Poor formatting of slice_patterns

[dtolnay]

#![feature(slice_patterns)]

fn main() {
    match b"123" {
        [0, ..] => {}
        _ => {}
    }
}

As of rustfmt 0.99.1-nightly (da17b68 2018-08-04), formatting produces the following diff which I believe is less readable than the original:

-         [0, ..] => {}
+         [0, _..] => {}

[faern]

Why is _.. even accepted syntax? It is not allowed in normal pattern matching:

let (a, _..) = (1, 2 ,3);

does not even compile.