Show detailed expected/found types in error message when trait paths are the same

[rhysd]

Fixes #65230.

Issue solved by this PR

trait T {
    type U;
    fn f(&self) -> Self::U;
}

struct X<'a>(&'a mut i32);

impl<'a> T for X<'a> {
    type U = &'a i32;
    fn f(&self) -> Self::U {
        self.0
    }
}

fn main() {}

Compiler generates the following note:

note: ...so that the types are compatible
  --> test.rs:10:28
   |
10 |       fn f(&self) -> Self::U {
   |  ____________________________^
11 | |         self.0
12 | |     }
   | |_____^
   = note: expected `T`
              found `T`

This note is not useful since the expected type and the found type are the same.

How this PR solve the issue

When the expected type and the found type are exactly the same in string representation, the note falls back to the detailed string representation of trait ref:

note: ...so that the types are compatible
  --> test.rs:10:28
   |
10 |       fn f(&self) -> Self::U {
   |  ____________________________^
11 | |         self.0
12 | |     }
   | |_____^
   = note: expected `<X<'a> as T>`
              found `<X<'_> as T>`

So that a user can notice what was different between the expected one and the found one.

[rust-highfive]

r? @petrochenkov

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[rhysd]

I believe this PR is ready for review.

[joshtriplett]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 7b9ddbd with merge 241eecf4e768114fe5702e365e7f003c1ada0208...

[bors]

☀️ Try build successful - checks-actions
Build commit: 241eecf4e768114fe5702e365e7f003c1ada0208 (241eecf4e768114fe5702e365e7f003c1ada0208)

[rust-timer]

Queued 241eecf4e768114fe5702e365e7f003c1ada0208 with parent f875143, future comparison URL.

[rust-timer]

Finished benchmarking commit (241eecf4e768114fe5702e365e7f003c1ada0208): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[petrochenkov]

@bors r+

[bors]

📌 Commit 7b9ddbd has been approved by petrochenkov

[bors]

⌛ Testing commit 7b9ddbd with merge c63fbaa9799dba29b73feb023d15c0cd8a4873f8...

[rhysd]

Thanks to CI, I found error messages change when -Zborrowck=mir is specified. I'll fix some test cases.

[rhysd]

@petrochenkov I updated the test to pass with -Z borrowck=mir. Would you review this again? I confirmed it passed with the following command locally:

./x.py test ./src/test/ui/issues/issue-65230.rs --compare-mode nll

[petrochenkov]

@bors r+

[bors]

📌 Commit 9211bee has been approved by petrochenkov

[bors]

⌛ Testing commit 9211bee with merge 68dfa07...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 68dfa07 to master...

[rust-timer]

Finished benchmarking commit (68dfa07): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression