When an ADT field is missing a stability attribute, look at its parent

[estebank]

This will allow us to remove stability attributes in fields in the stdlib which are shown in user visible output unnecessarily, obscuring the meaning of the span.

Partially address verbosity issues with the new output for #65386 introduced in #65912.

Once this is landed, we can manually modify the stdlib to no longer include stable attributes in tuple-like fields. This way the output can go from:

error[E0425]: cannot find function, tuple struct or tuple variant `OK` in this scope
   --> file.rs:2:11
    |
2   |   let _ = OK(());
    |           ^^ help: a tuple variant with a similar name exists (notice the capitalization): `Ok`
    |
   ::: src/libcore/result.rs:249:5
    |
249 |     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
    |     --------------------------------------------------- similarly named tuple variant `Ok` defined here

to

error[E0425]: cannot find function, tuple struct or tuple variant `OK` in this scope
   --> file.rs:2:11
    |
2   |   let _ = OK(());
    |           ^^ help: a tuple variant with a similar name exists (notice the capitalization): `Ok`
    |
   ::: src/libcore/result.rs:249:5
    |
249 |     Ok(T),
    |     ----- similarly named tuple variant `Ok` defined here

r? @petrochenkov

[petrochenkov]

I'd prefer to avoid adding arbitrary exceptions into the stability system without strong reasons.

Standard library is currently shipped with remapped paths and spans from it are not displayed, so this is not a pressing issue.

fn main() {
    ok;
}

---

error[E0425]: cannot find value `ok` in this scope
 --> src/main.rs:2:5
  |
2 |     ok;
  |     ^^
  |
help: a tuple variant with a similar name exists
  |
2 |     Ok;
  |     ^^
help: possible candidates are found in other modules, you can import them into scope
  |
1 | use futures::future::ok;
  |
1 | use futures_util::future::ok;
  |

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=dc9e9c25122f5ea56f62f14a12d7557a