Allow `as` in match statements

[strega-nil]

As in

let ptr = match mmap(...) {
    -1 as *mut c_void => panic!(),
    p => p,
}

This is very useful for FFI and #![no_std], I don't think it takes anything away, and it shouldn't be too difficult to add.

Possibilities right now include

let ptr = match mmap(...) as usize {
    -1 => panic!(),
    p => p as *mut c_void,
}

or

let ptr = match mmap(...) {
    p if p == -1 => panic!(),
    p => p,
}

Another possible idea, one that I doubt many people would like, is to allow *const and *mut pointers to be integrals. Then you could just do the C thing of

let ptr = match mmap(...) {
    -1 => panic!(), // -1 is a *mut c_void in this situation
    p => p,
}

[lilyball]

Perhaps type ascription is a better choice here? That RFC actually says

An earlier version of this RFC covered type ascription in patterns too, that has been postponed

So there may already be existing discussion about this.

[kennytm]

@kballard Type ascription asserts that the expression has the provided type, but it doesn't apply in this case: -1 is an integer, not a raw pointer. The casting is required.

That said, I think it is better to allow int ↔︎ raw pointer casting in CTFE to allow:

const INVALID_MMAP: *mut c_void = (-1usize) as *mut c_void;
match mmap(...) {
    INVALID_MMAP => ...
    p => ...
}

[lilyball]

Oh you're right, for some reason I was thinking that integral literals could be used where pointers are expected, but no, you need the explicit as there.

That said, I think it is better to allow int ↔︎ raw pointer casting in CTFE

Huh, that seems a bit surprising that it doesn't work. Is there an issue that tracks this? If not, one should be filed.

[scottmcm]

I think that with https://rust-lang.github.io/rfcs/2920-inline-const.html this will be doable as

let ptr = match mmap(...) {
    const { -1 as *mut c_void } => panic!(),
    p => p,
}

So I'll close this and people can follow rust-lang/rust#76001.