Unhelpful diagnostic for sort_by_key lifetime issue

[tornewuff]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=132fc68312d40c6ed0d878b1d5bdb711

fn parse_filename(file: &std::fs::DirEntry) -> &str {
    todo!()
}

fn foo(mut files: Vec<std::fs::DirEntry>) {
    files.sort_by_key(parse_filename);
}

The current output is:

error[E0308]: mismatched types
 --> src/lib.rs:6:11
  |
6 |     files.sort_by_key(parse_filename);
  |           ^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected associated type `<for<'r> fn(&'r DirEntry) -> &'r str {parse_filename} as FnOnce<(&DirEntry,)>>::Output`
             found associated type `<for<'r> fn(&'r DirEntry) -> &'r str {parse_filename} as FnOnce<(&DirEntry,)>>::Output`

For more information about this error, try `rustc --explain E0308`.

Ideally the output should explain the lifetime issue somehow rather than saying "one type is more general than the other" for two identical types. I understand that the reason why this code doesn't work is as described in https://stackoverflow.com/questions/47121985/why-cant-i-use-a-key-function-that-returns-a-reference-when-sorting-a-vector-wi but this error output wasn't informative :)

[BGR360]

@rustbot label +D-confusing +D-terse +A-lifetimes +A-associated-items

[panstromek]

I get a different error but similarly unhelpful

error[E0308]: mismatched types
   --> crates\rust-common\src\label.rs:703:9
    |
703 |         pattern.sort_by_key(sort_key);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
    = note: expected tuple `(_, _, &Node)`
               found tuple `(_, _, &Node)`
note: the lifetime requirement is introduced here
   --> C:\Users\panstromek\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\alloc\src\slice.rs:307:25
    |
307 |         F: FnMut(&T) -> K,
    |                         ^

code

        fn sort_key(pat: &(Node, Point)) -> (i64, i64, &Node) {
            todo!()
        }
        pattern.sort_by_key(sort_key);