Argument type error improvements

[compiler-errors]

Motivated by this interesting code snippet:

#[derive(Copy, Clone)]
struct Wrapper<T>(T);

fn foo(_: fn(i32), _: Wrapper<i32>) {}

fn f(_: u32) {}

fn main() {
    let w = Wrapper::<isize>(1isize);
    foo(f, w);
}

Which currently errors like:

error[E0308]: arguments to this function are incorrect
  --> src/main.rs:10:5
   |
10 |     foo(f, w);
   |     ^^^ -  - expected `i32`, found `isize`
   |         |
   |         expected `i32`, found `u32`
   |
   = note: expected fn pointer `fn(i32)`
                 found fn item `fn(u32) {f}`
   = note: expected struct `Wrapper<i32>`
              found struct `Wrapper<isize>`
note: function defined here
  --> src/main.rs:4:4
   |
4  | fn foo(_: fn(i32), _: Wrapper<i32>) {}
   |    ^^^ ----------  ---------------

Specifically, that double expected .. found .. which is very difficult to correlate to the types in the arguments. Also, the fact that "expected i32, found isize" and the other argument mismatch label don't even really explain what's going on here.

After this PR:

error[E0308]: arguments to this function are incorrect
  --> $DIR/two-mismatch-notes.rs:10:5
   |
LL |     foo(f, w);
   |     ^^^
   |
note: expected fn pointer, found fn item
  --> $DIR/two-mismatch-notes.rs:10:9
   |
LL |     foo(f, w);
   |         ^
   = note: expected fn pointer `fn(i32)`
                 found fn item `fn(u32) {f}`
note: expected struct `Wrapper`, found a different struct `Wrapper`
  --> $DIR/two-mismatch-notes.rs:10:12
   |
LL |     foo(f, w);
   |            ^
   = note: expected struct `Wrapper<i32>`
              found struct `Wrapper<isize>`
note: function defined here
  --> $DIR/two-mismatch-notes.rs:4:4
   |
LL | fn foo(_: fn(i32), _: Wrapper<i32>) {}
   |    ^^^ ----------  ---------------

error: aborting due to previous error

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

Yeah, it's a bit verbose, but much clearer IMO.

---

Open to discussions about how this could be further improved. Motivated by @jyn514's tweet here.

[rust-highfive]

r? @lcnr

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[lcnr]

small nit, then r=me

[jyn514]

😍😍😍

This is a great minimization, thank you! Unfortunately I'm not actually sure I find the error message more clear. It no longer clearly calls out the smallest types that don't match. In this example it's reasonably clear, but if the types of the arguments more complicated, like in my original example, or if the changes in #100473 don't land, I think it will be harder to spot why the argument types are mismatched.

That said, I absolutely love that it now points to the specific argument, rather than the entire function call.

[jyn514]

I think what would be helpful is to keep the new structure, where there's a separate note and label for each mismatched argument, but to keep the original message for secondary label:

note: expected struct `Wrapper<isize>`, found struct `Wrapper<i32>`
  --> $DIR/two-mismatch-notes.rs:10:12
   |
LL |     foo(f, w);
   |            ^
   = note: expected i32
              found isize

[compiler-errors]

@jyn514 thanks for the feedback, I can probably just rebase out the commit that changes the mismatched types being reported

[compiler-errors]

Now:

error[E0308]: arguments to this function are incorrect
  --> $DIR/two-mismatch-notes.rs:10:5
   |
LL |     foo(f, w);
   |     ^^^
   |
note: expected `i32`, found `u32`
  --> $DIR/two-mismatch-notes.rs:10:9
   |
LL |     foo(f, w);
   |         ^
   = note: expected fn pointer `fn(i32)`
                 found fn item `fn(u32) {f}`
note: expected `i32`, found `isize`
  --> $DIR/two-mismatch-notes.rs:10:12
   |
LL |     foo(f, w);
   |            ^
   = note: expected struct `Wrapper<i32>`
              found struct `Wrapper<isize>`
note: function defined here
  --> $DIR/two-mismatch-notes.rs:4:4
   |
LL | fn foo(_: fn(i32), _: Wrapper<i32>) {}
   |    ^^^ ----------  ---------------

[jyn514]

Amazing, thank you!! 😍

[compiler-errors]

@bors r=lcnr

[bors]

📌 Commit aa1a07f has been approved by lcnr

It is now in the queue for this repository.

[compiler-errors]

@bors rollup this just changes diagnostics anyways