"expected X found X" diagnostic for lifetime mismatch in method signature of trait impl

[SimonSapin]

With this code:

struct Foo(u32);

impl From<&Foo> for &u32 {
    fn from(foo: &Foo) -> &u32 {
        &foo.0
    }
}

The error message is:

error: `impl` item signature doesn't match `trait` item signature
   --> src/lib.rs:4:5
    |
4   |     fn from(foo: &Foo) -> &u32 {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&Foo) -> &u32`
    |
    = note: expected `fn(&Foo) -> &u32`
               found `fn(&Foo) -> &u32`
    = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
    = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output

Now, the two help lines do give relevant information on how to fix this, which is to have a single named lifetime and use it everywhere:

struct Foo(u32);

impl<'a> From<&'a Foo> for &'a u32 {
    fn from(foo: &'a Foo) -> &'a u32 {
        &foo.0
    }
}

However the part of the error that is more visible at first glance (at least to me) is:

    = note: expected `fn(&Foo) -> &u32`
               found `fn(&Foo) -> &u32`

My internal reaction was: if what was expected and found is the same thing, what’s the problem?

The two signature are in fact not the same because they each have their own implicit lifetime parameter which don’t necessarily match each other. Would it make sense to make those parameters explicit? Signatures in diagnostics don’t need to be entirely valid Rust syntax. Something like:

    = note: expected `fn(&'_1 Foo) -> &'_1 u32`
               found `fn(&'_2 Foo) -> &'_2 u32`

Meta

The output is the same in 1.48.0 and nightly-2020-11-30.

[jyn514]

Related: #76353, #41078

[estebank]

Current output:

error: `impl` item signature doesn't match `trait` item signature
 --> src/lib.rs:6:5
  |
6 |     fn from(foo: &Foo) -> &u32 {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 Foo) -> &'1 u32`
  |
  = note: expected `fn(&'1 Foo) -> &'2 u32`
             found `fn(&'1 Foo) -> &'1 u32`
  = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
  = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output