Auto merge of #116082 - compiler-errors:default-assoc-ty-msg, r=estebank

Tweak expected message to explain what it's actually signifying

r? `@estebank` since you added this

Author: bors

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -616,9 +616,13 @@ fn foo(&self) -> Self::T { String::new() }
                 for item in &items[..] {
                     if let hir::AssocItemKind::Type = item.kind {
                         let assoc_ty = tcx.type_of(item.id.owner_id).instantiate_identity();
-
-                        if self.infcx.can_eq(param_env, assoc_ty, found) {
-                            diag.span_label(item.span, "expected this associated type");
+                        if let hir::Defaultness::Default { has_value: true } = tcx.defaultness(item.id.owner_id)
+                            && self.infcx.can_eq(param_env, assoc_ty, found)
+                        {
+                            diag.span_label(
+                                item.span,
+                                format!("associated type is `default` and may be overridden"),
+                            );
                             return true;
                         }
                     }

tests/ui/associated-types/defaults-specialization.stderr

@@ -29,7 +29,7 @@ error[E0053]: method `make` has an incompatible type for trait
   --> $DIR/defaults-specialization.rs:35:18
    |
 LL |     default type Ty = bool;
-   |     ----------------------- expected this associated type
+   |     ----------------------- associated type is `default` and may be overridden
 LL |
 LL |     fn make() -> bool { true }
    |                  ^^^^
@@ -76,7 +76,7 @@ error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:44:29
    |
 LL |     default type Ty = bool;
-   |     ----------------------- expected this associated type
+   |     ----------------------- associated type is `default` and may be overridden
 LL |
 LL |     fn make() -> Self::Ty { true }
    |                  --------   ^^^^ expected associated type, found `bool`

tests/ui/specialization/specialization-default-types.stderr

@@ -12,7 +12,7 @@ error[E0308]: mismatched types
   --> $DIR/specialization-default-types.rs:15:9
    |
 LL |     default type Output = Box<T>;
-   |     ----------------------------- expected this associated type
+   |     ----------------------------- associated type is `default` and may be overridden
 LL |     default fn generate(self) -> Self::Output {
    |                                  ------------ expected `<T as Example>::Output` because of return type
 LL |         Box::new(self)