Elided lifetimes in trait impl error messages are confusing

Code

trait Trait<T> {
    fn method(self) -> T;
}
impl Trait<&i32> for &i32 {
    fn method(self) -> Self {self}
}

Current output

error[E0308]: method not compatible with trait
 --> src/lib.rs:7:5
  |
7 |     fn method(self) -> Self {self}
  |     ^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
  |
  = note: expected signature `fn(&_) -> &_`
             found signature `fn(&_) -> &_`
note: the anonymous lifetime as defined here...
 --> src/lib.rs:6:22
  |
6 | impl Trait<&i32> for &i32 {
  |                      ^
note: ...does not necessarily outlive the anonymous lifetime as defined here
 --> src/lib.rs:6:12
  |
6 | impl Trait<&i32> for &i32 {
  |            ^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (lib) due to 1 previous error

Desired output

Compiling playground v0.0.1 (/playground)
error: `impl` item signature doesn't match `trait` item signature
  --> src/lib.rs:10:5
   |
 4 |     fn method(x: T) -> Self;
   |     ------------------------ expected `fn(&'1 S) -> &'2 S`
...
 7 |     fn method(self) -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 S) -> &'1 S`
   |
   = note: expected signature `fn(&'1 S) -> &'2 S`
              found signature `fn(&'1 S) -> &'1 S`
help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
error: could not compile `playground` (lib) due to 1 previous error

Rationale and extra context

I first reported this in #87763, and the example there actually now has good error messages. This is not a regression of that issue, but something is still wrong in the general case.

I'm not sure what the right fix is (and not entirely sure I remember what the right types are), but the above "desired output" is similar to what you get in the case of #87763, so I'd expect something sort of similar here.

Other cases

If instead, you use the form from #87763, you get the following:


trait Trait<T> {
    fn method(_: T) -> Self;
}
impl Trait<&i32> for &i32 {
    fn method(x: &i32) -> &i32 {x}
}


->


   Compiling playground v0.0.1 (/playground)
error: `impl` item signature doesn't match `trait` item signature
 --> src/lib.rs:7:5
  |
4 |     fn method(_: T) -> Self;
  |     ------------------------ expected `fn(&'1 i32) -> &'2 i32`
...
7 |     fn method(x: &i32) -> &i32 {x}
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 i32) -> &'1 i32`
  |
  = note: expected signature `fn(&'1 i32) -> &'2 i32`
             found signature `fn(&'1 i32) -> &'1 i32`
help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
 --> src/lib.rs:4:18
  |
4 |     fn method(_: T) -> Self;
  |                  ^     ^^^^ consider borrowing this type parameter in the trait
  |                  |
  |                  consider borrowing this type parameter in the trait

error: could not compile `playground` (lib) due to 1 previous error


I bet there are many variations along a theme one could try here.

Rust Version

Rust playground 1.93.0

Anything else?

No response

Reproduce the two trait and impl examples in src/lib.rs with Rust Playground 1.93.0, comparing the current diagnostics with the desired output and the working form from #87763. Investigate the compiler diagnostic path for trait-impl signature mismatches, including how elided lifetimes are rendered. Done means the general case reports distinguishable lifetime signatures and explains the mismatch without misleading anonymous lifetimes.