Don't suggest `let` bindings if they don't help with borrows

[oli-obk]

fn foo(_: &'static u32) {}

fn unpromotable<T>(t: T) -> T { t }

fn main() {
    foo(&unpromotable(5u32));
}

suggests

error[E0597]: borrowed value does not live long enough
 --> src/main.rs:6:10
  |
6 |     foo(&unpromotable(5u32));
  |          ^^^^^^^^^^^^^^^^^^ - temporary value only lives until here
  |          |
  |          temporary value does not live long enough
  |
  = note: borrowed value must be valid for the static lifetime...
  = note: consider using a `let` binding to increase its lifetime

but

    let x = unpromotable(5u32);
    foo(&x);

still won't give us a 'static lifetime.

We should not emit the note if the required lifetime is 'static

Instructions

1. search for the note inside the source
2. I presume there's some code already checking for producing the "borrowed value must be valid for the static lifetime" message, find that code and bubble up the knowledge about that until you have it in the scope of the "consider using a let binding to increase its lifetime" message
3. Don't emit the message in that case
4. This'll already fail a bunch of tests, I don't think you need any new tests. Running ./x.py test --stage 1 src/test/{ui,compile-fail} --bless should succeed and give you a bunch of changes to commit. Check that all changes make sense and just commit them.

[PramodBisht]

@oli-obk can I take this on if nobody is working on this?

[oli-obk]

All yours!

[PramodBisht]

closing it, as required code change has been merged.