Slice patterns don't work well with ref mut

[clarfonthey]

Minimal example: https://is.gd/40m1am

#![feature(slice_patterns)]
fn main() {
    let mut buf = [0, 1];
    let [ref mut a, ref mut b] = buf;
}

This will complain about multiple mutable borrows, even though it should be valid.

[Stebalien]

Noted in #8636 (comment) (but may deserve a separate bug report?).

(FYI, slice patterns are unstable for a reason; they're pretty buggy at the moment).

[mikhail-m1]

should be closed after MIR borrowck will be by default