Don't document impossible to call default trait items on impls

compiler/rustc_middle/src/query/mod.rs

@@ -1951,6 +1951,14 @@ rustc_queries! {
         }
     }
 
+    query is_impossible_method(key: (DefId, DefId)) -> bool {
+        desc { |tcx|
+            "checking if {} is impossible to call within {}",
+            tcx.def_path_str(key.1),
+            tcx.def_path_str(key.0),
+        }
+    }
+
     query method_autoderef_steps(
         goal: CanonicalTyGoal<'tcx>
     ) -> MethodAutoderefStepsResult<'tcx> {

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -30,10 +30,14 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
+use rustc_infer::traits::TraitEngineExt as _;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
 use rustc_middle::ty::visit::TypeVisitable;
-use rustc_middle::ty::{self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry};
+use rustc_middle::ty::{
+    self, DefIdTree, GenericParamDefKind, Subst, ToPredicate, Ty, TyCtxt, TypeSuperVisitable,
+    VtblEntry,
+};
 use rustc_span::{sym, Span};
 use smallvec::SmallVec;
 
@@ -474,6 +478,77 @@ fn subst_and_check_impossible_predicates<'tcx>(
     result
 }
 
+/// Checks whether a trait's method is impossible to call on a given impl.
+///
+/// This only considers predicates that reference the impl's generics, and not
+/// those that reference the method's generics.
+fn is_impossible_method<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    (impl_def_id, trait_item_def_id): (DefId, DefId),
+) -> bool {
+    struct ReferencesOnlyParentGenerics<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        generics: &'tcx ty::Generics,
+        trait_item_def_id: DefId,
+    }
+    impl<'tcx> ty::TypeVisitor<'tcx> for ReferencesOnlyParentGenerics<'tcx> {
+        type BreakTy = ();
+        fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+            // If this is a parameter from the trait item's own generics, then bail
+            if let ty::Param(param) = t.kind()
+                && let param_def_id = self.generics.type_param(param, self.tcx).def_id
+                && self.tcx.parent(param_def_id) == self.trait_item_def_id
+            {
+                return ControlFlow::BREAK;
+            }
+            t.super_visit_with(self)
+        }
+        fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::ReEarlyBound(param) = r.kind()
+                && let param_def_id = self.generics.region_param(&param, self.tcx).def_id
+                && self.tcx.parent(param_def_id) == self.trait_item_def_id
+            {
+                return ControlFlow::BREAK;
+            }
+            r.super_visit_with(self)
+        }
+        fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::ConstKind::Param(param) = ct.kind()
+                && let param_def_id = self.generics.const_param(&param, self.tcx).def_id
+                && self.tcx.parent(param_def_id) == self.trait_item_def_id
+            {
+                return ControlFlow::BREAK;
+            }
+            ct.super_visit_with(self)
+        }
+    }
+
+    let generics = tcx.generics_of(trait_item_def_id);
+    let predicates = tcx.predicates_of(trait_item_def_id);
+    let impl_trait_ref =
+        tcx.impl_trait_ref(impl_def_id).expect("expected impl to correspond to trait");
+    let param_env = tcx.param_env(impl_def_id);
+
+    let mut visitor = ReferencesOnlyParentGenerics { tcx, generics, trait_item_def_id };
+    let predicates_for_trait = predicates.predicates.iter().filter_map(|(pred, span)| {
+        if pred.visit_with(&mut visitor).is_continue() {
+            Some(Obligation::new(
+                ObligationCause::dummy_with_span(*span),
+                param_env,
+                ty::EarlyBinder(*pred).subst(tcx, impl_trait_ref.substs),
+            ))
+        } else {
+            None
+        }
+    });
+
+    tcx.infer_ctxt().ignoring_regions().enter(|ref infcx| {
+        let mut fulfill_ctxt = <dyn TraitEngine<'_>>::new(tcx);
+        fulfill_ctxt.register_predicate_obligations(infcx, predicates_for_trait);
+        !fulfill_ctxt.select_all_or_error(infcx).is_empty()
+    })
+}
+
 #[derive(Clone, Debug)]
 enum VtblSegment<'tcx> {
     MetadataDSA,
@@ -854,6 +929,7 @@ pub fn provide(providers: &mut ty::query::Providers) {
         vtable_entries,
         vtable_trait_upcasting_coercion_new_vptr_slot,
         subst_and_check_impossible_predicates,
+        is_impossible_method,
         try_unify_abstract_consts: |tcx, param_env_and| {
             let (param_env, (a, b)) = param_env_and.into_parts();
             const_evaluatable::try_unify_abstract_consts(tcx, (a, b), param_env)

src/librustdoc/html/render/mod.rs

@@ -1550,6 +1550,15 @@ fn render_impl(
         rendering_params: ImplRenderingParameters,
     ) {
         for trait_item in &t.items {
+            // Skip over any default trait items that are impossible to call
+            // (e.g. if it has a `Self: Sized` bound on an unsized type).
+            if let Some(impl_def_id) = parent.item_id.as_def_id()
+                && let Some(trait_item_def_id) = trait_item.item_id.as_def_id()
+                && cx.tcx().is_impossible_method((impl_def_id, trait_item_def_id))
+            {
+                continue;
+            }
+
             let n = trait_item.name;
             if i.items.iter().any(|m| m.name == n) {
                 continue;

src/test/rustdoc/impossible-default.rs

@@ -0,0 +1,20 @@
+#![crate_name = "foo"]
+
+// Check that default trait items that are impossible to satisfy
+
+pub trait Foo {
+    fn needs_sized(&self)
+    where
+        Self: Sized,
+    {}
+
+    fn no_needs_sized(&self) {}
+}
+
+// @!has foo/struct.Bar.html '//*[@id="method.needs_sized"]//h4[@class="code-header"]' \
+// "fn needs_sized"
+// @has foo/struct.Bar.html '//*[@id="method.no_needs_sized"]//h4[@class="code-header"]' \
+// "fn no_needs_sized"
+pub struct Bar([u8]);
+
+impl Foo for Bar {}