one type is more general than the other: note: expected reference &_ found reference &_ (note: the lifetime requirement is introduced here: [arrow points at nothing])

[nabijaczleweli]

Code

fn main() {
    let outputs_rgb: &mut [[u8; 12]] = &mut [];
    fn gaming(_: &[u8; 12]) -> &[u8] {
        unimplemented!()
    }
    outputs_rgb.sort_unstable_by_key(gaming);
}

Current output

error[E0308]: mismatched types
    --> test.rs:6:5
     |
6    |     outputs_rgb.sort_unstable_by_key(gaming);
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
     |
     = note: expected reference `&_`
                found reference `&_`
note: the lifetime requirement is introduced here
    --> D:\Users\nabijaczleweli\.rustup\toolchains\stable-x86_64-pc-windows-gnu\lib/rustlib/src/rust\library\core\src\slice\mod.rs:3029:25
     |
3029 |         F: FnMut(&T) -> K,
     |                         ^

error: aborting due to 1 previous error

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

Desired output

idk. literally anything else i think lol

Rust Version

rustc 1.84.1 (e71f9a9a9 2025-01-27)
binary: rustc
commit-hash: e71f9a9a98b0faf423844bf0ba7438f29dc27d58
commit-date: 2025-01-27
host: x86_64-pc-windows-gnu
release: 1.84.1
LLVM version: 19.1.5

[nabijaczleweli]

On rustc 1.64.0-nightly (46b8c23f3 2022-07-01) I get

error[E0308]: mismatched types
 --> test.rs:6:5
  |
6 |     outputs_rgb.sort_unstable_by_key(gaming);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected trait `for<'r> <for<'r> fn(&'r [u8; 12]) -> &'r [u8] {gaming} as FnOnce<(&'r [u8; 12],)>>`
             found trait `for<'r> <for<'r> fn(&'r [u8; 12]) -> &'r [u8] {gaming} as FnOnce<(&'r [u8; 12],)>>`
note: the lifetime requirement is introduced here

which is... not worse?

[nabijaczleweli]

And on rustc 1.87.0-nightly (827a0d638 2025-02-18) I get

error[E0308]: mismatched types
 --> test.rs:6:5
  |
6 |     outputs_rgb.sort_unstable_by_key(gaming);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected reference `&_`
             found reference `&_`
note: the lifetime requirement is introduced here
 --> /rustc/827a0d638dabc9a22c56f9c37a557568f86ac76c\library\core\src\slice\mod.rs:3071:25

which is worse (observant readers may note that that path does not exist).

[nabijaczleweli]

Compare error from equivalent lambda:

outputs_rgb.par_sort_unstable_by_key(|rgb| &rgb[0..3]);

error: lifetime may not live long enough
   --> src\main.rs:287:60
    |
287 |                 outputs_rgb.par_sort_unstable_by_key(|rgb| &rgb[0..3] );
    |                                                       ---- ^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
    |                                                       |  |
    |                                                       |  return type of closure is &'2 [u8]
    |                                                       has type `&'1 [u8; 12]`

(acceptable)

[dagit]

I found a fairly minimal example to reproduce this:

#![allow(missing_docs)]

struct B {}
struct C<R> {
    r: R,
}

fn make_c(b: &B) -> C<&B> {
    C { r: b }
}

fn apply<P, S>(p: P)
where
    P: Fn(&B) -> S,
{
    p(&B {});
}

fn main() {
    apply(make_c);
}

On 1.85.1 that produces this output:

$ cargo check
    Checking type-error v0.1.0 (/home/dagit/local-data/tmp/type-error)
error[E0308]: mismatched types
  --> src/main.rs:20:5
   |
20 |     apply(make_c);
   |     ^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected struct `C<&B>`
              found struct `C<&B>`
note: the lifetime requirement is introduced here
  --> src/main.rs:14:18
   |
14 |     P: Fn(&B) -> S,
   |                  ^

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