Summary

Change array/slice patterns in the following ways:

  • Make them only match on arrays ([T; n] and [T]), not slices;
  • Make subslice matching yield a value of type [T; n] or [T], not &[T] or &mut [T];
  • Allow multiple mutable references to be made to different parts of the same array or slice in array patterns (resolving rust-lang/rust issue #8636).

Motivation

Before DST (and after the removal of ~[T]), there were only two types based on [T]: &[T] and &mut [T]. With DST, we can have many more types based on [T], Box<[T]> in particular, but theoretically any pointer type around a [T] could be used. However, array patterns still match on &[T], &mut [T], and [T; n] only, meaning that to match on a Box<[T]>, one must first convert it to a slice, which disallows moves. This may prove to significantly limit the amount of useful code that can be written using array patterns.

Another problem with today’s array patterns is in subslice matching, which specifies that the rest of a slice not matched on already in the pattern should be put into a variable:


# #![allow(unused_variables)]
#fn main() {
let foo = [1i, 2, 3];
match foo {
    [head, tail..] => {
        assert_eq!(head, 1);
        assert_eq!(tail, &[2, 3]);
    },
    _ => {},
}
#}

This makes sense, but still has a few problems. In particular, tail is a &[int], even though the compiler can always assert that it will have a length of 2, so there is no way to treat it like a fixed-length array. Also, all other bindings in array patterns are by-value, whereas bindings using subslice matching are by-reference (even though they don’t use ref). This can create confusing errors because of the fact that the .. syntax is the only way of taking a reference to something within a pattern without using the ref keyword.

Finally, the compiler currently complains when one tries to take multiple mutable references to different values within the same array in a slice pattern:


# #![allow(unused_variables)]
#fn main() {
let foo: &mut [int] = &mut [1, 2, 3];
match foo {
    [ref mut a, ref mut b] => ...,
    ...
}
#}

This fails to compile, because the compiler thinks that this would allow multiple mutable borrows to the same value (which is not the case).

Detailed design

  • Make array patterns match only on arrays ([T; n] and [T]). For example, the following code:

    
    # #![allow(unused_variables)]
    #fn main() {
    let foo: &[u8] = &[1, 2, 3];
    match foo {
        [a, b, c] => ...,
        ...
    }
    #}

    Would have to be changed to this:

    
    # #![allow(unused_variables)]
    #fn main() {
    let foo: &[u8] = &[1, 2, 3];
    match foo {
        &[a, b, c] => ...,
        ...
    }
    #}

    This change makes slice patterns mirror slice expressions much more closely.

  • Make subslice matching in array patterns yield a value of type [T; n] (if the array is of fixed size) or [T] (if not). This means changing most code that looks like this:

    
    # #![allow(unused_variables)]
    #fn main() {
    let foo: &[u8] = &[1, 2, 3];
    match foo {
        [a, b, c..] => ...,
        ...
    }
    #}

    To this:

    
    # #![allow(unused_variables)]
    #fn main() {
    let foo: &[u8] = &[1, 2, 3];
    match foo {
        &[a, b, ref c..] => ...,
        ...
    }
    #}

    It should be noted that if a fixed-size array is matched on using subslice matching, and ref is used, the type of the binding will be &[T; n], not &[T].

  • Improve the compiler’s analysis of multiple mutable references to the same value within array patterns. This would be done by allowing multiple mutable references to different elements of the same array (including bindings from subslice matching):

    
    # #![allow(unused_variables)]
    #fn main() {
    let foo: &mut [u8] = &mut [1, 2, 3, 4];
    match foo {
        &[ref mut a, ref mut b, ref c, ref mut d..] => ...,
        ...
    }
    #}

Drawbacks

  • This will break a non-negligible amount of code, requiring people to add &s and refs to their code.

  • The modifications to subslice matching will require ref or ref mut to be used in almost all cases. This could be seen as unnecessary.

Alternatives

  • Do a subset of this proposal; for example, the modifications to subslice matching in patterns could be removed.

Unresolved questions

  • What are the precise implications to the borrow checker of the change to multiple mutable borrows in the same array pattern? Since it is a backwards-compatible change, it can be implemented after 1.0 if it turns out to be difficult to implement.