`on_unimplemented` label and notes are not displayed for transitive bounds

[cramertj]

Code

struct S;

#[diagnostic::on_unimplemented(
    message = "unable to generate binding for function",
    label = "Some label",
    note = "note 1",
    note = "note 2",
    note = "note 3",
)]
trait Failure7<'a> {}

impl<'a> Clone for S where S: Failure7<'a> {
    fn clone(&self) -> Self {
        unreachable!()
    }
}

fn main() {
    fn take_clone(_: impl Clone) {}
    take_clone(S);
}

Current output

error[E0277]: the trait bound `S: Clone` is not satisfied
  --> src/main.rs:20:16
   |
20 |     take_clone(S);
   |     ---------- ^ the trait `Clone` is not implemented for `S`
   |     |
   |     required by a bound introduced by this call
   |
note: required for `S` to implement `Clone`
  --> src/main.rs:12:10
   |
12 | impl<'a> Clone for S where S: Failure7<'a> {
   |          ^^^^^     ^          ------------ unsatisfied trait bound introduced here
note: required by a bound in `take_clone`
  --> src/main.rs:19:27
   |
19 |     fn take_clone(_: impl Clone) {}
   |                           ^^^^^ required by this bound in `take_clone`
help: consider borrowing here
   |
20 |     take_clone(&S);
   |                +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Rationale and extra context

The output should include the provided label and note values, as it does when depending on the trait bound directly.

Rust Version

1.83.0

[compiler-errors]

This probably got messed up by #121826, which applied some really messy hacks to not note the transitive bound. I'd kinda prefer if we reverted that PR, frankly, rather than keep working around it.

We have #[diagnostic::do_not_recommend] for cases where we want to avoid recursing into unnecessary blanket impls, but the current state of the report_selection_error function makes it incredibly difficult to distinguish what the diagnostic is actually aiming to report.