Incorrect warn(unused_assignments) when assigned valued is compared with reference, but not read for value

[chrivers]

I found a spurious warn(unused_assignments) which I've condensed into the following small test case:

fn flip(bit: &mut bool)
{
    *bit = !*bit;
}
    
fn main() {
    let mut fullscreen = false;
    let mut old_fullscreen = fullscreen;
    flip(&mut fullscreen);
    if fullscreen != old_fullscreen {
        println!("flipped");
        old_fullscreen = fullscreen;
    }
}

$ rustc src/main2.rs
warning: value assigned to `old_fullscreen` is never read
  --> src/main2.rs:12:9
   |
12 |         old_fullscreen = fullscreen;
   |         ^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_assignments)] on by default
$ rustc --version
rustc 1.30.0-nightly (33b923fd4 2018-08-18)

It complains that the value of old_fullscreen is never read, but surely it is, since I'm comparing to it. In the real program, this is part of a main loop for a graphical application, and the value is checked like this in each iteration.

I checked for duplicated, and this does not seem to be related to #49171, which is about the ergonomics of a real warning, whereas this warning, if I understand correctly, should not be produced at all.

[ollie27]

The lint seems to be correct here. The line it's warning about is the assignment after the comparison which is unused. You might be better off posting the original code.

[Dylan-DPC-zz]

Closing this as it looks intended.