unsized_info[def_id] <- coerce_unsized_info);
                 }
             }
diff --git a/compiler/rustc_middle/src/query/erase.rs b/compiler/rustc_middle/src/query/erase.rs
index 778b4308d2ee8..1b01df6a18714 100644
--- a/compiler/rustc_middle/src/query/erase.rs
+++ b/compiler/rustc_middle/src/query/erase.rs
@@ -1,6 +1,7 @@
 use crate::mir;
 use crate::query::CyclePlaceholder;
 use crate::traits;
+use crate::ty::adjustment::CoerceUnsizedInfo;
 use crate::ty::{self, Ty};
 use std::intrinsics::transmute_unchecked;
 use std::mem::{size_of, MaybeUninit};
@@ -105,6 +106,10 @@ impl EraseType for Result<Option<ty::Instance<'_>>, rustc_errors::ErrorGuarantee
         [u8; size_of::<Result<Option<ty::Instance<'static>>, rustc_errors::ErrorGuaranteed>>()];
 }
 
+impl EraseType for Result<CoerceUnsizedInfo, rustc_errors::ErrorGuaranteed> {
+    type Result = [u8; size_of::<Result<CoerceUnsizedInfo, rustc_errors::ErrorGuaranteed>>()];
+}
+
 impl EraseType for Result<Option<ty::EarlyBinder<ty::Const<'_>>>, rustc_errors::ErrorGuaranteed> {
     type Result = [u8; size_of::<
         Result<Option<ty::EarlyBinder<ty::Const<'static>>>, rustc_errors::ErrorGuaranteed>,
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 23daefd5a65b9..2f7ae68c69211 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -977,10 +977,11 @@ rustc_queries! {
     }
 
     /// Caches `CoerceUnsized` kinds for impls on custom types.
-    query coerce_unsized_info(key: DefId) -> ty::adjustment::CoerceUnsizedInfo {
+    query coerce_unsized_info(key: DefId) -> Result<ty::adjustment::CoerceUnsizedInfo, ErrorGuaranteed> {
         desc { |tcx| "computing CoerceUnsized info for `{}`", tcx.def_path_str(key) }
         cache_on_disk_if { key.is_local() }
         separate_provide_extern
+        ensure_forwards_result_if_red
     }
 
     query typeck(key: LocalDefId) -> &'tcx ty::TypeckResults<'tcx> {
@@ -1000,8 +1001,9 @@ rustc_queries! {
         desc { |tcx| "checking whether `{}` has a body", tcx.def_path_str(def_id) }
     }
 
-    query coherent_trait(def_id: DefId) -> () {
+    query coherent_trait(def_id: DefId) -> Result<(), ErrorGuaranteed> {
         desc { |tcx| "coherence checking all impls of trait `{}`", tcx.def_path_str(def_id) }
+        ensure_forwards_result_if_red
     }
 
     /// Borrow-checks the function body. If this is a closure, returns
@@ -1032,6 +1034,7 @@ rustc_queries! {
             "checking whether impl `{}` follows the orphan rules",
             tcx.def_path_str(key),
         }
+        ensure_forwards_result_if_red
     }
 
     /// Check whether the function has any recursion that could cause the inliner to trigger
@@ -1300,6 +1303,7 @@ rustc_queries! {
     query specialization_graph_of(trait_id: DefId) -> Result<&'tcx specialization_graph::Graph, ErrorGuaranteed> {
         desc { |tcx| "building specialization graph of trait `{}`", tcx.def_path_str(trait_id) }
         cache_on_disk_if { true }
+        ensure_forwards_result_if_red
     }
     query object_safety_violations(trait_id: DefId) -> &'tcx [ObjectSafetyViolation] {
         desc { |tcx| "determining object safety of trait `{}`", tcx.def_path_str(trait_id) }
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index ef621f29bfd71..1f81678d6a54c 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -350,7 +350,7 @@ impl<'tcx> TyCtxt<'tcx> {
         validate: impl Fn(Self, DefId) -> Result<(), ErrorGuaranteed>,
     ) -> Option<ty::Destructor> {
         let drop_trait = self.lang_items().drop_trait()?;
-        self.ensure().coherent_trait(drop_trait);
+        self.ensure().coherent_trait(drop_trait).ok()?;
 
         let ty = self.type_of(adt_did).instantiate_identity();
         let mut dtor_candidate = None;
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 316f7d678c641..ad5fcc3acf12f 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -59,167 +59,154 @@ impl<'tcx> MirPass<'tcx> for InstrumentCoverage {
             _ => {}
         }
 
-        trace!("InstrumentCoverage starting for {def_id:?}");
-        Instrumentor::new(tcx, mir_body).inject_counters();
-        trace!("InstrumentCoverage done for {def_id:?}");
+        instrument_function_for_coverage(tcx, mir_body);
     }
 }
 
-struct Instrumentor<'a, 'tcx> {
-    tcx: TyCtxt<'tcx>,
-    mir_body: &'a mut mir::Body<'tcx>,
-    hir_info: ExtractedHirInfo,
-    basic_coverage_blocks: CoverageGraph,
-}
-
-impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
-    fn new(tcx: TyCtxt<'tcx>, mir_body: &'a mut mir::Body<'tcx>) -> Self {
-        let hir_info = extract_hir_info(tcx, mir_body.source.def_id().expect_local());
+fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir::Body<'tcx>) {
+    let def_id = mir_body.source.def_id();
+    let _span = debug_span!("instrument_function_for_coverage", ?def_id).entered();
 
-        debug!(?hir_info, "instrumenting {:?}", mir_body.source.def_id());
+    let hir_info = extract_hir_info(tcx, def_id.expect_local());
+    let basic_coverage_blocks = CoverageGraph::from_mir(mir_body);
 
-        let basic_coverage_blocks = CoverageGraph::from_mir(mir_body);
+    ////////////////////////////////////////////////////
+    // Compute coverage spans from the `CoverageGraph`.
+    let Some(coverage_spans) =
+        spans::generate_coverage_spans(mir_body, &hir_info, &basic_coverage_blocks)
+    else {
+        // No relevant spans were found in MIR, so skip instrumenting this function.
+        return;
+    };
 
-        Self { tcx, mir_body, hir_info, basic_coverage_blocks }
+    ////////////////////////////////////////////////////
+    // Create an optimized mix of `Counter`s and `Expression`s for the `CoverageGraph`. Ensure
+    // every coverage span has a `Counter` or `Expression` assigned to its `BasicCoverageBlock`
+    // and all `Expression` dependencies (operands) are also generated, for any other
+    // `BasicCoverageBlock`s not already associated with a coverage span.
+    let bcb_has_coverage_spans = |bcb| coverage_spans.bcb_has_coverage_spans(bcb);
+    let coverage_counters =
+        CoverageCounters::make_bcb_counters(&basic_coverage_blocks, bcb_has_coverage_spans);
+
+    let mappings = create_mappings(tcx, &hir_info, &coverage_spans, &coverage_counters);
+    if mappings.is_empty() {
+        // No spans could be converted into valid mappings, so skip this function.
+        debug!("no spans could be converted into valid mappings; skipping");
+        return;
     }
 
-    fn inject_counters(&'a mut self) {
-        ////////////////////////////////////////////////////
-        // Compute coverage spans from the `CoverageGraph`.
-        let Some(coverage_spans) = CoverageSpans::generate_coverage_spans(
-            self.mir_body,
-            &self.hir_info,
-            &self.basic_coverage_blocks,
-        ) else {
-            // No relevant spans were found in MIR, so skip instrumenting this function.
-            return;
-        };
-
-        ////////////////////////////////////////////////////
-        // Create an optimized mix of `Counter`s and `Expression`s for the `CoverageGraph`. Ensure
-        // every coverage span has a `Counter` or `Expression` assigned to its `BasicCoverageBlock`
-        // and all `Expression` dependencies (operands) are also generated, for any other
-        // `BasicCoverageBlock`s not already associated with a coverage span.
-        let bcb_has_coverage_spans = |bcb| coverage_spans.bcb_has_coverage_spans(bcb);
-        let coverage_counters = CoverageCounters::make_bcb_counters(
-            &self.basic_coverage_blocks,
-            bcb_has_coverage_spans,
-        );
+    inject_coverage_statements(
+        mir_body,
+        &basic_coverage_blocks,
+        bcb_has_coverage_spans,
+        &coverage_counters,
+    );
 
-        let mappings = self.create_mappings(&coverage_spans, &coverage_counters);
-        if mappings.is_empty() {
-            // No spans could be converted into valid mappings, so skip this function.
-            debug!("no spans could be converted into valid mappings; skipping");
-            return;
-        }
+    mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
+        function_source_hash: hir_info.function_source_hash,
+        num_counters: coverage_counters.num_counters(),
+        expressions: coverage_counters.into_expressions(),
+        mappings,
+    }));
+}
 
-        self.inject_coverage_statements(bcb_has_coverage_spans, &coverage_counters);
+/// For each coverage span extracted from MIR, create a corresponding
+/// mapping.
+///
+/// Precondition: All BCBs corresponding to those spans have been given
+/// coverage counters.
+fn create_mappings<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    hir_info: &ExtractedHirInfo,
+    coverage_spans: &CoverageSpans,
+    coverage_counters: &CoverageCounters,
+) -> Vec<Mapping> {
+    let source_map = tcx.sess.source_map();
+    let body_span = hir_info.body_span;
+
+    let source_file = source_map.lookup_source_file(body_span.lo());
+    use rustc_session::RemapFileNameExt;
+    let file_name = Symbol::intern(&source_file.name.for_codegen(tcx.sess).to_string_lossy());
+
+    let term_for_bcb = |bcb| {
+        coverage_counters
+            .bcb_counter(bcb)
+            .expect("all BCBs with spans were given counters")
+            .as_term()
+    };
 
-        self.mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
-            function_source_hash: self.hir_info.function_source_hash,
-            num_counters: coverage_counters.num_counters(),
-            expressions: coverage_counters.into_expressions(),
-            mappings,
-        }));
-    }
+    coverage_spans
+        .all_bcb_mappings()
+        .filter_map(|&BcbMapping { kind: bcb_mapping_kind, span }| {
+            let kind = match bcb_mapping_kind {
+                BcbMappingKind::Code(bcb) => MappingKind::Code(term_for_bcb(bcb)),
+            };
+            let code_region = make_code_region(source_map, file_name, span, body_span)?;
+            Some(Mapping { kind, code_region })
+        })
+        .collect::<Vec<_>>()
+}
 
-    /// For each coverage span extracted from MIR, create a corresponding
-    /// mapping.
-    ///
-    /// Precondition: All BCBs corresponding to those spans have been given
-    /// coverage counters.
-    fn create_mappings(
-        &self,
-        coverage_spans: &CoverageSpans,
-        coverage_counters: &CoverageCounters,
-    ) -> Vec<Mapping> {
-        let source_map = self.tcx.sess.source_map();
-        let body_span = self.hir_info.body_span;
-
-        let source_file = source_map.lookup_source_file(body_span.lo());
-        use rustc_session::RemapFileNameExt;
-        let file_name =
-            Symbol::intern(&source_file.name.for_codegen(self.tcx.sess).to_string_lossy());
-
-        let term_for_bcb = |bcb| {
-            coverage_counters
-                .bcb_counter(bcb)
-                .expect("all BCBs with spans were given counters")
-                .as_term()
+/// For each BCB node or BCB edge that has an associated coverage counter,
+/// inject any necessary coverage statements into MIR.
+fn inject_coverage_statements<'tcx>(
+    mir_body: &mut mir::Body<'tcx>,
+    basic_coverage_blocks: &CoverageGraph,
+    bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool,
+    coverage_counters: &CoverageCounters,
+) {
+    // Process the counters associated with BCB nodes.
+    for (bcb, counter_kind) in coverage_counters.bcb_node_counters() {
+        let do_inject = match counter_kind {
+            // Counter-increment statements always need to be injected.
+            BcbCounter::Counter { .. } => true,
+            // The only purpose of expression-used statements is to detect
+            // when a mapping is unreachable, so we only inject them for
+            // expressions with one or more mappings.
+            BcbCounter::Expression { .. } => bcb_has_coverage_spans(bcb),
         };
-
-        coverage_spans
-            .all_bcb_mappings()
-            .filter_map(|&BcbMapping { kind: bcb_mapping_kind, span }| {
-                let kind = match bcb_mapping_kind {
-                    BcbMappingKind::Code(bcb) => MappingKind::Code(term_for_bcb(bcb)),
-                };
-                let code_region = make_code_region(source_map, file_name, span, body_span)?;
-                Some(Mapping { kind, code_region })
-            })
-            .collect::<Vec<_>>()
+        if do_inject {
+            inject_statement(
+                mir_body,
+                make_mir_coverage_kind(counter_kind),
+                basic_coverage_blocks[bcb].leader_bb(),
+            );
+        }
     }
 
