Error when concrete type doesn't satisfy impl trait is poorly spanned

[SNCPlay42]

trait Trait {}

fn foo() -> impl Trait {
    0u8
}

Currently emits

error[E0277]: the trait bound `u8: Trait` is not satisfied
 --> src/lib.rs:3:13
  |
3 | fn foo() -> impl Trait {
  |             ^^^^^^^^^^ the trait `Trait` is not implemented for `u8`

It should probably point to the return value as the span of the actual error, with the current span appearing in a note, e.g.

error[E0277]: the trait bound `u8: Trait` is not satisfied
 --> src/lib.rs:4:4
  |
3 | fn foo() -> impl Trait {
  |             ---------- the return value of this function must implement `Trait`
4 |     0u8
  |     ^^^  the trait `Trait` is not implemented for `u8`

@rustbot label T-compiler A-impl-trait A-diagnostics D-terse

[SNCPlay42]

The problem is, the error comes from the obligation registered on the inference var created when instantiating the opaque type, which happens very early in type-checking, so when creating the obligation we don't yet have a good span for the actual return value(s) of the function (it's easy to find the function's body's tail, but what if it only returns from return statements?)