Invalid suggestion for mutability correction in error message

[Vtec234]

The compiler makes an invalid suggestion for how to correct the immutability of a reference in a match block. To reproduce, try to compile the following code:

use std::fs::File;
use std::io::Write;

fn main() {
    let mut op = Some(File::open("").unwrap());
    match op {
        Some(ref mut f) => { write!(f, "!").unwrap();  },
        None => {},
    }
}

On stable and beta, the compiler will print:

<anon>:7:33: 7:34 error: cannot borrow immutable local variable `f` as mutable
<anon>:7         Some(ref f) => { write!(f, "!").unwrap();  },
                                     ^
<anon>:7:26: 7:40 note: in this expansion of write! (defined in <std macros>)
<anon>:7:14: 7:19 help: to make the local variable mutable, use `mut` as shown:
<anon>:          Some(mut ref f) => { write!(f, "!").unwrap();  },

On nightly, it will print:

error: cannot borrow immutable local variable `f` as mutable
 --> <anon>:7:33
7 |>         Some(ref f) => { write!(f, "!").unwrap();  },
  |>              -----              ^ cannot borrow mutably
  |>              |
  |>              use `mut ref f` here to make mutable

Both of these suggestions are erroneous, as mut ref f is invalid syntax, and should be ref mut f.

[durka]

Actually if you write ref mut v it doesn't work either, as then v is an immutable &mut i32. So I guess no note should be printed, and possibly mut ref [mut] $ident should be allowed in patterns.

[Vtec234]

You are right, but it was just my mistake. I updated the original code to the simplest thing I could think of which invokes the right error message and is fixable by ref mut.

[sophiajt]

In this particular case, but is that true in the general case? The heuristics for when to suggest something are necessarily going to have corner cases (which hopefully will get smoothed over time)