-    /// For each BCB node or BCB edge that has an associated coverage counter,
-    /// inject any necessary coverage statements into MIR.
-    fn inject_coverage_statements(
-        &mut self,
-        bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool,
-        coverage_counters: &CoverageCounters,
-    ) {
-        // Process the counters associated with BCB nodes.
-        for (bcb, counter_kind) in coverage_counters.bcb_node_counters() {
-            let do_inject = match counter_kind {
-                // Counter-increment statements always need to be injected.
-                BcbCounter::Counter { .. } => true,
-                // The only purpose of expression-used statements is to detect
-                // when a mapping is unreachable, so we only inject them for
-                // expressions with one or more mappings.
-                BcbCounter::Expression { .. } => bcb_has_coverage_spans(bcb),
-            };
-            if do_inject {
-                inject_statement(
-                    self.mir_body,
-                    self.make_mir_coverage_kind(counter_kind),
-                    self.basic_coverage_blocks[bcb].leader_bb(),
-                );
-            }
+    // Process the counters associated with BCB edges.
+    for (from_bcb, to_bcb, counter_kind) in coverage_counters.bcb_edge_counters() {
+        let do_inject = match counter_kind {
+            // Counter-increment statements always need to be injected.
+            BcbCounter::Counter { .. } => true,
+            // BCB-edge expressions never have mappings, so they never need
+            // a corresponding statement.
+            BcbCounter::Expression { .. } => false,
+        };
+        if !do_inject {
+            continue;
         }
 
-        // Process the counters associated with BCB edges.
-        for (from_bcb, to_bcb, counter_kind) in coverage_counters.bcb_edge_counters() {
-            let do_inject = match counter_kind {
-                // Counter-increment statements always need to be injected.
-                BcbCounter::Counter { .. } => true,
-                // BCB-edge expressions never have mappings, so they never need
-                // a corresponding statement.
-                BcbCounter::Expression { .. } => false,
-            };
-            if !do_inject {
-                continue;
-            }
-
-            // We need to inject a coverage statement into a new BB between the
-            // last BB of `from_bcb` and the first BB of `to_bcb`.
-            let from_bb = self.basic_coverage_blocks[from_bcb].last_bb();
-            let to_bb = self.basic_coverage_blocks[to_bcb].leader_bb();
+        // We need to inject a coverage statement into a new BB between the
+        // last BB of `from_bcb` and the first BB of `to_bcb`.
+        let from_bb = basic_coverage_blocks[from_bcb].last_bb();
+        let to_bb = basic_coverage_blocks[to_bcb].leader_bb();
 
-            let new_bb = inject_edge_counter_basic_block(self.mir_body, from_bb, to_bb);
-            debug!(
-                "Edge {from_bcb:?} (last {from_bb:?}) -> {to_bcb:?} (leader {to_bb:?}) \
+        let new_bb = inject_edge_counter_basic_block(mir_body, from_bb, to_bb);
+        debug!(
+            "Edge {from_bcb:?} (last {from_bb:?}) -> {to_bcb:?} (leader {to_bb:?}) \
                 requires a new MIR BasicBlock {new_bb:?} for edge counter {counter_kind:?}",
-            );
+        );
 
-            // Inject a counter into the newly-created BB.
-            inject_statement(self.mir_body, self.make_mir_coverage_kind(counter_kind), new_bb);
-        }
+        // Inject a counter into the newly-created BB.
+        inject_statement(mir_body, make_mir_coverage_kind(counter_kind), new_bb);
     }
+}
 
-    fn make_mir_coverage_kind(&self, counter_kind: &BcbCounter) -> CoverageKind {
-        match *counter_kind {
-            BcbCounter::Counter { id } => CoverageKind::CounterIncrement { id },
-            BcbCounter::Expression { id } => CoverageKind::ExpressionUsed { id },
-        }
+fn make_mir_coverage_kind(counter_kind: &BcbCounter) -> CoverageKind {
+    match *counter_kind {
+        BcbCounter::Counter { id } => CoverageKind::CounterIncrement { id },
+        BcbCounter::Expression { id } => CoverageKind::ExpressionUsed { id },
     }
 }
 
diff --git a/compiler/rustc_mir_transform/src/coverage/spans.rs b/compiler/rustc_mir_transform/src/coverage/spans.rs
index 81f6c8312061b..dee6a3b7143f5 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans.rs
@@ -26,45 +26,6 @@ pub(super) struct CoverageSpans {
 }
 
 impl CoverageSpans {
-    /// Extracts coverage-relevant spans from MIR, and associates them with
-    /// their corresponding BCBs.
-    ///
-    /// Returns `None` if no coverage-relevant spans could be extracted.
-    pub(super) fn generate_coverage_spans(
-        mir_body: &mir::Body<'_>,
-        hir_info: &ExtractedHirInfo,
-        basic_coverage_blocks: &CoverageGraph,
-    ) -> Option<Self> {
-        let mut mappings = vec![];
-
-        let coverage_spans = CoverageSpansGenerator::generate_coverage_spans(
-            mir_body,
-            hir_info,
-            basic_coverage_blocks,
-        );
-        mappings.extend(coverage_spans.into_iter().map(|CoverageSpan { bcb, span, .. }| {
-            // Each span produced by the generator represents an ordinary code region.
-            BcbMapping { kind: BcbMappingKind::Code(bcb), span }
-        }));
-
-        if mappings.is_empty() {
-            return None;
-        }
-
-        // Identify which BCBs have one or more mappings.
-        let mut bcb_has_mappings = BitSet::new_empty(basic_coverage_blocks.num_nodes());
-        let mut insert = |bcb| {
-            bcb_has_mappings.insert(bcb);
-        };
-        for &BcbMapping { kind, span: _ } in &mappings {
-            match kind {
-                BcbMappingKind::Code(bcb) => insert(bcb),
-            }
-        }
-
-        Some(Self { bcb_has_mappings, mappings })
-    }
-
     pub(super) fn bcb_has_coverage_spans(&self, bcb: BasicCoverageBlock) -> bool {
         self.bcb_has_mappings.contains(bcb)
     }
@@ -74,6 +35,43 @@ impl CoverageSpans {
     }
 }
 
+/// Extracts coverage-relevant spans from MIR, and associates them with
+/// their corresponding BCBs.
+///
+/// Returns `None` if no coverage-relevant spans could be extracted.
+pub(super) fn generate_coverage_spans(
+    mir_body: &mir::Body<'_>,
+    hir_info: &ExtractedHirInfo,
+    basic_coverage_blocks: &CoverageGraph,
+) -> Option<CoverageSpans> {
+    let mut mappings = vec![];
+
+    let sorted_spans =
+        from_mir::mir_to_initial_sorted_coverage_spans(mir_body, hir_info, basic_coverage_blocks);
+    let coverage_spans = SpansRefiner::refine_sorted_spans(basic_coverage_blocks, sorted_spans);
+    mappings.extend(coverage_spans.into_iter().map(|CoverageSpan { bcb, span, .. }| {
+        // Each span produced by the generator represents an ordinary code region.
+        BcbMapping { kind: BcbMappingKind::Code(bcb), span }
+    }));
+
+    if mappings.is_empty() {
+        return None;
+    }
+
+    // Identify which BCBs have one or more mappings.
+    let mut bcb_has_mappings = BitSet::new_empty(basic_coverage_blocks.num_nodes());
+    let mut insert = |bcb| {
+        bcb_has_mappings.insert(bcb);
+    };
+    for &BcbMapping { kind, span: _ } in &mappings {
+        match kind {
+            BcbMappingKind::Code(bcb) => insert(bcb),
+        }
+    }
+
+    Some(CoverageSpans { bcb_has_mappings, mappings })
+}
+
 /// A BCB is deconstructed into one or more `Span`s. Each `Span` maps to a `CoverageSpan` that
 /// references the originating BCB and one or more MIR `Statement`s and/or `Terminator`s.
 /// Initially, the `Span`s come from the `Statement`s and `Terminator`s, but subsequent
@@ -130,7 +128,7 @@ impl CoverageSpan {
 ///  * Merge spans that represent continuous (both in source code and control flow), non-branching
 ///    execution
 ///  * Carve out (leave uncovered) any span that will be counted by another MIR (notably, closures)
-struct CoverageSpansGenerator<'a> {
+struct SpansRefiner<'a> {
     /// The BasicCoverageBlock Control Flow Graph (BCB CFG).
     basic_coverage_blocks: &'a CoverageGraph,
 
@@ -173,40 +171,15 @@ struct CoverageSpansGenerator<'a> {
     refined_spans: Vec<CoverageSpan>,
 }
 
-impl<'a> CoverageSpansGenerator<'a> {
-    /// Generate a minimal set of `CoverageSpan`s, each representing a contiguous code region to be
-    /// counted.
-    ///
-    /// The basic steps are:
-    ///
-    /// 1. Extract an initial set of spans from the `Statement`s and `Terminator`s of each
-    ///    `BasicCoverageBlockData`.
-    /// 2. Sort the spans by span.lo() (starting position). Spans that start at the same position
-    ///    are sorted with longer spans before shorter spans; and equal spans are sorted
-    ///    (deterministically) based on "dominator" relationship (if any).
-    /// 3. Traverse the spans in sorted order to identify spans that can be dropped (for instance,
-    ///    if another span or spans are already counting the same code region), or should be merged
-    ///    into a broader combined span (because it represents a contiguous, non-branching, and
-    ///    uninterrupted region of source code).
-    ///
-    ///    Closures are exposed in their enclosing functions as `Assign` `Rvalue`s, and since
-    ///    closures have their own MIR, their `Span` in their enclosing function should be left
-    ///    "uncovered".
-    ///
-    /// Note the resulting vector of `CoverageSpan`s may not be fully sorted (and does not need
-    /// to be).
-    pub(super) fn generate_coverage_spans(
-        mir_body: &mir::Body<'_>,
-        hir_info: &ExtractedHirInfo,
+impl<'a> SpansRefiner<'a> {
+    /// Takes the initial list of (sorted) spans extracted from MIR, and "refines"
+    /// them by merging compatible adjacent spans, removing redundant spans,
+    /// and carving holes in spans when they overlap in unwanted ways.
+    fn refine_sorted_spans(
         basic_coverage_blocks: &'a CoverageGraph,
+        sorted_spans: Vec<CoverageSpan>,
     ) -> Vec<CoverageSpan> {
-        let sorted_spans = from_mir::mir_to_initial_sorted_coverage_spans(
-            mir_body,
-            hir_info,
-            basic_coverage_blocks,
-        );
-
-        let coverage_spans = Self {
+        let this = Self {
             basic_coverage_blocks,
             sorted_spans_iter: sorted_spans.into_iter(),
             some_curr: None,
@@ -217,7 +190,7 @@ impl<'a> CoverageSpansGenerator<'a> {
             refined_spans: Vec::with_capacity(basic_coverage_blocks.num_nodes() * 2),
         };
 
-        coverage_spans.to_refined_spans()
+        this.to_refined_spans()
     }
 
     /// Iterate through the sorted `CoverageSpan`s, and return the refined list of merged and
diff --git a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
index 1b6dfccd57495..8d8e8e6132743 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
@@ -12,6 +12,12 @@ use crate::coverage::graph::{
 use crate::coverage::spans::CoverageSpan;
 use crate::coverage::ExtractedHirInfo;
 
