Confusing error message when eliding lifetimes in trait impl

[Aaron1011]

The following code:

trait Foo<T> {
    fn make_it(&self) -> T;
}

impl Foo<&u8> for () {
    fn make_it(&self) -> &u8 {
        &25
    }
}

produces the following error:

error: `impl` item signature doesn't match `trait` item signature
 --> src/lib.rs:6:5
  |
2 |     fn make_it(&self) -> T;
  |     ----------------------- expected fn(&()) -> &u8
...
6 |     fn make_it(&self) -> &u8 {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&()) -> &u8
  |
  = note: expected `fn(&()) -> &u8`
             found `fn(&()) -> &u8`

The issue is that the elided lifetimes in the impl header (impl Foo<&u8> for ()) and the method implementation fn make_it(&self) -> &u8 do not desugar to the same lifetime. However, we currently emit a nonsensical error message instead of explaining this.

We should explain the behavior of lifetime elision, and suggest introducing a lifetime parameter in the impl header.

[estebank]

Current output in nightly:

error: `impl` item signature doesn't match `trait` item signature
 --> src/lib.rs:6:5
  |
2 |     fn make_it(&self) -> T;
  |     ----------------------- expected `fn(&()) -> &u8`
...
6 |     fn make_it(&self) -> &u8 {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&()) -> &u8`
  |
  = note: expected `fn(&()) -> &u8`
             found `fn(&()) -> &u8`
help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
 --> src/lib.rs:2:26
  |
2 |     fn make_it(&self) -> T;
  |                          ^ consider borrowing this type parameter in the trait

[estebank]

Current output:

error: `impl` item signature doesn't match `trait` item signature
 --> src/lib.rs:6:5
  |
2 |     fn make_it(&self) -> T;
  |     ----------------------- expected `fn(&'1 ()) -> &'2 u8`
...
6 |     fn make_it(&self) -> &u8 {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 ()) -> &'1 u8`
  |
  = note: expected signature `fn(&'1 ()) -> &'2 u8`
             found signature `fn(&'1 ()) -> &'1 u8`
help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
 --> src/lib.rs:2:26
  |
2 |     fn make_it(&self) -> T;
  |                          ^ consider borrowing this type parameter in the trait