Account for common impl Trait/dyn Trait return type errors

[estebank]

• When all return paths have the same type, suggest impl Trait.
• When all return paths implement the expected trait, suggest Box<dyn Trait> and mention using an enum.
• When multiple different types are returned and impl Trait is expected, extend the explanation.
• When return type is impl Trait and the return paths do not implement Trait, point at the returned values.
• Split src/librustc/traits/error_reporting.rs into multiple files to keep size under control.

Fix #68110, cc #66523.

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[rust-highfive]

r? @eddyb

(rust_highfive has picked a reviewer for you, use r? to override)

[estebank]

The changes have reasonable single commit sizes, but the final diff is on the larger side.

CC @rust-lang/wg-diagnostics

[estebank]

Note that suggestions always come last, which messes up the flow a bit (I'd prefer the last thing in the diagnostic to be the links, but alas I won't be changing DiagnosticBuilder today).

[oli-obk]

r? @oli-obk

[oli-obk]

r? @Centril (reflecting reality here)

[estebank]

I'm looking at addressing the cases where T is not object safe to keep it out of the suggestion, but it is a hard error that doesn't even reach the unsized error:

error[E0038]: the trait `NotObjectSafe` cannot be made into an object
  --> src/test/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.rs:21:1
   |
3  |     fn foo() -> Self;
   |        --- associated function `foo` has no `self` parameter
...
21 | fn car() -> dyn NotObjectSafe {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object

error[E0038]: the trait `NotObjectSafe` cannot be made into an object
  --> src/test/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.rs:28:1
   |
3  |     fn foo() -> Self;
   |        --- associated function `foo` has no `self` parameter
...
28 | fn cat() -> Box<dyn NotObjectSafe> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object

The impl T case doesn't account for object safety yet.

[Centril]

@bors r+

[bors]

📌 Commit 029a9c6 has been approved by Centril

[Ixrec]

Do we have an issue for changing the "return type cannot be a bare trait" language to something more dyn-aware? (I couldn't find one)

[estebank]

It should be "unboxed trait object". Don't think there's a ticket.