+/// Traverses the MIR body to produce an initial collection of coverage-relevant
+/// spans, each associated with a node in the coverage graph (BCB) and possibly
+/// other metadata.
+///
+/// The returned spans are sorted in a specific order that is expected by the
+/// subsequent span-refinement step.
 pub(super) fn mir_to_initial_sorted_coverage_spans(
     mir_body: &mir::Body<'_>,
     hir_info: &ExtractedHirInfo,
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 8b991268819b3..b86244e5a4bfd 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -1113,7 +1113,13 @@ fn find_vtable_types_for_unsizing<'tcx>(
             assert_eq!(source_adt_def, target_adt_def);
 
             let CustomCoerceUnsized::Struct(coerce_index) =
-                crate::custom_coerce_unsize_info(tcx, source_ty, target_ty);
+                match crate::custom_coerce_unsize_info(tcx, source_ty, target_ty) {
+                    Ok(ccu) => ccu,
+                    Err(e) => {
+                        let e = Ty::new_error(tcx.tcx, e);
+                        return (e, e);
+                    }
+                };
 
             let source_fields = &source_adt_def.non_enum_variant().fields;
             let target_fields = &target_adt_def.non_enum_variant().fields;
diff --git a/compiler/rustc_monomorphize/src/lib.rs b/compiler/rustc_monomorphize/src/lib.rs
index 2f5f2d15cd456..3fa40db3c806e 100644
--- a/compiler/rustc_monomorphize/src/lib.rs
+++ b/compiler/rustc_monomorphize/src/lib.rs
@@ -15,6 +15,7 @@ use rustc_middle::query::{Providers, TyCtxtAt};
 use rustc_middle::traits;
 use rustc_middle::ty::adjustment::CustomCoerceUnsized;
 use rustc_middle::ty::{self, Ty};
+use rustc_span::ErrorGuaranteed;
 
 mod collector;
 mod errors;
@@ -28,7 +29,7 @@ fn custom_coerce_unsize_info<'tcx>(
     tcx: TyCtxtAt<'tcx>,
     source_ty: Ty<'tcx>,
     target_ty: Ty<'tcx>,
-) -> CustomCoerceUnsized {
+) -> Result<CustomCoerceUnsized, ErrorGuaranteed> {
     let trait_ref = ty::TraitRef::from_lang_item(
         tcx.tcx,
         LangItem::CoerceUnsized,
@@ -40,7 +41,7 @@ fn custom_coerce_unsize_info<'tcx>(
         Ok(traits::ImplSource::UserDefined(traits::ImplSourceUserDefinedData {
             impl_def_id,
             ..
-        })) => tcx.coerce_unsized_info(impl_def_id).custom_kind.unwrap(),
+        })) => Ok(tcx.coerce_unsized_info(impl_def_id)?.custom_kind.unwrap()),
         impl_source => {
             bug!("invalid `CoerceUnsized` impl_source: {:?}", impl_source);
         }
diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index eba71a23435a6..e94a0373c79ff 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -391,12 +391,18 @@ impl IntRange {
 /// first.
 impl fmt::Debug for IntRange {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if let Finite(lo) = self.lo {
+        if self.is_singleton() {
+            // Only finite ranges can be singletons.
+            let Finite(lo) = self.lo else { unreachable!() };
             write!(f, "{lo}")?;
-        }
-        write!(f, "{}", RangeEnd::Excluded)?;
-        if let Finite(hi) = self.hi {
-            write!(f, "{hi}")?;
+        } else {
+            if let Finite(lo) = self.lo {
+                write!(f, "{lo}")?;
+            }
+            write!(f, "{}", RangeEnd::Excluded)?;
+            if let Finite(hi) = self.hi {
+                write!(f, "{hi}")?;
+            }
         }
         Ok(())
     }
diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index 4fd01b5e638ce..a55d53dfdacde 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -101,15 +101,23 @@ pub trait TypeCx: Sized + fmt::Debug {
 
     /// The types of the fields for this constructor. The result must have a length of
     /// `ctor_arity()`.
-    fn ctor_sub_tys(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> &[Self::Ty];
+    fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a Constructor<Self>,
+        ty: &'a Self::Ty,
+    ) -> impl Iterator<Item = Self::Ty> + ExactSizeIterator + Captures<'a>;
 
     /// The set of all the constructors for `ty`.
     ///
     /// This must follow the invariants of `ConstructorSet`
     fn ctors_for_ty(&self, ty: &Self::Ty) -> Result<ConstructorSet<Self>, Self::Error>;
 
-    /// Best-effort `Debug` implementation.
-    fn debug_pat(f: &mut fmt::Formatter<'_>, pat: &DeconstructedPat<'_, Self>) -> fmt::Result;
+    /// Write the name of the variant represented by `pat`. Used for the best-effort `Debug` impl of
+    /// `DeconstructedPat`. Only invoqued when `pat.ctor()` is `Struct | Variant(_) | UnionField`.
+    fn write_variant_name(
+        f: &mut fmt::Formatter<'_>,
+        pat: &crate::pat::DeconstructedPat<'_, Self>,
+    ) -> fmt::Result;
 
     /// Raise a bug.
     fn bug(&self, fmt: fmt::Arguments<'_>) -> !;
diff --git a/compiler/rustc_pattern_analysis/src/pat.rs b/compiler/rustc_pattern_analysis/src/pat.rs
index 8cd0ecb073c2f..1cc3107455645 100644
--- a/compiler/rustc_pattern_analysis/src/pat.rs
+++ b/compiler/rustc_pattern_analysis/src/pat.rs
@@ -142,7 +142,75 @@ impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
 /// This is best effort and not good enough for a `Display` impl.
 impl<'p, Cx: TypeCx> fmt::Debug for DeconstructedPat<'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        Cx::debug_pat(f, self)
+        let pat = self;
+        let mut first = true;
+        let mut start_or_continue = |s| {
+            if first {
+                first = false;
+                ""
+            } else {
+                s
+            }
+        };
+        let mut start_or_comma = || start_or_continue(", ");
+
+        match pat.ctor() {
+            Struct | Variant(_) | UnionField => {
+                Cx::write_variant_name(f, pat)?;
+                // Without `cx`, we can't know which field corresponds to which, so we can't
+                // get the names of the fields. Instead we just display everything as a tuple
+                // struct, which should be good enough.
+                write!(f, "(")?;
+                for p in pat.iter_fields() {
+                    write!(f, "{}", start_or_comma())?;
+                    write!(f, "{p:?}")?;
+                }
+                write!(f, ")")
+            }
+            // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
+            // be careful to detect strings here. However a string literal pattern will never
+            // be reported as a non-exhaustiveness witness, so we can ignore this issue.
+            Ref => {
+                let subpattern = pat.iter_fields().next().unwrap();
+                write!(f, "&{:?}", subpattern)
+            }
+            Slice(slice) => {
+                let mut subpatterns = pat.iter_fields();
+                write!(f, "[")?;
+                match slice.kind {
+                    SliceKind::FixedLen(_) => {
+                        for p in subpatterns {
+                            write!(f, "{}{:?}", start_or_comma(), p)?;
+                        }
+                    }
+                    SliceKind::VarLen(prefix_len, _) => {
+                        for p in subpatterns.by_ref().take(prefix_len) {
+                            write!(f, "{}{:?}", start_or_comma(), p)?;
+                        }
+                        write!(f, "{}", start_or_comma())?;
+                        write!(f, "..")?;
+                        for p in subpatterns {
+                            write!(f, "{}{:?}", start_or_comma(), p)?;
+                        }
+                    }
+                }
+                write!(f, "]")
+            }
+            Bool(b) => write!(f, "{b}"),
+            // Best-effort, will render signed ranges incorrectly
+            IntRange(range) => write!(f, "{range:?}"),
+            F32Range(lo, hi, end) => write!(f, "{lo}{end}{hi}"),
+            F64Range(lo, hi, end) => write!(f, "{lo}{end}{hi}"),
+            Str(value) => write!(f, "{value:?}"),
+            Opaque(..) => write!(f, "<constant pattern>"),
+            Or => {
+                for pat in pat.iter_fields() {
+                    write!(f, "{}{:?}", start_or_continue(" | "), pat)?;
+                }
+                Ok(())
+            }
+            Wildcard | Missing { .. } | NonExhaustive | Hidden => write!(f, "_ : {:?}", pat.ty()),
+        }
     }
 }
 
@@ -241,8 +309,7 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
     /// For example, if `ctor` is a `Constructor::Variant` for `Option::Some`, we get the pattern
     /// `Some(_)`.
     pub(crate) fn wild_from_ctor(pcx: &PlaceCtxt<'_, Cx>, ctor: Constructor<Cx>) -> Self {
-        let field_tys = pcx.ctor_sub_tys(&ctor);
-        let fields = field_tys.iter().cloned().map(|ty| Self::wildcard(ty)).collect();
+        let fields = pcx.ctor_sub_tys(&ctor).map(|ty| Self::wildcard(ty)).collect();
         Self::new(ctor, fields, pcx.ty.clone())
     }
 
diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index d8c3c010a2a93..23cab5f43ab6b 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -3,7 +3,6 @@ use std::fmt;
 use std::iter::once;
 
 use rustc_arena::{DroplessArena, TypedArena};
-use rustc_data_structures::captures::Captures;
 use rustc_hir::def_id::DefId;
 use rustc_hir::HirId;
 use rustc_index::{Idx, IndexVec};
@@ -20,7 +19,7 @@ use rustc_target::abi::{FieldIdx, Integer, VariantIdx, FIRST_VARIANT};
 use crate::constructor::{
     IntRange, MaybeInfiniteInt, OpaqueId, RangeEnd, Slice, SliceKind, VariantVisibility,
 };
-use crate::{errors, TypeCx};
+use crate::{errors, Captures, TypeCx};
 
 use crate::constructor::Constructor::*;
 
@@ -210,11 +209,11 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
     /// Returns the types of the fields for a given constructor. The result must have a length of
     /// `ctor.arity()`.
     #[instrument(level = "trace", skip(self))]
-    pub(crate) fn ctor_sub_tys(
-        &self,
-        ctor: &Constructor<'p, 'tcx>,
+    pub(crate) fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a Constructor<'p, 'tcx>,
         ty: RevealedTy<'tcx>,
-    ) -> &[RevealedTy<'tcx>] {
+    ) -> impl Iterator<Item = RevealedTy<'tcx>> + ExactSizeIterator + Captures<'a> {
         fn reveal_and_alloc<'a, 'tcx>(
             cx: &'a RustcMatchCheckCtxt<'_, 'tcx>,
             iter: impl Iterator<Item = Ty<'tcx>>,
@@ -222,7 +221,7 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
             cx.dropless_arena.alloc_from_iter(iter.map(|ty| cx.reveal_opaque_ty(ty)))
         }
         let cx = self;
-        match ctor {
+        let slice = match ctor {
             Struct | Variant(_) | UnionField => match ty.kind() {
                 ty::Tuple(fs) => reveal_and_alloc(cx, fs.iter()),
                 ty::Adt(adt, args) => {
@@ -263,7 +262,8 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
             Or => {
                 bug!("called `Fields::wildcards` on an `Or` ctor")
             }
-        }
+        };
+        slice.iter().copied()
     }
 
     /// The number of fields for this constructor.
@@ -850,103 +850,6 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
 
         Pat { ty: pat.ty().inner(), span: DUMMY_SP, kind }
     }
-
-    /// Best-effort `Debug` implementation.
-    pub(crate) fn debug_pat(
-        f: &mut fmt::Formatter<'_>,
-        pat: &crate::pat::DeconstructedPat<'_, Self>,
-    ) -> fmt::Result {
-        let mut first = true;
-        let mut start_or_continue = |s| {
-            if first {
-                first = false;
-                ""
-            } else {
-                s
-            }
-        };
-        let mut start_or_comma = || start_or_continue(", ");
-
-        match pat.ctor() {
-            Struct | Variant(_) | UnionField => match pat.ty().kind() {
-                ty::Adt(def, _) if def.is_box() => {
-                    // Without `box_patterns`, the only legal pattern of type `Box` is `_` (outside
-                    // of `std`). So this branch is only reachable when the feature is enabled and
-                    // the pattern is a box pattern.
-                    let subpattern = pat.iter_fields().next().unwrap();
-                    write!(f, "box {subpattern:?}")
-                }
-                ty::Adt(..) | ty::Tuple(..) => {
-                    let variant =
-                        match pat.ty().kind() {
-                            ty::Adt(adt, _) => Some(adt.variant(
-                                RustcMatchCheckCtxt::variant_index_for_adt(pat.ctor(), *adt),
-                            )),
-                            ty::Tuple(_) => None,
-                            _ => unreachable!(),
-                        };
-
-                    if let Some(variant) = variant {
-                        write!(f, "{}", variant.name)?;
-                    }
-
-                    // Without `cx`, we can't know which field corresponds to which, so we can't
-                    // get the names of the fields. Instead we just display everything as a tuple
-                    // struct, which should be good enough.
-                    write!(f, "(")?;
-                    for p in pat.iter_fields() {
-                        write!(f, "{}", start_or_comma())?;
-                        write!(f, "{p:?}")?;
-                    }
-                    write!(f, ")")
-                }
-                _ => write!(f, "_"),
-            },
-            // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
-            // be careful to detect strings here. However a string literal pattern will never
-            // be reported as a non-exhaustiveness witness, so we can ignore this issue.
-            Ref => {
-                let subpattern = pat.iter_fields().next().unwrap();
-                write!(f, "&{:?}", subpattern)
-            }
-            Slice(slice) => {
-                let mut subpatterns = pat.iter_fields();
-                write!(f, "[")?;
-                match slice.kind {
-                    SliceKind::FixedLen(_) => {
-                        for p in subpatterns {
-                            write!(f, "{}{:?}", start_or_comma(), p)?;
-                        }
-                    }
-                    SliceKind::VarLen(prefix_len, _) => {
-                        for p in subpatterns.by_ref().take(prefix_len) {
-                            write!(f, "{}{:?}", start_or_comma(), p)?;
-                        }
-                        write!(f, "{}", start_or_comma())?;
-                        write!(f, "..")?;
-                        for p in subpatterns {
-                            write!(f, "{}{:?}", start_or_comma(), p)?;
-                        }
-                    }
-                }
-                write!(f, "]")
-            }
-            Bool(b) => write!(f, "{b}"),
-            // Best-effort, will render signed ranges incorrectly
-            IntRange(range) => write!(f, "{range:?}"),
-            F32Range(lo, hi, end) => write!(f, "{lo}{end}{hi}"),
-            F64Range(lo, hi, end) => write!(f, "{lo}{end}{hi}"),
-            Str(value) => write!(f, "{value}"),
-            Opaque(..) => write!(f, "<constant pattern>"),
-            Or => {
-                for pat in pat.iter_fields() {
-                    write!(f, "{}{:?}", start_or_continue(" | "), pat)?;
-                }
-                Ok(())
-            }
-            Wildcard | Missing { .. } | NonExhaustive | Hidden => write!(f, "_ : {:?}", pat.ty()),
-        }
-    }
 }
 
 impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
@@ -964,11 +867,11 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
     fn ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: &Self::Ty) -> usize {
         self.ctor_arity(ctor, *ty)
     }
-    fn ctor_sub_tys(
-        &self,
-        ctor: &crate::constructor::Constructor<Self>,
-        ty: &Self::Ty,
-    ) -> &[Self::Ty] {
+    fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a crate::constructor::Constructor<Self>,
+        ty: &'a Self::Ty,
+    ) -> impl Iterator<Item = Self::Ty> + ExactSizeIterator + Captures<'a> {
         self.ctor_sub_tys(ctor, *ty)
     }
     fn ctors_for_ty(
@@ -978,12 +881,21 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
         self.ctors_for_ty(*ty)
     }
 
-    fn debug_pat(
+    fn write_variant_name(
         f: &mut fmt::Formatter<'_>,
         pat: &crate::pat::DeconstructedPat<'_, Self>,
     ) -> fmt::Result {
-        Self::debug_pat(f, pat)
+        if let ty::Adt(adt, _) = pat.ty().kind() {
+            if adt.is_box() {
+                write!(f, "Box")?
+            } else {
+                let variant = adt.variant(Self::variant_index_for_adt(pat.ctor(), *adt));
+                write!(f, "{}", variant.name)?;
+            }
+        }
+        Ok(())
     }
+
     fn bug(&self, fmt: fmt::Arguments<'_>) -> ! {
         span_bug!(self.scrut_span, "{}", fmt)
     }
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index d7852a2b2cbfc..0de48c9f3c0d0 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -750,7 +750,10 @@ impl<'a, Cx: TypeCx> PlaceCtxt<'a, Cx> {
     pub(crate) fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize {
         self.mcx.tycx.ctor_arity(ctor, self.ty)
     }
-    pub(crate) fn ctor_sub_tys(&self, ctor: &Constructor<Cx>) -> &[Cx::Ty] {
+    pub(crate) fn ctor_sub_tys(
+        &'a self,
+        ctor: &'a Constructor<Cx>,
+    ) -> impl Iterator<Item = Cx::Ty> + ExactSizeIterator + Captures<'a> {
         self.mcx.tycx.ctor_sub_tys(ctor, self.ty)
     }
     pub(crate) fn ctors_for_ty(&self) -> Result<ConstructorSet<Cx>, Cx::Error> {
@@ -1058,8 +1061,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
     ) -> Matrix<'p, Cx> {
         let ctor_sub_tys = pcx.ctor_sub_tys(ctor);
         let arity = ctor_sub_tys.len();
-        let specialized_place_ty =
-            ctor_sub_tys.iter().chain(self.place_ty[1..].iter()).cloned().collect();
+        let specialized_place_ty = ctor_sub_tys.chain(self.place_ty[1..].iter().cloned()).collect();
         let ctor_sub_validity = self.place_validity[0].specialize(ctor);
         let specialized_place_validity = std::iter::repeat(ctor_sub_validity)
             .take(arity)
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 24aa336c68fe9..891a83ce80cb8 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -332,20 +332,6 @@ impl Session {
         }
     }
 
-    // FIXME(matthewjasper) Remove this method, it should never be needed.
-    pub fn track_errors<F, T>(&self, f: F) -> Result<T, ErrorGuaranteed>
-    where
-        F: FnOnce() -> T,
-    {
-        let old_count = self.dcx().err_count();
-        let result = f();
-        if self.dcx().err_count() == old_count {
-            Ok(result)
-        } else {
-            Err(self.dcx().delayed_bug("`self.err_count()` changed but an error was not emitted"))
-        }
-    }
-
     /// Used for code paths of expensive computations that should only take place when
     /// warnings or errors are emitted. If no messages are emitted ("good path"), then
     /// it's likely a bug.
@@ -1524,16 +1510,25 @@ pub trait RemapFileNameExt {
     where
         Self: 'a;
 
-    fn for_scope(&self, sess: &Session, scopes: RemapPathScopeComponents) -> Self::Output<'_>;
+    /// Returns a possibly remapped filename based on the passed scope and remap cli options.
+    ///
+    /// One and only one scope should be passed to this method. For anything related to
+    /// "codegen" see the [`RemapFileNameExt::for_codegen`] method.
+    fn for_scope(&self, sess: &Session, scope: RemapPathScopeComponents) -> Self::Output<'_>;
 
+    /// Return a possibly remapped filename, to be used in "codegen" related parts.
     fn for_codegen(&self, sess: &Session) -> Self::Output<'_>;
 }
 
 impl RemapFileNameExt for rustc_span::FileName {
     type Output<'a> = rustc_span::FileNameDisplay<'a>;
 
-    fn for_scope(&self, sess: &Session, scopes: RemapPathScopeComponents) -> Self::Output<'_> {
-        if sess.opts.unstable_opts.remap_path_scope.contains(scopes) {
+    fn for_scope(&self, sess: &Session, scope: RemapPathScopeComponents) -> Self::Output<'_> {
+        assert!(
+            scope.bits().count_ones() == 1,
+            "one and only one scope should be passed to for_scope"
+        );
+        if sess.opts.unstable_opts.remap_path_scope.contains(scope) {
             self.prefer_remapped_unconditionaly()
         } else {
             self.prefer_local()
@@ -1552,8 +1547,12 @@ impl RemapFileNameExt for rustc_span::FileName {
 impl RemapFileNameExt for rustc_span::RealFileName {
     type Output<'a> = &'a Path;
 
-    fn for_scope(&self, sess: &Session, scopes: RemapPathScopeComponents) -> Self::Output<'_> {
-        if sess.opts.unstable_opts.remap_path_scope.contains(scopes) {
+    fn for_scope(&self, sess: &Session, scope: RemapPathScopeComponents) -> Self::Output<'_> {
+        assert!(
+            scope.bits().count_ones() == 1,
+            "one and only one scope should be passed to for_scope"
+        );
+        if sess.opts.unstable_opts.remap_path_scope.contains(scope) {
             self.remapped_path_if_available()
         } else {
             self.local_path_if_available()
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 68a83aadeba7a..efd5e3727b9fc 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -425,8 +425,14 @@ symbols! {
         assume,
         assume_init,
         async_await,
+        async_call,
+        async_call_mut,
+        async_call_once,
         async_closure,
+        async_fn,
         async_fn_in_trait,
+        async_fn_mut,
+        async_fn_once,
         async_fn_track_caller,
         async_for_loop,
         async_iterator,
diff --git a/compiler/rustc_target/src/spec/base/wasm.rs b/compiler/rustc_target/src/spec/base/wasm.rs
index 87ade9e58cf45..4b4d2aca26e4a 100644
--- a/compiler/rustc_target/src/spec/base/wasm.rs
+++ b/compiler/rustc_target/src/spec/base/wasm.rs
@@ -38,9 +38,6 @@ pub fn options() -> TargetOptions {
                 // supposed to be imported and have all other symbols generate errors if
                 // they remain undefined.
                 concat!($prefix, "--allow-undefined"),
-                // Rust code should never have warnings, and warnings are often
-                // indicative of bugs, let's prevent them.
-                concat!($prefix, "--fatal-warnings"),
                 // LLD only implements C++-like demangling, which doesn't match our own
                 // mangling scheme. Tell LLD to not demangle anything and leave it up to
                 // us to demangle these symbols later. Currently rustc does not perform
diff --git a/compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs b/compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs
index 394f02ecac495..e9e874297d39b 100644
--- a/compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs
+++ b/compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs
@@ -5,10 +5,7 @@ use crate::spec::{
 pub fn target() -> Target {
     // Reset flags for non-Em flavors back to empty to satisfy sanity checking tests.
     let pre_link_args = LinkArgs::new();
-    let post_link_args = TargetOptions::link_args(
-        LinkerFlavor::EmCc,
-        &["-sABORTING_MALLOC=0", "-Wl,--fatal-warnings"],
-    );
+    let post_link_args = TargetOptions::link_args(LinkerFlavor::EmCc, &["-sABORTING_MALLOC=0"]);
 
     let opts = TargetOptions {
         os: "emscripten".into(),
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index f3b04b55a35c5..6a40ffebdada8 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -7,7 +7,7 @@ use super::{
 
 use crate::errors;
 use crate::infer::InferCtxt;
-use crate::traits::{NormalizeExt, ObligationCtxt};
+use crate::traits::{ImplDerivedObligationCause, NormalizeExt, ObligationCtxt};
 
 use hir::def::CtorOf;
 use rustc_data_structures::fx::FxHashSet;
@@ -2973,7 +2973,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::ObjectTypeBound(..) => {}
             ObligationCauseCode::RustCall => {
                 if let Some(pred) = predicate.to_opt_poly_trait_pred()
-                    && Some(pred.def_id()) == self.tcx.lang_items().sized_trait()
+                    && Some(pred.def_id()) == tcx.lang_items().sized_trait()
                 {
                     err.note("argument required to be sized due to `extern \"rust-call\"` ABI");
                 }
@@ -3022,15 +3022,15 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         let def_id = trait_pred.def_id();
                         let visible_item = if let Some(local) = def_id.as_local() {
                             // Check for local traits being reachable.
-                            let vis = &self.tcx.resolutions(()).effective_visibilities;
+                            let vis = &tcx.resolutions(()).effective_visibilities;
                             // Account for non-`pub` traits in the root of the local crate.
-                            let is_locally_reachable = self.tcx.parent(def_id).is_crate_root();
+                            let is_locally_reachable = tcx.parent(def_id).is_crate_root();
                             vis.is_reachable(local) || is_locally_reachable
                         } else {
                             // Check for foreign traits being reachable.
-                            self.tcx.visible_parent_map(()).get(&def_id).is_some()
+                            tcx.visible_parent_map(()).get(&def_id).is_some()
                         };
-                        if Some(def_id) == self.tcx.lang_items().sized_trait()
+                        if Some(def_id) == tcx.lang_items().sized_trait()
                             && let Some(hir::Node::TraitItem(hir::TraitItem {
                                 ident,
                                 kind: hir::TraitItemKind::Type(bounds, None),
@@ -3039,7 +3039,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             // Do not suggest relaxing if there is an explicit `Sized` obligation.
                             && !bounds.iter()
                                 .filter_map(|bound| bound.trait_ref())
-                                .any(|tr| tr.trait_def_id() == self.tcx.lang_items().sized_trait())
+                                .any(|tr| tr.trait_def_id() == tcx.lang_items().sized_trait())
                         {
                             let (span, separator) = if let [.., last] = bounds {
                                 (last.span().shrink_to_hi(), " +")
@@ -3102,10 +3102,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
             ObligationCauseCode::Coercion { source, target } => {
                 let mut file = None;
-                let source =
-                    self.tcx.short_ty_string(self.resolve_vars_if_possible(source), &mut file);
-                let target =
-                    self.tcx.short_ty_string(self.resolve_vars_if_possible(target), &mut file);
+                let source = tcx.short_ty_string(self.resolve_vars_if_possible(source), &mut file);
+                let target = tcx.short_ty_string(self.resolve_vars_if_possible(target), &mut file);
                 err.note(with_forced_trimmed_paths!(format!(
                     "required for the cast from `{source}` to `{target}`",
                 )));
@@ -3158,7 +3156,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     err.help("see https://doc.rust-lang.org/stable/std/array/fn.from_fn.html for more information");
                 }
 
