Regression in helpful compiler error message when working with lifetimes

[petergarnaes]

So I have the following code:

struct Article {
    proof_reader: ProofReader,
}

struct ProofReader {
    name: String,
}

pub trait HaveRelationship<To> {
    fn get_relation(&self) -> To;
}

impl HaveRelationship<&ProofReader> for Article {
    fn get_relation(&self) -> &ProofReader {
        &self.proof_reader
    }
}

Which on the stable compiler gives this helpful error message:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements
  --> src/main.rs:14:5
   |
14 |     fn get_relation(&self) -> &ProofReader {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 14:5...
  --> src/main.rs:14:5
   |
14 | /     fn get_relation(&self) -> &ProofReader {
15 | |         &self.proof_reader
16 | |     }
   | |_____^
note: ...but the lifetime must also be valid for the lifetime '_ as defined on the impl at 13:23...
  --> src/main.rs:13:23
   |
13 | impl HaveRelationship<&ProofReader> for Article {
   |                       ^
   = note: ...so that the method type is compatible with trait:
           expected fn(&Article) -> &ProofReader
              found fn(&Article) -> &ProofReader

Yet on the nightly compiler the compiler error does not inform me about lifetimes, but tells me the signature needs to be what it already is:

error: `impl` item signature doesn't match `trait` item signature
  --> src/main.rs:14:5
   |
10 |     fn get_relation(&self) -> To;
   |     ----------------------------- expected fn(&Article) -> &ProofReader
...
14 |     fn get_relation(&self) -> &ProofReader {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&Article) -> &ProofReader
   |
   = note: expected `fn(&Article) -> &ProofReader`
              found `fn(&Article) -> &ProofReader`

playground stable version
playground nightly version

[jonas-schievink]

Happens on beta too

[estebank]

For what is worth, the correct code would be closer to:

pub trait HaveRelationship<'a, To: 'a> {
    fn get_relation(&'a self) -> To;
}

impl<'a> HaveRelationship<'a, &'a ProofReader> for Article {
    fn get_relation(&'a self) -> &'a ProofReader {
        &self.proof_reader
    }
}

The error is not incorrect, as there is a discrepancy between the trait and the impl, but it should provide more information in this case, particularly to show that the lifetimes in expected and found are not evaluated the same.

[estebank]

This is caused by #65068. I believe this is now a duplicate of #41343.

CC @nikomatsakis do you have any ideas of what the appropriate output should be for these cases?

[pnkfelix]

triage: P-medium, removing nomination