Don't add specialized notes to error messages pointing at a Type

Fixes rust-lang#121398

compiler/rustc_middle/src/hir/map/mod.rs

@@ -312,6 +312,14 @@ impl<'hir> Map<'hir> {
         Some(body_id)
     }
 
+    /// Check if a given `LocalDefId` is in an impl block.
+    pub fn is_impl_block(self, id: LocalDefId) -> bool {
+        matches!(
+            self.get_if_local(id.to_def_id()),
+            Some(Node::Item(Item { kind: ItemKind::Impl(_), .. }))
+        )
+    }
+
     /// Given a body owner's id, returns the `BodyId` associated with it.
     #[track_caller]
     pub fn body_owned_by(self, id: LocalDefId) -> BodyId {

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -150,16 +150,20 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         ty::print::with_no_trimmed_paths!(ty::print::with_no_visible_paths!({
             let generics = self.tcx.generics_of(def_id);
             let self_ty = trait_ref.self_ty();
-            // This is also included through the generics list as `Self`,
-            // but the parser won't allow you to use it
-            flags.push((sym::_Self, Some(self_ty.to_string())));
-            if let Some(def) = self_ty.ty_adt_def() {
-                // We also want to be able to select self's original
-                // signature with no type arguments resolved
-                flags.push((
-                    sym::_Self,
-                    Some(self.tcx.type_of(def.did()).instantiate_identity().to_string()),
-                ));
+
+            // Don't add specialized notes on types
+            if !self.tcx.hir().is_impl_block(obligation.cause.body_id) {
+                // This is also included through the generics list as `Self`,
+                // but the parser won't allow you to use it
+                flags.push((sym::_Self, Some(self_ty.to_string())));
+                if let Some(def) = self_ty.ty_adt_def() {
+                    // We also want to be able to select self's original
+                    // signature with no type arguments resolved
+                    flags.push((
+                        sym::_Self,
+                        Some(self.tcx.type_of(def.did()).instantiate_identity().to_string()),
+                    ));
+                }
             }
 
             for param in generics.params.iter() {

tests/ui/errors/traits/no-specialized-error-for-types-121398.rs

@@ -5,7 +5,7 @@ impl Bar for String {} //~ ERROR `String` is not an iterator [E0277]
 impl<T> Bar for Vec<T> {} //~ ERROR `Vec<T>` is not an iterator [E0277]
 
 use std::ops::IndexMut;
-trait Foo<Idx>: IndexMut<Idx> {} 
+trait Foo<Idx>: IndexMut<Idx> {}
 
 impl<Idx> Foo<Idx> for String {} //~ ERROR the type `String` cannot be mutably indexed by `Idx` [E0277]
 

tests/ui/errors/traits/no-specialized-error-for-types-121398.stderr

@@ -0,0 +1,46 @@
+error[E0277]: `String` is not an iterator
+  --> $DIR/no-specialized-error-for-types-121398.rs:3:14
+   |
+LL | impl Bar for String {}
+   |              ^^^^^^ `String` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `String`
+note: required by a bound in `Bar`
+  --> $DIR/no-specialized-error-for-types-121398.rs:1:12
+   |
+LL | trait Bar: Iterator {}
+   |            ^^^^^^^^ required by this bound in `Bar`
+
+error[E0277]: `Vec<T>` is not an iterator
+  --> $DIR/no-specialized-error-for-types-121398.rs:5:17
+   |
+LL | impl<T> Bar for Vec<T> {}
+   |                 ^^^^^^ `Vec<T>` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `Vec<T>`
+note: required by a bound in `Bar`
+  --> $DIR/no-specialized-error-for-types-121398.rs:1:12
+   |
+LL | trait Bar: Iterator {}
+   |            ^^^^^^^^ required by this bound in `Bar`
+
+error[E0277]: the type `String` cannot be mutably indexed by `Idx`
+  --> $DIR/no-specialized-error-for-types-121398.rs:10:24
+   |
+LL | impl<Idx> Foo<Idx> for String {}
+   |                        ^^^^^^ `String` cannot be mutably indexed by `Idx`
+   |
+   = help: the trait `IndexMut<Idx>` is not implemented for `String`
+note: required by a bound in `Foo`
+  --> $DIR/no-specialized-error-for-types-121398.rs:8:17
+   |
+LL | trait Foo<Idx>: IndexMut<Idx> {}
+   |                 ^^^^^^^^^^^^^ required by this bound in `Foo`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | impl<Idx> Foo<Idx> for String where String: IndexMut<Idx> {}
+   |                               +++++++++++++++++++++++++++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/errors/traits/specialized-error-for-expr.stderr

@@ -0,0 +1,12 @@
+error[E0277]: `String` is not an iterator
+  --> $DIR/specialized-error-for-expr.rs:3:14
+   |
+LL |     for _ in s {}
+   |              ^ `String` is not an iterator; try calling `.chars()` or `.bytes()`
+   |
+   = help: the trait `Iterator` is not implemented for `String`, which is required by `String: IntoIterator`
+   = note: required for `String` to implement `IntoIterator`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.