CI on https://github.com/async-rs/async-std suddenly started failing as we run with deny warnings.

the error is

error: variable does not need to be mutable
##[error]  --> src/io/stderr.rs:92:9
   |
92 |         mut self: Pin<&mut Self>,
   |         ----^^^^
   |         |
   |         help: remove this `mut`
   |
   = note: `-D unused-mut` implied by `-D warnings`

error: variable does not need to be mutable

for the following code

impl Write for Stderr {
    fn poll_write(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        let state = &mut *self.0.lock().unwrap();

    // ... 
}

which stops compiling if the referenced mut is removed.

(Sorry in advance if this is known, but I couldn't find a matching issue)

CI run: https://github.com/async-rs/async-std/pull/822/checks?check_run_id=793134400
Source Code: https://github.com/async-rs/async-std/blob/master/src/io/stderr.rs#L90-L96

Edit: It seems that the nightly version might not be what triggered it, but rather a change in the underlying implementation of the lock that is being called above.

No warnings under

• rustc 1.45.0-nightly (1836e3b 2020-05-06)
• rustc 1.44.1 (c7087fe 2020-06-17)

This issue has been assigned to @nbdd0121 via this comment.