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