Clarify some diagnostic messages

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2940,8 +2940,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
  // though we can easily give a hint that ought to be
  // relevant.
  err.note(
- "lifetimes appearing in an associated type are not considered constrained",
+ "lifetimes appearing in an associated or opaque type are not considered constrained",
  );
+ err.note("consider introducing a named lifetime parameter");
  }
 
  err.emit();

src/test/ui/associated-types/issue-62200.rs

@@ -10,6 +10,7 @@ impl T<'_> for S {
 
 fn foo(x: impl Fn(<S as T<'_>>::A) -> <S as T<'_>>::A) {}
 //~^ ERROR binding for associated type `Output` references an anonymous lifetime
-//~^^ NOTE lifetimes appearing in an associated type are not considered constrained
+//~| NOTE lifetimes appearing in an associated or opaque type are not considered constrained
+//~| NOTE consider introducing a named lifetime parameter
 
 fn main() {}

src/test/ui/associated-types/issue-62200.stderr

@@ -4,7 +4,8 @@ error[E0582]: binding for associated type `Output` references an anonymous lifet
 LL | fn foo(x: impl Fn(<S as T<'_>>::A) -> <S as T<'_>>::A) {}
  | ^^^^^^^^^^^^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-47511.stderr

@@ -4,7 +4,8 @@ error[E0581]: return type references an anonymous lifetime, which is not constra
 LL | fn f(_: X) -> X {
  | ^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error[E0581]: return type references lifetime `'a`, which is not constrained by the fn input types
  --> $DIR/issue-47511.rs:12:23

src/test/ui/type-alias-impl-trait/constrain_inputs.rs

@@ -12,15 +12,15 @@ mod lifetime_params {
  //~^ ERROR binding for associated type `Output` references an anonymous lifetime
 }
 
-mod type_params {
+mod lifetime_params_2 {
  type Ty<'a> = impl FnOnce() -> &'a str;
  fn defining(s: &str) -> Ty<'_> { move || s }
  fn execute(ty: Ty<'_>) -> &str { ty() }
  //~^ ERROR return type references an anonymous lifetime, which is not constrained by the fn input types
 }
 
 // regression test for https://github.com/rust-lang/rust/issues/97104
-mod type_params2 {
+mod type_params {
  type Ty<T> = impl Sized;
  fn define<T>(s: T) -> Ty<T> { s }
 

src/test/ui/type-alias-impl-trait/constrain_inputs.stderr

@@ -4,47 +4,53 @@ error[E0581]: return type references an anonymous lifetime, which is not constra
 LL | fn execute(ty: Ty<'_>) -> &str { todo!() }
  | ^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error[E0581]: return type references an anonymous lifetime, which is not constrained by the fn input types
  --> $DIR/constrain_inputs.rs:9:35
  |
 LL | type BadFnSig = fn(Ty<'_>) -> &str;
  | ^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error[E0582]: binding for associated type `Output` references an anonymous lifetime, which does not appear in the trait input types
  --> $DIR/constrain_inputs.rs:11:42
  |
 LL | type BadTraitRef = dyn Fn(Ty<'_>) -> &str;
  | ^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error[E0581]: return type references an anonymous lifetime, which is not constrained by the fn input types
  --> $DIR/constrain_inputs.rs:18:31
  |
 LL | fn execute(ty: Ty<'_>) -> &str { ty() }
  | ^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error[E0581]: return type references an anonymous lifetime, which is not constrained by the fn input types
  --> $DIR/constrain_inputs.rs:27:37
  |
 LL | type BadFnSig = fn(Ty<&str>) -> &str;
  | ^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error[E0582]: binding for associated type `Output` references an anonymous lifetime, which does not appear in the trait input types
  --> $DIR/constrain_inputs.rs:29:44
  |
 LL | type BadTraitRef = dyn Fn(Ty<&str>) -> &str;
  | ^^^^
  |
- = note: lifetimes appearing in an associated type are not considered constrained
+ = note: lifetimes appearing in an associated or opaque type are not considered constrained
+ = note: consider introducing a named lifetime parameter
 
 error: aborting due to 6 previous errors