useless_asref false positive on `&mut &[T]`

Summary

Clippy complains about calling as_ref() on a mut slice when it shouldn't.

Lint Name

useless_asref

Reproducer

use std::{error::Error, io::BufRead};

fn main() {
  let buf = get_buf();

  // clippy complains with: "this call to `as_ref` does nothing"
  test(&mut buf.as_ref()).unwrap();

  // this is fine
  test(&mut &*buf).unwrap();
}

fn get_buf<'a>() -> &'a mut [u8] {
  todo!()
}

fn test<R>(_input: &mut R) -> Result<(), Box<dyn Error>>
where
  R: BufRead + Send + Sync,
{
  Ok(())
}

Version

rustc 1.59.0 (9d1b2106e 2022-02-23)
binary: rustc
commit-hash: 9d1b2106e23b1abd32fce1f17267604a5102f57a
commit-date: 2022-02-23
host: x86_64-unknown-linux-gnu
release: 1.59.0
LLVM version: 13.0.0

Additional Labels

No response

Start by running the supplied Rust reproducer and locating the implementation and tests for the useless_asref lint. The fix is done when the &mut &[u8] case no longer produces a false positive while the existing valid-case behavior remains covered.