Added shadowed hint for overlapping associated types

Added help message to suggest renaming

Added rename help message on subtrait

Remove rename suggestions on unrelated associated types

Added ui test

Removed call to filter_by_name_unhygienic

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -9,7 +9,7 @@ use rustc_errors::{pluralize, struct_span_err, Applicability, Diagnostic, ErrorG
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::traits::FulfillmentError;
-use rustc_middle::ty::{self, suggest_constraining_type_param, Ty, TyCtxt};
+use rustc_middle::ty::{self, suggest_constraining_type_param, AssocItem, AssocKind, Ty, TyCtxt};
 use rustc_session::parse::feature_err;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{sym, Ident};
@@ -513,6 +513,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         if associated_types.values().all(|v| v.is_empty()) {
             return;
         }
+
         let tcx = self.tcx();
         // FIXME: Marked `mut` so that we can replace the spans further below with a more
         // appropriate one, but this should be handled earlier in the span assignment.
@@ -585,6 +586,32 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
         }
 
+        // We get all the associated items that _are_ set,
+        // so that we can check if any of their names match one of the ones we are missing.
+        // This would mean that they are shadowing the associated type we are missing,
+        // and we can then use their span to indicate this to the user.
+        let bound_names = trait_bounds
+            .iter()
+            .filter_map(|poly_trait_ref| {
+                poly_trait_ref.trait_ref.path.segments.last().and_then(|path| path.args)
+            })
+            .flat_map(|args| {
+                args.bindings.iter().map(|binding| {
+                    let ident = binding.ident;
+                    let trait_def = poly_trait_ref.trait_ref.path.res.opt_def_id();
+                    let assoc_item = trait_def.and_then(|did| {
+                        tcx.associated_items(did).find_by_name_and_kinds(
+                            tcx,
+                            ident,
+                            &[AssocKind::Fn, AssocKind::Type, AssocKind::Const],
+                            did,
+                        );
+                    });
+                    (ident.name, assoc_item)
+                })
+            })
+            .collect::<FxHashMap<Symbol, Option<&AssocItem>>>();
+
         let mut names = names
             .into_iter()
             .map(|(trait_, mut assocs)| {
@@ -614,6 +641,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             pluralize!(names_len),
             names,
         );
+        let mut rename_suggestions = vec![];
         let mut suggestions = vec![];
         let mut types_count = 0;
         let mut where_constraints = vec![];
@@ -625,23 +653,47 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 *names.entry(item.name).or_insert(0) += 1;
             }
             let mut dupes = false;
+            let mut shadows = false;
             for item in assoc_items {
                 let prefix = if names[&item.name] > 1 {
                     let trait_def_id = item.container_id(tcx);
                     dupes = true;
                     format!("{}::", tcx.def_path_str(trait_def_id))
+                } else if bound_names.contains_key(&item.name) {
+                    let trait_def_id = item.container_id(tcx);
+                    shadows = true;
+                    format!("{}::", tcx.def_path_str(trait_def_id))
                 } else {
                     String::new()
                 };
                 if let Some(sp) = tcx.hir().span_if_local(item.def_id) {
                     err.span_label(sp, format!("`{}{}` defined here", prefix, item.name));
                 }
+
+                if let Some(Some(assoc_item)) = bound_names.get(&item.name) {
+                    err.span_label(
+                        tcx.def_span(assoc_item.def_id),
+                        format!("`{}{}` shadowed here", prefix, item.name),
+                    );
+                }
             }
             if potential_assoc_types.len() == assoc_items.len() {
                 // When the amount of missing associated types equals the number of
                 // extra type arguments present. A suggesting to replace the generic args with
                 // associated types is already emitted.
                 already_has_generics_args_suggestion = true;
+            } else if shadows {
+                for item in assoc_items {
+                    if let Some(Some(assoc_item)) = bound_names.get(&item.name) {
+                        if let Some(sp) = tcx.hir().span_if_local(item.def_id) {
+                            rename_suggestions.push(sp);
+                        }
+
+                        if let Some(sp) = tcx.hir().span_if_local(assoc_item.def_id) {
+                            rename_suggestions.push(sp);
+                        }
+                    }
+                }
             } else if let (Ok(snippet), false) =
                 (tcx.sess.source_map().span_to_snippet(*span), dupes)
             {
@@ -725,6 +777,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 err.span_help(where_constraints, where_msg);
             }
         }
+
+        for span in rename_suggestions {
+            err.span_help(span, "consider renaming this associated type");
+        }
         err.emit();
     }
 }

tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr

@@ -6,6 +6,11 @@ LL |     inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>
    |             |                  |
    |             |                  associated types `Item`, `IntoIter` must be specified
    |             associated types `Item`, `IntoIter` must be specified
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+   = note: `IntoIterator::Item` shadowed here
+   |
+   = note: `IntoIterator::Item` shadowed here
 
 error[E0223]: ambiguous associated type
   --> $DIR/overlaping-bound-suggestion.rs:7:13

tests/ui/associated-types/associated-type-shadowed-from-supertrait.rs

@@ -0,0 +1,19 @@
+// Test Setting the value of an associated type
+// that is shadowed from a supertrait
+
+pub trait Super {
+    type X;
+}
+
+pub trait Sub: Super {
+    type X; //~INFO shadowed
+}
+
+//~ ERROR missing type
+impl<T> Clone for Box<dyn Sub<X = T>> {
+    fn clone(&self) -> Self {
+        unimplemented!();
+    }
+}
+
+pub fn main() {}

tests/ui/associated-types/associated-type-shadowed-from-supertrait.stderr

@@ -0,0 +1,26 @@
+error[E0191]: the value of the associated type `X` in `Super` must be specified
+  --> $DIR/associated-type-shadowed-from-supertrait.rs:13:27
+   |
+LL |     type X;
+   |     ------ `Super::X` defined here
+...
+LL |     type X;
+   |     ------ `Super::X` shadowed here
+...
+LL | impl<T> Clone for Box<dyn Sub<X = T>> {
+   |                           ^^^^^^^^^^ associated type `X` must be specified
+   |
+help: consider renaming this associated type
+  --> $DIR/associated-type-shadowed-from-supertrait.rs:5:5
+   |
+LL |     type X;
+   |     ^^^^^^
+help: consider renaming this associated type
+  --> $DIR/associated-type-shadowed-from-supertrait.rs:9:5
+   |
+LL |     type X;
+   |     ^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0191`.