Point out shadowed associated types

When trying to set the value of a shadowed associated type,
the error message was confusing, as it simply said that there is a
missing type. Now the compiler notifies the user about the shadow,
and suggests renaming as a fix

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 +586,35 @@ 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| {
+                    let args = path.args?;
+                    Some((path, args))
+                })
+            })
+            .flat_map(|(path, args)| {
+                args.bindings.iter().map(|binding| {
+                    let ident = binding.ident;
+                    let trait_def = 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 +644,7 @@ 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 +656,47 @@ 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 +780,10 @@ 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;
+}
+
+impl<T> Clone for Box<dyn Sub<X = T>> {
+    //~^ ERROR associated type `X` in `Super` must be specified
+    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:12:27
+   |
+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`.