Resolve assoc item bindings by namespace

If a const is expected, resolve a const.
If a type is expected, resolve a type.
Don't try to resolve a type first falling back to consts.

compiler/rustc_hir_analysis/messages.ftl

@@ -1,9 +1,24 @@
+hir_analysis_ambiguous_assoc_item = ambiguous associated {$assoc_kind} `{$assoc_name}` in bounds of `{$ty_param_name}`
+    .label = ambiguous associated {$assoc_kind} `{$assoc_name}`
+
 hir_analysis_ambiguous_lifetime_bound =
     ambiguous lifetime bound, explicit lifetime bound required
 
 hir_analysis_assoc_bound_on_const = expected associated type, found {$descr}
     .note = trait bounds not allowed on {$descr}
 
+hir_analysis_assoc_item_not_found = associated {$assoc_kind} `{$assoc_name}` not found for `{$ty_param_name}`
+
+hir_analysis_assoc_item_not_found_found_in_other_trait_label = there is {$identically_named ->
+        [true] an
+        *[false] a similarly named
+    } associated {$assoc_kind} `{$suggested_name}` in the trait `{$trait_name}`
+hir_analysis_assoc_item_not_found_label = associated {$assoc_kind} `{$assoc_name}` not found
+hir_analysis_assoc_item_not_found_other_sugg = `{$ty_param_name}` has the following associated {$assoc_kind}
+hir_analysis_assoc_item_not_found_similar_in_other_trait_sugg = change the associated {$assoc_kind} name to use `{$suggested_name}` from `{$trait_name}`
+hir_analysis_assoc_item_not_found_similar_in_other_trait_with_bound_sugg = and also change the associated {$assoc_kind} name
+hir_analysis_assoc_item_not_found_similar_sugg = there is an associated {$assoc_kind} with a similar name
+
 hir_analysis_assoc_type_binding_not_allowed =
     associated type bindings are not allowed here
     .label = associated type not allowed here
@@ -284,10 +299,6 @@ hir_analysis_placeholder_not_allowed_item_signatures = the placeholder `_` is no
 
 hir_analysis_requires_note = the `{$trait_name}` impl for `{$ty}` requires that `{$error_predicate}`
 
-hir_analysis_return_type_notation_conflicting_bound =
-    ambiguous associated function `{$assoc_name}` for `{$ty_name}`
-    .note = `{$assoc_name}` is declared in two supertraits: `{$first_bound}` and `{$second_bound}`
-
 hir_analysis_return_type_notation_equality_bound =
     return type notation is not allowed to use type equality
 
@@ -298,9 +309,6 @@ hir_analysis_return_type_notation_illegal_param_type =
     return type notation is not allowed for functions that have type parameters
     .label = type parameter declared here
 
-hir_analysis_return_type_notation_missing_method =
-    cannot find associated function `{$assoc_name}` for `{$ty_name}`
-
 hir_analysis_return_type_notation_on_non_rpitit =
     return type notation used on function that is not `async` and does not return `impl Trait`
     .note = function returns `{$ty}`, which is not compatible with associated type return bounds

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -256,44 +256,35 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
 
         let tcx = self.tcx();
 
-        let return_type_notation =
-            binding.gen_args.parenthesized == hir::GenericArgsParentheses::ReturnTypeNotation;
-
-        let candidate = if return_type_notation {
-            if self.trait_defines_associated_item_named(
-                trait_ref.def_id(),
-                ty::AssocKind::Fn,
-                binding.item_name,
-            ) {
-                trait_ref
+        let assoc_kind =
+            if binding.gen_args.parenthesized == hir::GenericArgsParentheses::ReturnTypeNotation {
+                ty::AssocKind::Fn
+            } else if let ConvertedBindingKind::Equality(term) = binding.kind
+                && let ty::TermKind::Const(_) = term.unpack()
+            {
+                ty::AssocKind::Const
             } else {
-                self.one_bound_for_assoc_method(
-                    traits::supertraits(tcx, trait_ref),
-                    trait_ref.print_only_trait_path(),
-                    binding.item_name,
-                    path_span,
-                )?
-            }
-        } else if self.trait_defines_associated_item_named(
+                ty::AssocKind::Type
+            };
+
+        let candidate = if self.trait_defines_associated_item_named(
             trait_ref.def_id(),
-            ty::AssocKind::Type,
+            assoc_kind,
             binding.item_name,
         ) {
-            // Simple case: X is defined in the current trait.
+            // Simple case: The assoc item is defined in the current trait.
             trait_ref
         } else {
             // Otherwise, we have to walk through the supertraits to find
-            // those that do.
-            self.one_bound_for_assoc_type(
+            // those that do contain it.
+            self.one_bound_for_assoc_item(
                 || traits::supertraits(tcx, trait_ref),
                 trait_ref.skip_binder().print_only_trait_name(),
                 None,
+                assoc_kind,
                 binding.item_name,
                 path_span,
-                match binding.kind {
-                    ConvertedBindingKind::Equality(term) => Some(term),
-                    _ => None,
-                },
+                Some(&binding.kind),
             )?
         };
 
@@ -302,18 +293,11 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
 
         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
         // of calling `filter_by_name_and_kind`.
-        let find_item_of_kind = |kind| {
-            tcx.associated_items(candidate.def_id())
-                .filter_by_name_unhygienic(assoc_ident.name)
-                .find(|i| i.kind == kind && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident)
-        };
-        let assoc_item = if return_type_notation {
-            find_item_of_kind(ty::AssocKind::Fn)
-        } else {
-            find_item_of_kind(ty::AssocKind::Type)
-                .or_else(|| find_item_of_kind(ty::AssocKind::Const))
-        }
-        .expect("missing associated type");
+        let assoc_item = tcx
+            .associated_items(candidate.def_id())
+            .filter_by_name_unhygienic(assoc_ident.name)
+            .find(|i| i.kind == assoc_kind && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident)
+            .expect("missing associated item");
 
         if !assoc_item.visibility(tcx).is_accessible_from(def_scope, tcx) {
             tcx.sess
@@ -340,7 +324,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 .or_insert(binding.span);
         }
 
-        let projection_ty = if return_type_notation {
+        let projection_ty = if let ty::AssocKind::Fn = assoc_kind {
             let mut emitted_bad_param_err = false;
             // If we have an method return type bound, then we need to substitute
             // the method's early bound params with suitable late-bound params.
@@ -495,7 +479,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
         let assoc_item_def_id = projection_ty.skip_binder().def_id;
         let def_kind = tcx.def_kind(assoc_item_def_id);
         match binding.kind {
-            ConvertedBindingKind::Equality(..) if return_type_notation => {
+            ConvertedBindingKind::Equality(..) if let ty::AssocKind::Fn = assoc_kind => {
                 return Err(self.tcx().sess.emit_err(
                     crate::errors::ReturnTypeNotationEqualityBound { span: binding.span },
                 ));
@@ -505,6 +489,9 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 // the "projection predicate" for:
                 //
                 // `<T as Iterator>::Item = u32`
+                // FIXME(fmease): This is no longer reachable since we no longer fall back to
+                // consts from types above. Ideally, we would start a second "probe" here
+                // where AssocKind is not fixed and then provide a nice error message.
                 match (def_kind, term.unpack()) {
                     (DefKind::AssocTy, ty::TermKind::Ty(_))
                     | (DefKind::AssocConst, ty::TermKind::Const(_)) => (),