Wrong error message when Fn parameter doesn't match expected signature in call to function

[mqudsi]

A minimal complete test case:

fn takes_callback<F,T,R>(callback: F, t: T)
    where F: FnOnce(T) -> R
{
    callback(t);
}

fn callback(s1: &str, s2: &str) {
    println!("Hello {} {}", s1, s2);
}

fn main() {
    takes_callback(callback, "rust!");
}

Attempting to compile this under nightly gives the following:

mqudsi@ZBook /m/c/U/M/g/iMessageCore> rustc +nightly ./test.rs
error[E0593]: function is expected to take 1 argument, but it takes 2 arguments
  --> ./test.rs:12:5
   |
12 |     takes_callback(callback, "rust!");
   |     ^^^^^^^^^^^^^^ expected function that takes 1 argument
   |
   = note: required by `takes_callback`

error: aborting due to previous error

The error E0593 is correct, a function is expected to take 1 argument but it takes 2 arguments. But the "function" that is failing this expectation/test is not takes_callback but rather callback.

The error message "expected function that takes 1 argument" in the diagram below the error points to takes_callback which definitely was not expected to take only one argument (since it takes two arguments, FnOnce and T).

As best as I can tell, this error should be

mqudsi@ZBook /m/c/U/M/g/iMessageCore> rustc +nightly ./test.rs
error[E0593]: function is expected to take 1 argument, but it takes 2 arguments
  --> ./test.rs:12:5
   |
12 |     takes_callback(callback, "rust!");
   |                       ^^^^^ expected closure that takes 1 argument
   |
   = note: required by `takes_callback`

error: aborting due to previous error

[mqudsi]

I updated the bug description to the real bug; the issue at hand was more subtle than I had realized when I first filed

[estebank]

Proposed output:

error[E0593]: function is expected to take 1 argument, but it takes 2 arguments
  --> src/main.rs:12:5
   |
1  | fn takes_callback<F,T,R>(callback: F, t: T)
   |                          -------- expected function that takes 1 argument here...
2  |     where F: FnOnce(T) -> R
   |           ----------------- ...because of this binding
...
12 |     takes_callback(callback, "rust!");
   |     ^^^^^^^^^^^^^^ -------- this function takes 2 arguments
   |     |
   |     expected function that takes 1 argument
   |
   = note: required by `takes_callback`

[shepmaster]

Duplicate of #42855 ?

[estebank]

The current output doesn't point directly at F: FnOnce(T) -> R, but it does point at the entire method:

error[E0593]: function is expected to take 1 argument, but it takes 2 arguments
  --> src/main.rs:12:5
   |
7  | fn callback(s1: &str, s2: &str) {
   | ------------------------------- takes 2 arguments
...
12 |     takes_callback(callback, "rust!");
   |     ^^^^^^^^^^^^^^ expected function that takes 1 argument
   |
note: required by `takes_callback`
  --> src/main.rs:1:1
   |
1  | / fn takes_callback<F,T,R>(callback: F, t: T)
2  | |     where F: FnOnce(T) -> R
3  | | {
4  | |     callback(t);
5  | | }
   | |_^

Closing as duplicate as per @shepmaster's comment.