-                if self.tcx.sess.is_nightly_build()
+                if tcx.sess.is_nightly_build()
                     && matches!(is_constable, IsConstable::Fn | IsConstable::Ctor)
                 {
                     err.help(
@@ -3168,8 +3166,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
             ObligationCauseCode::VariableType(hir_id) => {
-                let parent_node = self.tcx.hir().parent_id(hir_id);
-                match self.tcx.opt_hir_node(parent_node) {
+                let parent_node = tcx.hir().parent_id(hir_id);
+                match tcx.opt_hir_node(parent_node) {
                     Some(Node::Local(hir::Local { ty: Some(ty), .. })) => {
                         err.span_suggestion_verbose(
                             ty.span.shrink_to_lo(),
@@ -3207,7 +3205,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         err.note("all local variables must have a statically known size");
                     }
                 }
-                if !self.tcx.features().unsized_locals {
+                if !tcx.features().unsized_locals {
                     err.help("unsized locals are gated as an unstable feature");
                 }
             }
@@ -3289,7 +3287,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     err.note("all function arguments must have a statically known size");
                 }
                 if tcx.sess.opts.unstable_features.is_nightly_build()
-                    && !self.tcx.features().unsized_fn_params
+                    && !tcx.features().unsized_fn_params
                 {
                     err.help("unsized fn params are gated as an unstable feature");
                 }
@@ -3358,7 +3356,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     "all values captured by value by a closure must have a statically known size",
                 );
                 let hir::ExprKind::Closure(closure) =
-                    self.tcx.hir_node_by_def_id(closure_def_id).expect_expr().kind
+                    tcx.hir_node_by_def_id(closure_def_id).expect_expr().kind
                 else {
                     bug!("expected closure in SizedClosureCapture obligation");
                 };
@@ -3369,7 +3367,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
             ObligationCauseCode::SizedCoroutineInterior(coroutine_def_id) => {
-                let what = match self.tcx.coroutine_kind(coroutine_def_id) {
+                let what = match tcx.coroutine_kind(coroutine_def_id) {
                     None
                     | Some(hir::CoroutineKind::Coroutine(_))
                     | Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _)) => {
@@ -3420,10 +3418,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 'print: {
                     if !is_upvar_tys_infer_tuple {
                         let mut file = None;
-                        let ty_str = self.tcx.short_ty_string(ty, &mut file);
+                        let ty_str = tcx.short_ty_string(ty, &mut file);
                         let msg = format!("required because it appears within the type `{ty_str}`");
                         match ty.kind() {
-                            ty::Adt(def, _) => match self.tcx.opt_item_ident(def.did()) {
+                            ty::Adt(def, _) => match tcx.opt_item_ident(def.did()) {
                                 Some(ident) => err.span_note(ident.span, msg),
                                 None => err.note(msg),
                             },
@@ -3446,7 +3444,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 {
                                     break 'print;
                                 }
-                                err.span_note(self.tcx.def_span(def_id), msg)
+                                err.span_note(tcx.def_span(def_id), msg)
                             }
                             ty::CoroutineWitness(def_id, args) => {
                                 use std::fmt::Write;
@@ -3463,7 +3461,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 err.note(msg.trim_end_matches(", ").to_string())
                             }
                             ty::Coroutine(def_id, _) => {
-                                let sp = self.tcx.def_span(def_id);
+                                let sp = tcx.def_span(def_id);
 
                                 // Special-case this to say "async block" instead of `[static coroutine]`.
                                 let kind = tcx.coroutine_kind(def_id).unwrap();
@@ -3475,7 +3473,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 )
                             }
                             ty::Closure(def_id, _) => err.span_note(
-                                self.tcx.def_span(def_id),
+                                tcx.def_span(def_id),
                                 "required because it's used within this closure",
                             ),
                             ty::Str => err.note("`str` is considered to contain a `[u8]` slice for auto trait purposes"),
@@ -3519,14 +3517,12 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     self.resolve_vars_if_possible(data.derived.parent_trait_pred);
                 let parent_def_id = parent_trait_pred.def_id();
                 let mut file = None;
-                let self_ty =
-                    self.tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file);
-                let msg = format!(
-                    "required for `{self_ty}` to implement `{}`",
-                    parent_trait_pred.print_modifiers_and_trait_path()
-                );
+                let self_ty_str =
+                    tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file);
+                let trait_name = parent_trait_pred.print_modifiers_and_trait_path().to_string();
+                let msg = format!("required for `{self_ty_str}` to implement `{trait_name}`");
                 let mut is_auto_trait = false;
-                match self.tcx.hir().get_if_local(data.impl_or_alias_def_id) {
+                match tcx.hir().get_if_local(data.impl_or_alias_def_id) {
                     Some(Node::Item(hir::Item {
                         kind: hir::ItemKind::Trait(is_auto, ..),
                         ident,
@@ -3538,7 +3534,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         err.span_note(ident.span, msg);
                     }
                     Some(Node::Item(hir::Item {
-                        kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }),
+                        kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, generics, .. }),
                         ..
                     })) => {
                         let mut spans = Vec::with_capacity(2);
@@ -3565,6 +3561,15 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             );
                         }
                         err.span_note(spans, msg);
+                        point_at_assoc_type_restriction(
+                            tcx,
+                            err,
+                            &self_ty_str,
+                            &trait_name,
+                            predicate,
+                            &generics,
+                            &data,
+                        );
                     }
                     _ => {
                         err.note(msg);
@@ -3618,9 +3623,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         pluralize!(count)
                     ));
                     let mut file = None;
-                    let self_ty = self
-                        .tcx
-                        .short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file);
+                    let self_ty =
+                        tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file);
                     err.note(format!(
                         "required for `{self_ty}` to implement `{}`",
                         parent_trait_pred.print_modifiers_and_trait_path()
@@ -3678,10 +3682,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 multispan.push_span_label(span, "required by this bound");
                 err.span_note(
                     multispan,
-                    format!(
-                        "required by a bound on the type alias `{}`",
-                        self.infcx.tcx.item_name(def_id)
-                    ),
+                    format!("required by a bound on the type alias `{}`", tcx.item_name(def_id)),
                 );
             }
             ObligationCauseCode::FunctionArgumentObligation {
@@ -3712,25 +3713,23 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 });
             }
             ObligationCauseCode::CompareImplItemObligation { trait_item_def_id, kind, .. } => {
-                let item_name = self.tcx.item_name(trait_item_def_id);
+                let item_name = tcx.item_name(trait_item_def_id);
                 let msg = format!(
                     "the requirement `{predicate}` appears on the `impl`'s {kind} \
                      `{item_name}` but not on the corresponding trait's {kind}",
                 );
-                let sp = self
-                    .tcx
+                let sp = tcx
                     .opt_item_ident(trait_item_def_id)
                     .map(|i| i.span)
-                    .unwrap_or_else(|| self.tcx.def_span(trait_item_def_id));
+                    .unwrap_or_else(|| tcx.def_span(trait_item_def_id));
                 let mut assoc_span: MultiSpan = sp.into();
                 assoc_span.push_span_label(
                     sp,
                     format!("this trait's {kind} doesn't have the requirement `{predicate}`"),
                 );
-                if let Some(ident) = self
-                    .tcx
+                if let Some(ident) = tcx
                     .opt_associated_item(trait_item_def_id)
-                    .and_then(|i| self.tcx.opt_item_ident(i.container_id(self.tcx)))
+                    .and_then(|i| tcx.opt_item_ident(i.container_id(tcx)))
                 {
                     assoc_span.push_span_label(ident.span, "in this trait");
                 }
@@ -4820,6 +4819,29 @@ fn hint_missing_borrow<'tcx>(
     }
 }
 
+/// Collect all the paths that reference `Self`.
+/// Used to suggest replacing associated types with an explicit type in `where` clauses.
+#[derive(Debug)]
+pub struct SelfVisitor<'v> {
+    pub paths: Vec<&'v hir::Ty<'v>>,
+    pub name: Option<Symbol>,
+}
+
+impl<'v> Visitor<'v> for SelfVisitor<'v> {
+    fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
+        if let hir::TyKind::Path(path) = ty.kind
+            && let hir::QPath::TypeRelative(inner_ty, segment) = path
+            && (Some(segment.ident.name) == self.name || self.name.is_none())
+            && let hir::TyKind::Path(inner_path) = inner_ty.kind
+            && let hir::QPath::Resolved(None, inner_path) = inner_path
+            && let Res::SelfTyAlias { .. } = inner_path.res
+        {
+            self.paths.push(ty);
+        }
+        hir::intravisit::walk_ty(self, ty);
+    }
+}
+
 /// Collect all the returned expressions within the input expression.
 /// Used to point at the return spans when we want to suggest some change to them.
 #[derive(Default)]
@@ -5064,6 +5086,134 @@ pub fn suggest_desugaring_async_fn_to_impl_future_in_trait<'tcx>(
     Some(sugg)
 }
 
+/// On `impl` evaluation cycles, look for `Self::AssocTy` restrictions in `where` clauses, explain
+/// they are not allowed and if possible suggest alternatives.
+fn point_at_assoc_type_restriction(
+    tcx: TyCtxt<'_>,
+    err: &mut Diagnostic,
+    self_ty_str: &str,
+    trait_name: &str,
+    predicate: ty::Predicate<'_>,
+    generics: &hir::Generics<'_>,
+    data: &ImplDerivedObligationCause<'_>,
+) {
+    let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder() else {
+        return;
+    };
+    let ty::ClauseKind::Projection(proj) = clause else {
+        return;
+    };
+    let name = tcx.item_name(proj.projection_ty.def_id);
+    let mut predicates = generics.predicates.iter().peekable();
+    let mut prev: Option<&hir::WhereBoundPredicate<'_>> = None;
+    while let Some(pred) = predicates.next() {
+        let hir::WherePredicate::BoundPredicate(pred) = pred else {
+            continue;
+        };
+        let mut bounds = pred.bounds.iter().peekable();
+        while let Some(bound) = bounds.next() {
+            let Some(trait_ref) = bound.trait_ref() else {
+                continue;
+            };
+            if bound.span() != data.span {
+                continue;
+            }
+            if let hir::TyKind::Path(path) = pred.bounded_ty.kind
+                && let hir::QPath::TypeRelative(ty, segment) = path
+                && segment.ident.name == name
+                && let hir::TyKind::Path(inner_path) = ty.kind
+                && let hir::QPath::Resolved(None, inner_path) = inner_path
+                && let Res::SelfTyAlias { .. } = inner_path.res
+            {
+                // The following block is to determine the right span to delete for this bound
+                // that will leave valid code after the suggestion is applied.
+                let span = if pred.origin == hir::PredicateOrigin::WhereClause
+                    && generics
+                        .predicates
+                        .iter()
+                        .filter(|p| {
+                            matches!(
+                                p,
+                                hir::WherePredicate::BoundPredicate(p)
+                                if hir::PredicateOrigin::WhereClause == p.origin
+                            )
+                        })
+                        .count()
+                        == 1
+                {
+                    // There's only one `where` bound, that needs to be removed. Remove the whole
+                    // `where` clause.
+                    generics.where_clause_span
+                } else if let Some(hir::WherePredicate::BoundPredicate(next)) = predicates.peek()
+                    && pred.origin == next.origin
+                {
+                    // There's another bound, include the comma for the current one.
+                    pred.span.until(next.span)
+                } else if let Some(prev) = prev
+                    && pred.origin == prev.origin
+                {
+                    // Last bound, try to remove the previous comma.
+                    prev.span.shrink_to_hi().to(pred.span)
+                } else if pred.origin == hir::PredicateOrigin::WhereClause {
+                    pred.span.with_hi(generics.where_clause_span.hi())
+                } else {
+                    pred.span
+                };
+
+                err.span_suggestion_verbose(
+                    span,
+                    "associated type for the current `impl` cannot be restricted in `where` \
+                     clauses, remove this bound",
+                    "",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            if let Some(new) =
+                tcx.associated_items(data.impl_or_alias_def_id).find_by_name_and_kind(
+                    tcx,
+                    Ident::with_dummy_span(name),
+                    ty::AssocKind::Type,
+                    data.impl_or_alias_def_id,
+                )
+            {
+                // The associated type is specified in the `impl` we're
+                // looking at. Point at it.
+                let span = tcx.def_span(new.def_id);
+                err.span_label(
+                    span,
+                    format!(
+                        "associated type `<{self_ty_str} as {trait_name}>::{name}` is specified \
+                         here",
+                    ),
+                );
+                // Search for the associated type `Self::{name}`, get
+                // its type and suggest replacing the bound with it.
+                let mut visitor = SelfVisitor { paths: vec![], name: Some(name) };
+                visitor.visit_trait_ref(trait_ref);
+                for path in visitor.paths {
+                    err.span_suggestion_verbose(
+                        path.span,
+                        "replace the associated type with the type specified in this `impl`",
+                        tcx.type_of(new.def_id).skip_binder().to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+            } else {
+                let mut visitor = SelfVisitor { paths: vec![], name: None };
+                visitor.visit_trait_ref(trait_ref);
+                let span: MultiSpan =
+                    visitor.paths.iter().map(|p| p.span).collect::<Vec<Span>>().into();
+                err.span_note(
+                    span,
+                    "associated types for the current `impl` cannot be restricted in `where` \
+                     clauses",
+                );
+            }
+        }
+        prev = Some(pred);
+    }
+}
+
 fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
     let mut refs = vec![];
 
