allow slice patterns to match non-fixed/generic length arrays

[Skgland]

Currently slice patterns can match fixed sized arrays and dynamic sized slices. It would be useful in code over generic length array to also be able to match on dynamic sized/non-fixed length/generic length arrays.

fn example<const N: usize>(arr: /* &[u8; N] | &mut [u8; N] | */ [u8; N]) {
    match arr {
        [] => {
            println!("None");
        }
        [_] => {
            println!("Some");
        }
        [_,_,_rest@..] => {
            println!("Many");
        }
    }
}

play-ground

Currently this results in [E7030](https://doc.rust-lang.org/stable/error_codes/E0730.html) and [E0308](https://doc.rust-lang.org/stable/error_codes/E0730.html)

```log Compiling playground v0.0.1 (/playground) error[E0308]: mismatched types --> src/main.rs:14:9 | 14 | [] => { | ^^ expected `0`, found `N` | = note: expected array `[u8; 0]` found array `[u8; N]` error[E0308]: mismatched types --> src/main.rs:17:9 | 17 | [_] => { | ^^^ expected `1`, found `N` | = note: expected array `[u8; 1]` found array `[u8; N]` error[E0730]: cannot pattern-match on an array without a fixed length --> src/main.rs:20:9 | 20 | [_,_,..] => { | ^^^^^^^^ Some errors have detailed explanations: E0308, E0730. For more information about an error, try `rustc --explain E0308`. error: could not compile `playground` (bin "playground") due to 3 previous errors ```

For arrays behind references one can mostly workaround this by turning the array reference into a slice reference.

fn example<const N: usize>(arr: /*  &mut [u8; N] | */ &[u8; N]) {
   let slice : &[_] = arr;
    match slice {
        [] => {
            println!("None");
        }
        [_] => {
            println!("Some");
        }
        [_,_,_rest@..] => {
            println!("Many");
        }
    }
}

But this leaves out the owned case and for arrays I would expect sub-array patterns to be arrays rather than slices, i.e. for input type &[u8;N] the type of _rest for the first example would be &[u8;N - 2] rather than &[u8].

For exhaustiveness checking generic arrays would be treated like slices i.e. all lengths (0..=usize::MAX) needs to be covered.

[Skgland]

@kennytm you reacted with the confused emoji, could you elaborate what confuses you?

[kennytm]

@Skgland do you have any concrete example that actually needing the type of rest to be known as [u8; N-2]?

[fmease]

I find this feature request not unreasonable. However, this is pretty much blocked on GCE (generic_const_exprs) as such a match introduces values of type [$T; N - $M].

[Skgland]

Skgland do you have any concrete example that actually needing the type of rest to be known as [u8; N-2]?

All of them would have been replaced with workarounds by now, so no I don't. Though I don't think I ever required to bind the rest on a pattern match on owned array of generic length. But I have run into the general problem of wanting to match a generic length array a couple of times, which is why I searched for an existing Issue/PR, found none and opened this.

I included it even though I don't have a specific use case because leaving out sub-array patterns seamed to leave the feature incomplete. Given that it already works like that for arrays of known size playground that appears to be the reasonable choice for the type.

I find this feature request not unreasonable. However, this is pretty much blocked on GCE (generic_const_exprs) as such a match introduces values of type [$T; N - $M].

Could an initial implementation disallow binding the sub-array i.e. .. is allowed but var@.. isn't. Would that even help? So that only sub-array bindings are blocked on GCE, while destructing the array as a whole isn't.

[kennytm]

Suppose this RFC is accepted, will the following still result in E0527/E0528? Or those will be demoted to "unreachable pattern" warnings?

fn main() {
    match [1] {
        [] => unreachable!(),
        [_] => println!("ok"),
        [_, _, ..] => unreachable!(),
    }
}

[Skgland]

I don't think there would ever be a reason to manually write those, but I could see it making writing a macro easier/possible by turning those cases from errors to warnings. Maybe a specialized deny by default lint would make sense.