Addition to E0582 explanation

Code

trait Foo {
    type Assoc<'a>;
}

#[allow(dead_code)]
struct Bar<X, F>
where
    X: Foo,
    F: for<'a> Fn(X::Assoc<'a>) -> &'a i32
{
    x: X,
    f: F
}

Current output

error[E0582]: binding for associated type `Output` references lifetime `'a`, which does not appear in the trait input types
  --> src/lib.rs:23:42
   |
23 |         F: for<'a> Fn(X::Content<'a>) -> &'a i32
   |                                          ^^^^^^^

For more information about this error, try `rustc --explain E0582`.

Desired output

No response

Rationale and extra context

Problem

This error message could be more helpful.

• While 'a appears in the parameter for the associated type, there is no guarantee that an impl for the associated type actually uses the parameter, therefore 'a is not guaranteed to be used in the input for any impl Foo

Suggestion

1. Either add extra explanation to the error when with regards to associated type usage (e.g. the above problem annotating the X::Assoc<'a> (requires time to implement
2. Update the rustc --explain E0582 to include this example.
3. No changes, skill issue

Other cases

No response

Rust Version

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2

Anything else?

No response

Start with the E0582 entry used by rustc --explain E0582 and compare it with the reported associated-type example. Clarify whether the accepted scope is an explanation update or a diagnostic change; done means the explanation covers why the lifetime is not guaranteed and the relevant tests or documentation checks pass.