Improve type mismatch w/ function signatures

[WaffleLapkin]

This PR makes use of note: expected/found (instead of labeling types in labels) in type mismatch with function signatures. Pros: it's easier to compare the signatures, cons: the error is a little more verbose now.

This is especially nice when

• The signatures differ in a small subset of parameters (same parameters are elided)
• The difference is in details, for example isize vs usize (there is a better chance that the types align)

Also this PR fixes the inconsistency in variable names in the edited code (expected and found).

A zulip thread from which this pr started: [link].

An example diagnostic:

this pr

nightly

error[E0631]: type mismatch in function arguments --> ./t.rs:4:12 | 4 | expect(&f); | ------ ^^ expected due to this | | | required by a bound introduced by this call ... 10 | fn f(_: isize, _: u8, _: Vec<u32>) {} | ---------------------------------- found signature defined here | = note: expected function signature `fn(usize, _, Vec<u64>) -> _` found function signature `fn(isize, _, Vec<u32>) -> _` note: required because of the requirements on the impl of `Trait` for `fn(isize, u8, Vec<u32>) {f}` --> ./t.rs:8:9 | 8 | impl<F> Trait for F where F: Fn(usize, u8, Vec<u64>) -> u8 {} | ^^^^^ ^ = note: required for the cast from `fn(isize, u8, Vec<u32>) {f}` to the object type `dyn Trait`

error[E0631]: type mismatch in function arguments --> ./t.rs:4:12 | 4 | expect(&f); | ------ ^^ expected signature of `fn(usize, u8, Vec<u64>) -> _` | | | required by a bound introduced by this call ... 10 | fn f(_: isize, _: u8, _: Vec<u32>) {} | ---------------------------------- found signature of `fn(isize, u8, Vec<u32>) -> _` | note: required because of the requirements on the impl of `Trait` for `fn(isize, u8, Vec<u32>) {f}` --> ./t.rs:8:9 | 8 | impl<F> Trait for F where F: Fn(usize, u8, Vec<u64>) -> u8 {} | ^^^^^ ^ = note: required for the cast to the object type `dyn Trait`

code

fn main() {
    fn expect(_: &dyn Trait) {}

    expect(&f);
}

trait Trait {}
impl<F> Trait for F where F: Fn(usize, u8, Vec<u64>) -> u8 {}

fn f(_: isize, _: u8, _: Vec<u32>) {}

r? @compiler-errors

[WaffleLapkin]

As usual I'm not sure about the wording, but other than that I think this is an improvement :")

[WaffleLapkin]

Not sure what's up with these notes, they don't even render in console...

[compiler-errors]

@WaffleLapkin actually I do prefer mentioning "expected closure signature", since otherwise people may think we're comparing fn ptrs. Sorry for the back and forth 😿

r=me after reverting back to 7da578b

@bors delegate+

[bors]

✌️ @WaffleLapkin can now approve this pull request

[WaffleLapkin]

@bors r=compiler-errors

[bors]

📌 Commit 7da578b has been approved by compiler-errors

It is now in the queue for this repository.