Autoderef chooses deref() when deref_mut() is needed for ref mut pattern

[kmcallister]

use std::cell::RefCell;

struct S {
    node: E,
}

enum E {
    Foo(uint),
    Bar,
}

fn main() {
    let x = RefCell::new(S { node: Foo(0) });

    let mut b = x.borrow_mut();
    match b.node {
        Foo(ref mut n) => *n += 1,
        _ => (),
    }
}

$ rustc -v
rustc 0.11.0 (49bc17bfdd7143909aede81652d7d624cecd8a70 2014-07-07 17:16:34 +0000)
$ rustc foo.rs
foo.rs:17:13: 17:22 error: cannot borrow immutable anonymous field as mutable
foo.rs:17         Foo(ref mut n) => *n += 1,
                      ^~~~~~~~~

Changing to match b.deref_mut().node fixes the error.

[huonw]

This is possibly related to #12825.

[eddyb]

Ah, I completely missed the fact that expressions matched over are lvalues that could require mutability (so they should always prefer DerefMut under the current scheme).

[japaric]

Triage: Still occurs with:

rustc 0.13.0-dev (97a57ec90 2014-11-07 11:31:25 +0000)

#12825 has been closed as fixed.

[steveklabnik]

Triage: still happens

[SimonSapin]

Still an issue in rustc 1.0.0-nightly (083b8a4 2015-04-05) (built 2015-04-05)

[bluss]

For those who wonder: The fix will be in Rust 1.2.