Give return-position impl traits in trait a (synthetic) name to avoid name collisions with new lowering strategy

[spastorino]

The only needed commit from this PR is the last one.

r? @compiler-errors

Needs #109455.

[compiler-errors]

Since this is waiting on that other PR, gonna mark it as waiting on author

@rustbot author

[spastorino]

@rustbot ready

I took over @compiler-errors' #109455 and changed the last commit according to @petrochenkov's review.

Opened it for discussions but still have 2 problems to address.

1. We need to give ty::AssocItem a proper name here https://github.com/rust-lang/rust/pull/109499/files#diff-03c3ed2f6e27e58df63382e00323eea6fd1ce3a4f9546941b2b717fead25e520R1076, unsure what to do there.
2. We need to generate associated item names like Foo::bar::{opaque#0} and we are giving Foo::{opaque#0} to match diagnostics here https://github.com/rust-lang/rust/pull/109499/files#diff-2245ba262b8ba2866c27f1ac3f76db96146870beb91a6ac7ce621ca2cbb7c501R9. I'm not sure why https://github.com/rust-lang/rust/pull/109499/files#diff-f7bb866d465985edefe49cace054fae026f7ca4321a0db60294f1cbdac847ef7R356 that starts from opaque_ty_def_id is not grabbing correctly the fn name. Need to debug.

[spastorino]

I see now what happens in problem 2, the name is set correctly on AssocItem but that's not what's used on diagnostics. What we use is https://github.com/spastorino/rust/blob/034aae297dadc7b3884f711c6c7d5c7071b34675/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs#L2805.
I guess we may need to change https://github.com/spastorino/rust/blob/034aae297dadc7b3884f711c6c7d5c7071b34675/compiler/rustc_middle/src/ty/print/mod.rs#L102 or we can also just leave it as Foo::{opaque#0}?

[spastorino]

Problem 2 fixed by making default_print_def_path account for RPITITs.

[spastorino]

Ok, I've tidy up everything and left only the solution that makes error reports do Trait::{opaque#}.

Given that @petrochenkov removed their assignment I guess you're happy now with the changes, correct?. If so, this is now up to @compiler-errors.

[compiler-errors]

I'm happy with this for now.

@bors r+

[bors]

📌 Commit 2ca350c has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2ca350c with merge f2d9a3d...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing f2d9a3d to master...

[rust-timer]

Finished benchmarking commit (f2d9a3d): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.2%	[1.0%, 3.4%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-2.4%	[-3.0%, -1.8%]	2
All ❌✅ (primary)	0.6%	[-2.6%, 3.4%]	3

Cycles

This benchmark run did not return any relevant results for this metric.