diff --git a/library/core/src/ops/async_function.rs b/library/core/src/ops/async_function.rs
new file mode 100644
index 0000000000000..965873f163eff
--- /dev/null
+++ b/library/core/src/ops/async_function.rs
@@ -0,0 +1,108 @@
+use crate::future::Future;
+use crate::marker::Tuple;
+
+/// An async-aware version of the [`Fn`](crate::ops::Fn) trait.
+///
+/// All `async fn` and functions returning futures implement this trait.
+#[unstable(feature = "async_fn_traits", issue = "none")]
+#[rustc_paren_sugar]
+#[fundamental]
+#[must_use = "async closures are lazy and do nothing unless called"]
+#[cfg_attr(not(bootstrap), lang = "async_fn")]
+pub trait AsyncFn<Args: Tuple>: AsyncFnMut<Args> {
+    /// Future returned by [`AsyncFn::async_call`].
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type CallFuture<'a>: Future<Output = Self::Output>
+    where
+        Self: 'a;
+
+    /// Call the [`AsyncFn`], returning a future which may borrow from the called closure.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    extern "rust-call" fn async_call(&self, args: Args) -> Self::CallFuture<'_>;
+}
+
+/// An async-aware version of the [`FnMut`](crate::ops::FnMut) trait.
+///
+/// All `async fn` and functions returning futures implement this trait.
+#[unstable(feature = "async_fn_traits", issue = "none")]
+#[rustc_paren_sugar]
+#[fundamental]
+#[must_use = "async closures are lazy and do nothing unless called"]
+#[cfg_attr(not(bootstrap), lang = "async_fn_mut")]
+pub trait AsyncFnMut<Args: Tuple>: AsyncFnOnce<Args> {
+    /// Future returned by [`AsyncFnMut::async_call_mut`].
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type CallMutFuture<'a>: Future<Output = Self::Output>
+    where
+        Self: 'a;
+
+    /// Call the [`AsyncFnMut`], returning a future which may borrow from the called closure.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallMutFuture<'_>;
+}
+
+/// An async-aware version of the [`FnOnce`](crate::ops::FnOnce) trait.
+///
+/// All `async fn` and functions returning futures implement this trait.
+#[unstable(feature = "async_fn_traits", issue = "none")]
+#[rustc_paren_sugar]
+#[fundamental]
+#[must_use = "async closures are lazy and do nothing unless called"]
+#[cfg_attr(not(bootstrap), lang = "async_fn_once")]
+pub trait AsyncFnOnce<Args: Tuple> {
+    /// Future returned by [`AsyncFnOnce::async_call_once`].
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type CallOnceFuture: Future<Output = Self::Output>;
+
+    /// Output type of the called closure's future.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type Output;
+
+    /// Call the [`AsyncFnOnce`], returning a future which may move out of the called closure.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture;
+}
+
+mod impls {
+    use super::{AsyncFn, AsyncFnMut, AsyncFnOnce};
+    use crate::future::Future;
+    use crate::marker::Tuple;
+
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    impl<F: Fn<A>, A: Tuple> AsyncFn<A> for F
+    where
+        <F as FnOnce<A>>::Output: Future,
+    {
+        type CallFuture<'a> = <F as FnOnce<A>>::Output where Self: 'a;
+
+        extern "rust-call" fn async_call(&self, args: A) -> Self::CallFuture<'_> {
+            self.call(args)
+        }
+    }
+
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    impl<F: FnMut<A>, A: Tuple> AsyncFnMut<A> for F
+    where
+        <F as FnOnce<A>>::Output: Future,
+    {
+        type CallMutFuture<'a> = <F as FnOnce<A>>::Output where Self: 'a;
+
+        extern "rust-call" fn async_call_mut(&mut self, args: A) -> Self::CallMutFuture<'_> {
+            self.call_mut(args)
+        }
+    }
+
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    impl<F: FnOnce<A>, A: Tuple> AsyncFnOnce<A> for F
+    where
+        <F as FnOnce<A>>::Output: Future,
+    {
+        type CallOnceFuture = <F as FnOnce<A>>::Output;
+
+        type Output = <<F as FnOnce<A>>::Output as Future>::Output;
+
+        extern "rust-call" fn async_call_once(self, args: A) -> Self::CallOnceFuture {
+            self.call_once(args)
+        }
+    }
+}
diff --git a/library/core/src/ops/mod.rs b/library/core/src/ops/mod.rs
index 35654d0b853b7..4289a86f89b06 100644
--- a/library/core/src/ops/mod.rs
+++ b/library/core/src/ops/mod.rs
@@ -139,6 +139,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 mod arith;
+mod async_function;
 mod bit;
 mod control_flow;
 mod coroutine;
@@ -173,6 +174,9 @@ pub use self::drop::Drop;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::function::{Fn, FnMut, FnOnce};
 
+#[unstable(feature = "async_fn_traits", issue = "none")]
+pub use self::async_function::{AsyncFn, AsyncFnMut, AsyncFnOnce};
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::index::{Index, IndexMut};
 
diff --git a/tests/ui/associated-consts/issue-105330.rs b/tests/ui/associated-consts/issue-105330.rs
index 6c6dae864f340..285e89cce4985 100644
--- a/tests/ui/associated-consts/issue-105330.rs
+++ b/tests/ui/associated-consts/issue-105330.rs
@@ -14,6 +14,5 @@ fn foo<A: TraitWAssocConst<A=32>>() { //~ ERROR E0658
 
 fn main<A: TraitWAssocConst<A=32>>() {
     //~^ ERROR E0658
-    //~| ERROR E0131
     foo::<Demo>();
 }
diff --git a/tests/ui/associated-consts/issue-105330.stderr b/tests/ui/associated-consts/issue-105330.stderr
index b4c021d0f4fdc..452367bed1201 100644
--- a/tests/ui/associated-consts/issue-105330.stderr
+++ b/tests/ui/associated-consts/issue-105330.stderr
@@ -43,13 +43,7 @@ LL | impl TraitWAssocConst for impl Demo {
    |
    = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
-error[E0131]: `main` function is not allowed to have generic parameters
-  --> $DIR/issue-105330.rs:15:8
-   |
-LL | fn main<A: TraitWAssocConst<A=32>>() {
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` cannot have generic parameters
-
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0131, E0404, E0562, E0658.
-For more information about an error, try `rustc --explain E0131`.
+Some errors have detailed explanations: E0404, E0562, E0658.
+For more information about an error, try `rustc --explain E0404`.
diff --git a/tests/ui/associated-types/impl-wf-cycle-1.stderr b/tests/ui/associated-types/impl-wf-cycle-1.stderr
index 5fa36733f6677..d642f9f0e36a7 100644
--- a/tests/ui/associated-types/impl-wf-cycle-1.stderr
+++ b/tests/ui/associated-types/impl-wf-cycle-1.stderr
@@ -7,6 +7,9 @@ LL | | where
 LL | |     Self::A: Baz,
 LL | |     Self::B: Fiz,
    | |_________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
    |
 note: required for `(T,)` to implement `Grault`
   --> $DIR/impl-wf-cycle-1.rs:15:17
@@ -18,6 +21,10 @@ LL |     Self::A: Baz,
    |              --- unsatisfied trait bound introduced here
    = note: 1 redundant requirement hidden
    = note: required for `(T,)` to implement `Grault`
+help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
+   |
+LL -     Self::A: Baz,
+   |
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/associated-types/impl-wf-cycle-2.stderr b/tests/ui/associated-types/impl-wf-cycle-2.stderr
index 17a96bbd93430..9454d1d669792 100644
--- a/tests/ui/associated-types/impl-wf-cycle-2.stderr
+++ b/tests/ui/associated-types/impl-wf-cycle-2.stderr
@@ -6,6 +6,9 @@ LL | |
 LL | | where
 LL | |     Self::A: Copy,
    | |__________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
    |
 note: required for `(T,)` to implement `Grault`
   --> $DIR/impl-wf-cycle-2.rs:7:17
@@ -15,6 +18,11 @@ LL | impl<T: Grault> Grault for (T,)
 ...
 LL |     Self::A: Copy,
    |              ---- unsatisfied trait bound introduced here
+help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
+   |
+LL - where
+LL -     Self::A: Copy,
+   |
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/associated-types/impl-wf-cycle-3.rs b/tests/ui/associated-types/impl-wf-cycle-3.rs
new file mode 100644
index 0000000000000..044033c5e8ab1
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-3.rs
@@ -0,0 +1,13 @@
+trait A<T> {}
+
+trait B {
+    type Type;
+}
+
+impl<T> B for T //~ ERROR overflow evaluating the requirement
+where
+    T: A<Self::Type>,
+{
+    type Type = bool;
+}
+fn main() {}
diff --git a/tests/ui/associated-types/impl-wf-cycle-3.stderr b/tests/ui/associated-types/impl-wf-cycle-3.stderr
new file mode 100644
index 0000000000000..d3ca06f890b34
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-3.stderr
@@ -0,0 +1,27 @@
+error[E0275]: overflow evaluating the requirement `<T as B>::Type == <T as B>::Type`
+  --> $DIR/impl-wf-cycle-3.rs:7:1
+   |
+LL | / impl<T> B for T
+LL | | where
+LL | |     T: A<Self::Type>,
+   | |_____________________^
+LL |   {
+LL |       type Type = bool;
+   |       --------- associated type `<T as B>::Type` is specified here
+   |
+note: required for `T` to implement `B`
+  --> $DIR/impl-wf-cycle-3.rs:7:9
+   |
+LL | impl<T> B for T
+   |         ^     ^
+LL | where
+LL |     T: A<Self::Type>,
+   |        ------------- unsatisfied trait bound introduced here
+help: replace the associated type with the type specified in this `impl`
+   |
+LL |     T: A<bool>,
+   |          ~~~~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.
diff --git a/tests/ui/associated-types/impl-wf-cycle-4.rs b/tests/ui/associated-types/impl-wf-cycle-4.rs
new file mode 100644
index 0000000000000..bfa8adc71a11d
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-4.rs
@@ -0,0 +1,15 @@
+trait Filter {
+    type ToMatch;
+}
+
+impl<T> Filter for T //~ ERROR overflow evaluating the requirement
+where
+    T: Fn(Self::ToMatch),
+{
+}
+
+struct JustFilter<F: Filter> {
+    filter: F,
+}
+
+fn main() {}
diff --git a/tests/ui/associated-types/impl-wf-cycle-4.stderr b/tests/ui/associated-types/impl-wf-cycle-4.stderr
new file mode 100644
index 0000000000000..cdbac267d34dc
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-4.stderr
@@ -0,0 +1,25 @@
+error[E0275]: overflow evaluating the requirement `<T as Filter>::ToMatch == <T as Filter>::ToMatch`
+  --> $DIR/impl-wf-cycle-4.rs:5:1
+   |
+LL | / impl<T> Filter for T
+LL | | where
+LL | |     T: Fn(Self::ToMatch),
+   | |_________________________^
+   |
+note: required for `T` to implement `Filter`
+  --> $DIR/impl-wf-cycle-4.rs:5:9
+   |
+LL | impl<T> Filter for T
+   |         ^^^^^^     ^
+LL | where
+LL |     T: Fn(Self::ToMatch),
+   |        ----------------- unsatisfied trait bound introduced here
+note: associated types for the current `impl` cannot be restricted in `where` clauses
+  --> $DIR/impl-wf-cycle-4.rs:7:11
+   |
+LL |     T: Fn(Self::ToMatch),
+   |           ^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.
diff --git a/tests/ui/associated-types/impl-wf-cycle-5.fixed b/tests/ui/associated-types/impl-wf-cycle-5.fixed
new file mode 100644
index 0000000000000..bff6ca90975d2
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-5.fixed
@@ -0,0 +1,31 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+    T: Grault,
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}
diff --git a/tests/ui/associated-types/impl-wf-cycle-5.rs b/tests/ui/associated-types/impl-wf-cycle-5.rs
new file mode 100644
index 0000000000000..a822e1fb0081b
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-5.rs
@@ -0,0 +1,32 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+    T: Grault,
+    Self::A: Baz,
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}
diff --git a/tests/ui/associated-types/impl-wf-cycle-5.stderr b/tests/ui/associated-types/impl-wf-cycle-5.stderr
new file mode 100644
index 0000000000000..284a50bb9a3be
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-5.stderr
@@ -0,0 +1,31 @@
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-5.rs:20:1
+   |
+LL | / impl<T> Grault for (T,)
+LL | |
+LL | | where
+LL | |     T: Grault,
+LL | |     Self::A: Baz,
+   | |_________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
+   |
+note: required for `(T,)` to implement `Grault`
+  --> $DIR/impl-wf-cycle-5.rs:20:9
+   |
+LL | impl<T> Grault for (T,)
+   |         ^^^^^^     ^^^^
+...
+LL |     Self::A: Baz,
+   |              --- unsatisfied trait bound introduced here
+help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
+   |
+LL -     T: Grault,
+LL -     Self::A: Baz,
+LL +     T: Grault,
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.
diff --git a/tests/ui/associated-types/impl-wf-cycle-6.fixed b/tests/ui/associated-types/impl-wf-cycle-6.fixed
new file mode 100644
index 0000000000000..73ed64f7ce381
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-6.fixed
@@ -0,0 +1,30 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T: Grault> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}
diff --git a/tests/ui/associated-types/impl-wf-cycle-6.rs b/tests/ui/associated-types/impl-wf-cycle-6.rs
new file mode 100644
index 0000000000000..20d635cac5d6f
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-6.rs
@@ -0,0 +1,31 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T: Grault> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+    Self::A: Baz,
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}
diff --git a/tests/ui/associated-types/impl-wf-cycle-6.stderr b/tests/ui/associated-types/impl-wf-cycle-6.stderr
new file mode 100644
index 0000000000000..c9b5d8060be68
--- /dev/null
+++ b/tests/ui/associated-types/impl-wf-cycle-6.stderr
@@ -0,0 +1,29 @@
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-6.rs:20:1
+   |
+LL | / impl<T: Grault> Grault for (T,)
+LL | |
+LL | | where
+LL | |     Self::A: Baz,
+   | |_________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
+   |
+note: required for `(T,)` to implement `Grault`
+  --> $DIR/impl-wf-cycle-6.rs:20:17
+   |
+LL | impl<T: Grault> Grault for (T,)
+   |                 ^^^^^^     ^^^^
+...
+LL |     Self::A: Baz,
+   |              --- unsatisfied trait bound introduced here
+help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
+   |
+LL - where
+LL -     Self::A: Baz,
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.
diff --git a/tests/ui/associated-types/issue-38821.rs b/tests/ui/associated-types/issue-38821.rs
index 6753860e9ff80..34c35d7acf1db 100644
--- a/tests/ui/associated-types/issue-38821.rs
+++ b/tests/ui/associated-types/issue-38821.rs
@@ -22,7 +22,22 @@ pub trait Column: Expression {}
 
