Argument type error improvements
Motivated by this interesting code snippet:
```rust
#[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](https://mobile.twitter.com/joshuayn514/status/1558042020601634816) here.
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