An array without a fixed length was pattern-matched.

Example of erroneous code:

```compile_fail,E0730
#![feature(const_generics)]

fn is_123<const N: usize>(x: [u32; N]) -> bool {
    match x {
        [1, 2, 3] => true, // error: cannot pattern-match on an
                           //        array without a fixed length
        _ => false
    }
}
```

Ensure that the pattern is consistent with the size of the matched
array. Additional elements can be matched with `..`:

```
#![feature(slice_patterns)]

let r = &[1, 2, 3, 4];
match r {
    &[a, b, ..] => { // ok!
        println!("a={}, b={}", a, b);
    }
}
```