 #[derive(Debug, Copy, Clone)]
 //~^ ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
 pub enum ColumnInsertValue<Col, Expr> where
+//~^ ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+//~| ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
     Col: Column,
     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>,
 {
diff --git a/tests/ui/associated-types/issue-38821.stderr b/tests/ui/associated-types/issue-38821.stderr
index 8b628f5ae6d19..0e34f541ea303 100644
--- a/tests/ui/associated-types/issue-38821.stderr
+++ b/tests/ui/associated-types/issue-38821.stderr
@@ -17,6 +17,272 @@ help: consider further restricting the associated type
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
    |                                                                       +++++++++++++++++++++++++++++++++++++++
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:38:1
+   |
+LL | pub enum ColumnInsertValue<Col, Expr> where
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull
+   |                                                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:38:1
+   |
+LL | / pub enum ColumnInsertValue<Col, Expr> where
+LL | |
+LL | |
+LL | |     Col: Column,
+...  |
+LL | |     Default(Col),
+LL | | }
+   | |_^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull
+   |                                                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:10
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |          ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider further restricting the associated type
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
+   |                                                                       +++++++++++++++++++++++++++++++++++++++
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:10
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |          ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider further restricting the associated type
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
+   |                                                                       +++++++++++++++++++++++++++++++++++++++
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:10
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |          ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:10
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |          ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:17
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                 ^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Copy` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider further restricting the associated type
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
+   |                                                                       +++++++++++++++++++++++++++++++++++++++
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:23
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                       ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider further restricting the associated type
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
+   |                                                                       +++++++++++++++++++++++++++++++++++++++
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:23
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                       ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider further restricting the associated type
+   |
+LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
+   |                                                                       +++++++++++++++++++++++++++++++++++++++
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:23
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                       ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:23
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                       ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:10
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |          ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:10
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |          ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:23
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                       ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
+  --> $DIR/issue-38821.rs:23:23
+   |
+LL | #[derive(Debug, Copy, Clone)]
+   |                       ^^^^^ the trait `NotNull` is not implemented for `<Col as Expression>::SqlType`
+   |
+note: required for `<Col as Expression>::SqlType` to implement `IntoNullable`
+  --> $DIR/issue-38821.rs:9:18
+   |
+LL | impl<T: NotNull> IntoNullable for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 16 previous errors
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/async-await/async-fn/simple.rs b/tests/ui/async-await/async-fn/simple.rs
new file mode 100644
index 0000000000000..99a5d56a3093b
--- /dev/null
+++ b/tests/ui/async-await/async-fn/simple.rs
@@ -0,0 +1,16 @@
+// edition: 2021
+// check-pass
+
+#![feature(async_fn_traits)]
+
+use std::ops::AsyncFn;
+
+async fn foo() {}
+
+async fn call_asyncly(f: impl AsyncFn(i32) -> i32) -> i32 {
+    f(1).await
+}
+
+fn main() {
+    let fut = call_asyncly(|x| async move { x + 1 });
+}
diff --git a/tests/ui/coercion/issue-53475.rs b/tests/ui/coercion/issue-53475.rs
index 3770c024fb92e..f389169f95405 100644
--- a/tests/ui/coercion/issue-53475.rs
+++ b/tests/ui/coercion/issue-53475.rs
@@ -3,7 +3,7 @@
 use std::any::Any;
 use std::ops::CoerceUnsized;
 
-struct Foo<T> {
+struct Foo<T: ?Sized> {
     data: Box<T>,
 }
 
diff --git a/tests/ui/const-generics/bad-generic-in-copy-impl.rs b/tests/ui/const-generics/bad-generic-in-copy-impl.rs
index b5663464cf422..092a7af236bad 100644
--- a/tests/ui/const-generics/bad-generic-in-copy-impl.rs
+++ b/tests/ui/const-generics/bad-generic-in-copy-impl.rs
@@ -2,6 +2,7 @@
 pub struct Foo {
     x: [u8; SIZE],
     //~^ ERROR mismatched types
+    //~| ERROR mismatched types
 }
 
 const SIZE: u32 = 1;
diff --git a/tests/ui/const-generics/bad-generic-in-copy-impl.stderr b/tests/ui/const-generics/bad-generic-in-copy-impl.stderr
index d60d8ec0e90d3..fbd546d42fb52 100644
--- a/tests/ui/const-generics/bad-generic-in-copy-impl.stderr
+++ b/tests/ui/const-generics/bad-generic-in-copy-impl.stderr
@@ -4,6 +4,14 @@ error[E0308]: mismatched types
 LL |     x: [u8; SIZE],
    |             ^^^^ expected `usize`, found `u32`
 
-error: aborting due to 1 previous error
+error[E0308]: mismatched types
+  --> $DIR/bad-generic-in-copy-impl.rs:3:13
+   |
+LL |     x: [u8; SIZE],
+   |             ^^^^ expected `usize`, found `u32`
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/did_you_mean/bad-assoc-ty.stderr b/tests/ui/did_you_mean/bad-assoc-ty.stderr
index 3c474d19d1d05..eed01267224d3 100644
--- a/tests/ui/did_you_mean/bad-assoc-ty.stderr
+++ b/tests/ui/did_you_mean/bad-assoc-ty.stderr
@@ -191,7 +191,14 @@ error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:33:10
    |
 LL | type H = Fn(u8) -> (u8)::Output;
-   |          ^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output`
+   |          ^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: use fully-qualified syntax
+   |
+LL | type H = <(dyn Fn(u8) -> u8 + 'static) as AsyncFnOnce>::Output;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL | type H = <(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:39:19
diff --git a/tests/ui/dst/dst-bad-assign-3.stderr b/tests/ui/dst/dst-bad-assign-3.stderr
index 6dd3434fd21be..007f6b898bee5 100644
--- a/tests/ui/dst/dst-bad-assign-3.stderr
+++ b/tests/ui/dst/dst-bad-assign-3.stderr
@@ -8,6 +8,7 @@ LL |     f5.2 = Bar1 {f: 36};
    |
    = note: expected trait object `dyn ToBar`
                     found struct `Bar1`
+   = help: `Bar1` implements `ToBar` so you could box the found value and coerce it to the trait object `Box<dyn ToBar>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `dyn ToBar` cannot be known at compilation time
   --> $DIR/dst-bad-assign-3.rs:33:5
diff --git a/tests/ui/dst/dst-bad-assign.stderr b/tests/ui/dst/dst-bad-assign.stderr
index d8d1057876f4f..f935d27e96e2f 100644
--- a/tests/ui/dst/dst-bad-assign.stderr
+++ b/tests/ui/dst/dst-bad-assign.stderr
@@ -8,6 +8,7 @@ LL |     f5.ptr = Bar1 {f: 36};
    |
    = note: expected trait object `dyn ToBar`
                     found struct `Bar1`
+   = help: `Bar1` implements `ToBar` so you could box the found value and coerce it to the trait object `Box<dyn ToBar>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `dyn ToBar` cannot be known at compilation time
   --> $DIR/dst-bad-assign.rs:35:5
diff --git a/tests/ui/dyn-star/no-implicit-dyn-star.stderr b/tests/ui/dyn-star/no-implicit-dyn-star.stderr
index 06c7bcc574825..bea334a8a69a8 100644
--- a/tests/ui/dyn-star/no-implicit-dyn-star.stderr
+++ b/tests/ui/dyn-star/no-implicit-dyn-star.stderr
@@ -8,6 +8,7 @@ LL |     dyn_star_foreign::require_dyn_star_display(1usize);
    |
    = note: expected trait object `(dyn* std::fmt::Display + 'static)`
                       found type `usize`
+   = help: `usize` implements `Display`, `#[feature(dyn_star)]` is likely not enabled; that feature it is currently incomplete
 note: function defined here
   --> $DIR/auxiliary/dyn-star-foreign.rs:6:8
    |
diff --git a/tests/ui/enum-discriminant/issue-72554.rs b/tests/ui/enum-discriminant/issue-72554.rs
index 54f7e9ac592eb..1fe9a5f4faa7d 100644
--- a/tests/ui/enum-discriminant/issue-72554.rs
+++ b/tests/ui/enum-discriminant/issue-72554.rs
@@ -3,6 +3,7 @@ use std::collections::BTreeSet;
 #[derive(Hash)]
 pub enum ElemDerived {
     //~^ ERROR recursive type `ElemDerived` has infinite size
+    //~| ERROR cycle detected
     A(ElemDerived)
 }
 
diff --git a/tests/ui/enum-discriminant/issue-72554.stderr b/tests/ui/enum-discriminant/issue-72554.stderr
index 381f24d351e9c..648680c6031da 100644
--- a/tests/ui/enum-discriminant/issue-72554.stderr
+++ b/tests/ui/enum-discriminant/issue-72554.stderr
@@ -3,7 +3,7 @@ error[E0072]: recursive type `ElemDerived` has infinite size
    |
 LL | pub enum ElemDerived {
    | ^^^^^^^^^^^^^^^^^^^^
-LL |
+...
 LL |     A(ElemDerived)
    |       ----------- recursive without indirection
    |
@@ -12,6 +12,21 @@ help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
 LL |     A(Box<ElemDerived>)
    |       ++++           +
 
