On type mismatch involving fn call, point at fn definition

[estebank]

Given

fn foo() -> u32 {
    1
}

fn bar() {
    let x: usize = foo();
}

we now present:

error[E0308]: mismatched types
 --> src/lib.rs:6:20
  |
6 |     let x: usize = foo();
  |                    ^^^^^ expected usize, found u32
help: you can convert an `u32` to `usize` and panic if the converted value wouldn't fit
  |
6 |     let x: usize = foo().try_into().unwrap();
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^

We should ideally also point at foo and maybe tell the user to consider changing it.

[lolbinarycat]

this is especially odd considering we do point at the function when the problem is in the argument type:

fn foo(_x: u16) -> usize {
    1
}

fn bar() {
    let x: usize = foo(1_u8);
}

[oriongonza]

@compiler-errors commented:

I believe it inverts the normal "source of truth" we use (the signature) when suggesting to change a function signature that may have other call-sites, and I am skeptical that adding this information on every mismatch is worthwhile. Does it account for the other call-sites of the function that may break when this signature is changed?

Which I believe to be reasonable.