Wrong error message with closures on nightly

[mcarton]

The following example

fn main() {
    let a = vec![(1, 2)];
    
    let b: Vec<_> = a.iter().map(|x: (u32, u32)| 45).collect(); 
}

gives the following error on nightly:

error[E0593]: closure takes 2 arguments but 1 argument is required
 --> src/main.rs:4:30
  |
4 |     let b: Vec<_> = a.iter().map(|x: (u32, u32)| 45).collect(); 
  |                              ^^^ ------------------ takes 2 arguments
  |                              |
  |                              expected closure that takes 1 argument

which is wrong (map is expecting a function with one argument, which is what this closure is) and the following correct message on stable/beta:

error[E0281]: type mismatch: the type `[closure@src/main.rs:4:34: 4:52]` implements the trait `std::ops::FnMut<((u32, u32),)>`, but the trait `std::ops::FnMut<(&({integer}, {integer}),)>` is required (expected reference, found tuple)
 --> src/main.rs:4:30
  |
4 |     let b: Vec<_> = a.iter().map(|x: (u32, u32)| 45).collect(); 
  |                              ^^^

[rbalicki2]

How does one fix this? I have run into this problem as well, and have tried all 8 variants of |&x: &(&u32, &u32)|, toggling each of the 3 placements of ampersands.

It looked like, at some point, that .map(|x: &(i32, i32)| 42) fixed it, but I could not get that to work for group_by of itertools. Perhaps these are separate issues.

[estebank]

Introduced in #41488.

[Emilgardis]

I would like to tackle this if no one already has begun doing it.

[estebank]

@Emilgardis go ahead! You should add a new test for this case, and you'll need to add an extra check somewhere in report_selection_error. Don't hesitate to ask for help either here, in IRC or https://internals.rust-lang.org.

[Emilgardis]

@estebank Do I add a type_mismatch .rs test or add to an existing test file?

[Emilgardis]

pr on #42270