borrow_deref_ref breaks code

[matthiaskrgr]

Summary

.

Lint Name

borrow_deref_ref

Reproducer

I tried this code:

fn main() {
    let x: &'static str = "x";

    {
        let y = "y".to_string();
        let ref mut x = &*x;
        *x = &*y;
    }

    assert_eq!(x, "x");
}

I saw this happen:
cargo clippy --fix -- -Aclippy::all -Wclippy::borrow_deref_ref

after fixes were automatically applied the compiler reported errors within these files:

  * src/main.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
 --> src/main.rs:6:13
  |
2 |     let x: &'static str = "x";
  |         - help: consider changing this to be mutable: `mut x`
...
6 |         let ref mut x = x;
  |             ^^^^^^^^^ cannot borrow as mutable

error[E0597]: `y` does not live long enough
 --> src/main.rs:7:16
  |
2 |     let x: &'static str = "x";
  |            ------------ type annotation requires that `y` is borrowed for `'static`
...
7 |         *x = &*y;
  |                ^ borrowed value does not live long enough
8 |     }
  |     - `y` dropped here while still borrowed

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0596, E0597.
For more information about an error, try `rustc --explain E0596`.
Original diagnostics will follow.

warning: deref on an immutable reference
 --> src/main.rs:6:25
  |
6 |         let ref mut x = &*x;
  |                         ^^^ help: if you would like to reborrow, try removing `&*`: `x`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_deref_ref
  = note: requested on the command line with `-W clippy::borrow-deref-ref`

warning: `clpy` (bin "clpy" test) generated 1 warning (run `cargo fix --bin "clpy" --tests` to apply 1 suggestion)
warning: `clpy` (bin "clpy") generated 1 warning (1 duplicate)
    Finished dev [unoptimized + debuginfo] target(s) in 0.61s

I expected to see this happen:

Version

rustc 1.67.0-nightly (c5d82ed7a 2022-11-19)
binary: rustc
commit-hash: c5d82ed7a4ad94a538bb87e5016e7d5ce0bd434b
commit-date: 2022-11-19
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4

Additional Labels

No response

[roy-work]

I think I've hit another instance of this? We've the following type:

/// Abstracts over `stdio` and files.
#[derive(Debug)]
pub enum File {
  Stdio,
  File(fs::File),
}

Someone implemented the following on this type:

impl io::Read for &'_ File {
  fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
    match self {
      File::Stdio => io::stdin().read(buf),
      File::File(file) => (&*file).read(buf),
    }
  }
}

This triggers the clippy lint:

warning: deref on an immutable reference
    |
117 |       File::File(file) => (&*file).read(buf),
    |                           ^^^^^^^^ help: if you would like to reborrow, try removing `&*`: `file`

If you attempt to take Clippy's advice, however, and change the expression to file.read(buf), you get:

error[E0596]: cannot borrow `file` as mutable, as it is not declared as mutable
    |
117 |       File::File(file) => file.read(buf),
    |                           ^^^^^^^^^^^^^^ cannot borrow as mutable

I'm not sure how else to write this code except as it was originally written.