-error: aborting due to 1 previous error
+error[E0391]: cycle detected when computing drop-check constraints for `ElemDerived`
+  --> $DIR/issue-72554.rs:4:1
+   |
+LL | pub enum ElemDerived {
+   | ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which immediately requires computing drop-check constraints for `ElemDerived` again
+note: cycle used when computing drop-check constraints for `Elem`
+  --> $DIR/issue-72554.rs:11:1
+   |
+LL | pub enum Elem {
+   | ^^^^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0072`.
+Some errors have detailed explanations: E0072, E0391.
+For more information about an error, try `rustc --explain E0072`.
diff --git a/tests/ui/generic-associated-types/issue-79422.extended.stderr b/tests/ui/generic-associated-types/issue-79422.extended.stderr
index 14492266cdaf3..ae1526296a795 100644
--- a/tests/ui/generic-associated-types/issue-79422.extended.stderr
+++ b/tests/ui/generic-associated-types/issue-79422.extended.stderr
@@ -27,6 +27,7 @@ LL |     type VRefCont<'a> = &'a V where Self: 'a;
    |                         ^^^^^
    = note: expected trait object `(dyn RefCont<'_, u8> + 'static)`
                  found reference `&u8`
+   = help: `&u8` implements `RefCont` so you could box the found value and coerce it to the trait object `Box<dyn RefCont>`, you will have to change the expected type as well
    = note: required for the cast from `Box<BTreeMap<u8, u8>>` to `Box<dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>>`
 
 error: aborting due to 2 previous errors
diff --git a/tests/ui/impl-trait/dyn-impl-type-mismatch.rs b/tests/ui/impl-trait/dyn-impl-type-mismatch.rs
new file mode 100644
index 0000000000000..c6170abb58285
--- /dev/null
+++ b/tests/ui/impl-trait/dyn-impl-type-mismatch.rs
@@ -0,0 +1,18 @@
+trait Trait {}
+struct Struct;
+impl Trait for Struct {}
+fn foo() -> impl Trait {
+    Struct
+}
+fn main() {
+    let a: Box<dyn Trait> = if true {
+        Box::new(Struct)
+    } else {
+        foo() //~ ERROR E0308
+    };
+    let a: dyn Trait = if true {
+        Struct //~ ERROR E0308
+    } else {
+        foo() //~ ERROR E0308
+    };
+}
diff --git a/tests/ui/impl-trait/dyn-impl-type-mismatch.stderr b/tests/ui/impl-trait/dyn-impl-type-mismatch.stderr
new file mode 100644
index 0000000000000..ddbdaf3eb4b03
--- /dev/null
+++ b/tests/ui/impl-trait/dyn-impl-type-mismatch.stderr
@@ -0,0 +1,43 @@
+error[E0308]: mismatched types
+  --> $DIR/dyn-impl-type-mismatch.rs:11:9
+   |
+LL | fn foo() -> impl Trait {
+   |             ---------- the found opaque type
+...
+LL |         foo()
+   |         ^^^^^ expected `Box<dyn Trait>`, found opaque type
+   |
+   = note:   expected struct `Box<dyn Trait>`
+           found opaque type `impl Trait`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+help: store this in the heap by calling `Box::new`
+   |
+LL |         Box::new(foo())
+   |         +++++++++     +
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-impl-type-mismatch.rs:14:9
+   |
+LL |         Struct
+   |         ^^^^^^ expected `dyn Trait`, found `Struct`
+   |
+   = note: expected trait object `dyn Trait`
+                    found struct `Struct`
+   = help: `Struct` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-impl-type-mismatch.rs:16:9
+   |
+LL | fn foo() -> impl Trait {
+   |             ---------- the found opaque type
+...
+LL |         foo()
+   |         ^^^^^ expected `dyn Trait`, found opaque type
+   |
+   = note: expected trait object `dyn Trait`
+               found opaque type `impl Trait`
+   = help: you can box the `impl Trait` to coerce it to `Box<dyn Trait>`, but you'll have to change the expected type as well
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr b/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
index 4a52f82540d83..9cc199d1e0ec6 100644
--- a/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
+++ b/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
@@ -6,6 +6,7 @@ LL | fn fuz() -> (usize, Trait) { (42, Struct) }
    |
    = note: expected trait object `(dyn Trait + 'static)`
                     found struct `Struct`
+   = help: `Struct` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:13
@@ -27,6 +28,7 @@ LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
    |
    = note: expected trait object `(dyn Trait + 'static)`
                     found struct `Struct`
+   = help: `Struct` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:13
diff --git a/tests/ui/methods/issues/issue-61525.stderr b/tests/ui/methods/issues/issue-61525.stderr
index a329b52e37375..2670a3e4755b1 100644
--- a/tests/ui/methods/issues/issue-61525.stderr
+++ b/tests/ui/methods/issues/issue-61525.stderr
@@ -27,6 +27,7 @@ LL |         1.query::<dyn ToString>("")
    |
    = note: expected trait object `dyn ToString`
                  found reference `&'static str`
+   = help: `&'static str` implements `ToString` so you could box the found value and coerce it to the trait object `Box<dyn ToString>`, you will have to change the expected type as well
 note: method defined here
   --> $DIR/issue-61525.rs:2:8
    |
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/specializing-constness-2.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/specializing-constness-2.stderr
index e5b9493b3ce11..5210a6942010d 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/specializing-constness-2.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/specializing-constness-2.stderr
@@ -7,16 +7,6 @@ LL | impl<T: Default> A for T {
 LL | impl<T: Default + ~const Sup> const A for T {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
 
-error[E0308]: mismatched types
-  --> $DIR/specializing-constness-2.rs:27:5
-   |
-LL |     <T as A>::a();
-   |     ^^^^^^^^^^^^^ expected `host`, found `true`
-   |
-   = note: expected constant `host`
-              found constant `true`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0119, E0308.
-For more information about an error, try `rustc --explain E0119`.
+For more information about this error, try `rustc --explain E0119`.
diff --git a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs
index 8ceecd801d3a7..a0ee771441772 100644
--- a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs
+++ b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs
@@ -6,7 +6,6 @@
 struct S<const L: usize>;
 
 impl<const N: i32> Copy for S<N> {}
-//~^ ERROR the constant `N` is not of type `usize`
 impl<const M: usize> Copy for S<M> {}
 //~^ ERROR: conflicting implementations of trait `Copy` for type `S<_>`
 
diff --git a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr
index 3268a8767339f..2953bc95917c9 100644
--- a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr
+++ b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr
@@ -1,29 +1,11 @@
 error[E0119]: conflicting implementations of trait `Copy` for type `S<_>`
-  --> $DIR/bad-const-wf-doesnt-specialize.rs:10:1
+  --> $DIR/bad-const-wf-doesnt-specialize.rs:9:1
    |
 LL | impl<const N: i32> Copy for S<N> {}
    | -------------------------------- first implementation here
-LL |
 LL | impl<const M: usize> Copy for S<M> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `S<_>`
 
-error: the constant `N` is not of type `usize`
-  --> $DIR/bad-const-wf-doesnt-specialize.rs:8:29
-   |
-LL | impl<const N: i32> Copy for S<N> {}
-   |                             ^^^^ expected `usize`, found `i32`
-   |
-note: required for `S<N>` to implement `Clone`
-  --> $DIR/bad-const-wf-doesnt-specialize.rs:5:10
-   |
-LL | #[derive(Clone)]
-   |          ^^^^^
-LL | struct S<const L: usize>;
-   |          ----- unsatisfied trait bound introduced in this `derive` macro
-note: required by a bound in `Copy`
-  --> $SRC_DIR/core/src/marker.rs:LL:COL
-   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/tests/ui/typeck/typeck_type_placeholder_item.rs b/tests/ui/typeck/typeck_type_placeholder_item.rs
index f6d1fb6a23d4c..4eba14f5a93fb 100644
--- a/tests/ui/typeck/typeck_type_placeholder_item.rs
+++ b/tests/ui/typeck/typeck_type_placeholder_item.rs
@@ -198,7 +198,6 @@ trait Qux {
     //~^ ERROR the placeholder `_` is not allowed within types on item signatures for associated types
 }
 impl Qux for Struct {
-    //~^ ERROR: not all trait items implemented, missing: `F`
     type A = _;
     //~^ ERROR the placeholder `_` is not allowed within types on item signatures for associated types
     type B = _;
diff --git a/tests/ui/typeck/typeck_type_placeholder_item.stderr b/tests/ui/typeck/typeck_type_placeholder_item.stderr
index bfcc76c1dae7d..a4325b01f02c4 100644
--- a/tests/ui/typeck/typeck_type_placeholder_item.stderr
+++ b/tests/ui/typeck/typeck_type_placeholder_item.stderr
@@ -29,7 +29,7 @@ LL | struct BadStruct2<_, T>(_, T);
    |                   ^ expected identifier, found reserved identifier
 
 error: associated constant in `impl` without body
-  --> $DIR/typeck_type_placeholder_item.rs:206:5
+  --> $DIR/typeck_type_placeholder_item.rs:205:5
    |
 LL |     const C: _;
    |     ^^^^^^^^^^-
@@ -411,7 +411,7 @@ LL | type Y = impl Trait<_>;
    |                     ^ not allowed in type signatures
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
-  --> $DIR/typeck_type_placeholder_item.rs:217:31
+  --> $DIR/typeck_type_placeholder_item.rs:216:31
    |
 LL | fn value() -> Option<&'static _> {
    |               ----------------^-
@@ -420,7 +420,7 @@ LL | fn value() -> Option<&'static _> {
    |               help: replace with the correct return type: `Option<&'static u8>`
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
-  --> $DIR/typeck_type_placeholder_item.rs:222:10
+  --> $DIR/typeck_type_placeholder_item.rs:221:10
    |
 LL | const _: Option<_> = map(value);
    |          ^^^^^^^^^
@@ -429,7 +429,7 @@ LL | const _: Option<_> = map(value);
    |          help: replace with the correct type: `Option<u8>`
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
-  --> $DIR/typeck_type_placeholder_item.rs:225:31
+  --> $DIR/typeck_type_placeholder_item.rs:224:31
    |
 LL | fn evens_squared(n: usize) -> _ {
    |                               ^
@@ -438,13 +438,13 @@ LL | fn evens_squared(n: usize) -> _ {
    |                               help: replace with an appropriate return type: `impl Iterator<Item = usize>`
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
-  --> $DIR/typeck_type_placeholder_item.rs:230:10
+  --> $DIR/typeck_type_placeholder_item.rs:229:10
    |
 LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
    |          ^ not allowed in type signatures
    |
-note: however, the inferred type `Map<Filter<Range<i32>, {closure@typeck_type_placeholder_item.rs:230:29}>, {closure@typeck_type_placeholder_item.rs:230:49}>` cannot be named
-  --> $DIR/typeck_type_placeholder_item.rs:230:14
+note: however, the inferred type `Map<Filter<Range<i32>, {closure@typeck_type_placeholder_item.rs:229:29}>, {closure@typeck_type_placeholder_item.rs:229:49}>` cannot be named
+  --> $DIR/typeck_type_placeholder_item.rs:229:14
    |
 LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -631,25 +631,25 @@ LL |         fn clone_from(&mut self, other: &FnTest9) { *self = FnTest9; }
    |                                         ~~~~~~~~
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated types
-  --> $DIR/typeck_type_placeholder_item.rs:202:14
+  --> $DIR/typeck_type_placeholder_item.rs:201:14
    |
 LL |     type A = _;
    |              ^ not allowed in type signatures
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated types
-  --> $DIR/typeck_type_placeholder_item.rs:204:14
+  --> $DIR/typeck_type_placeholder_item.rs:203:14
    |
 LL |     type B = _;
    |              ^ not allowed in type signatures
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated constants
-  --> $DIR/typeck_type_placeholder_item.rs:206:14
+  --> $DIR/typeck_type_placeholder_item.rs:205:14
    |
 LL |     const C: _;
    |              ^ not allowed in type signatures
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated constants
-  --> $DIR/typeck_type_placeholder_item.rs:209:14
+  --> $DIR/typeck_type_placeholder_item.rs:208:14
    |
 LL |     const D: _ = 42;
    |              ^
@@ -657,16 +657,7 @@ LL |     const D: _ = 42;
    |              not allowed in type signatures
    |              help: replace with the correct type: `i32`
 
-error[E0046]: not all trait items implemented, missing: `F`
-  --> $DIR/typeck_type_placeholder_item.rs:200:1
-   |
-LL |     type F: std::ops::Fn(_);
-   |     ----------------------- `F` from trait
-...
-LL | impl Qux for Struct {
-   | ^^^^^^^^^^^^^^^^^^^ missing `F` in implementation
-
-error: aborting due to 72 previous errors
+error: aborting due to 71 previous errors
 
-Some errors have detailed explanations: E0046, E0121, E0282, E0403.
-For more information about an error, try `rustc --explain E0046`.
+Some errors have detailed explanations: E0121, E0282, E0403.
+For more information about an error, try `rustc --explain E0121`.