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

3 years ago

source link: https://github.com/rust-lang/rust/pull/89633
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rhysd commented 13 days ago

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.

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