Borrow checker gives E0499 in loops with mutable borrows, but it is misleading

[TheButlah]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6a4dbffaa1c7bfe6e167b40eacaca921
The current output is:

error[E0499]: cannot borrow `decoder` as mutable more than once at a time
  --> src/main.rs:15:29
   |
15 |     while let Some(frame) = decoder.read_next_frame().unwrap() {
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^ `decoder` was mutably borrowed here in the previous iteration of the loop

It looks like the error is pointing to the wrong spot in the code and is quite misleading.
Ideally the output should mention that decoder is still mutably borrowed because it holds a reference to frame between loops. Or something, im not actually sure :P

I have confirmed that this occurs in both the latest stable (1.62.1) and nightly (1.64.0-nightly 4d6d601c8 2022-07-26)

[yanchen4791]

@rustbot claim

[yanchen4791]

With the proposed fix #102080, the output will be like this:

error[E0499]: cannot borrow `decoder` as mutable more than once at a time
  --> main.rs:15:29
   |
15 |     while let Some(frame) = decoder.read_next_frame().unwrap() {
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^ `decoder` was mutably borrowed here in the previous iteration of the loop
16 |         frames.push(frame);  // this line is the culprit
   |         ------------------ first borrow later used here

error: aborting due to previous error