Missing AsMut<[T;N]> for [T;N]

[alexkazik]

I tried this code:

fn foo<T: AsMut<[u8; 12]>>(mut t: T) {
    t.as_mut()[0] = 0;
}
fn bar(x: &mut [u8; 12]) {
    foo(x);
}

I expected it to compile.

Instead, this happened:

error[E0277]: the trait bound `[u8; 12]: AsMut<[u8; 12]>` is not satisfied
 --> crate/src/it.rs:5:9
  |
5 |     foo(x);
  |     --- ^ the trait `AsMut<[u8; 12]>` is not implemented for `[u8; 12]`, which is required by `&mut [u8; 12]: AsMut<[u8; 12]>`
  |     |
  |     required by a bound introduced by this call
  |
  = help: the following other types implement trait `AsMut<T>`:
            [T; N]
            [T]
  = note: required for `&mut [u8; 12]` to implement `AsMut<[u8; 12]>`
note: required by a bound in `foo`
 --> crate/src/it.rs:1:11
  |
1 | fn foo<T: AsMut<[u8; 12]>>(mut t: T) {
  |           ^^^^^^^^^^^^^^^ required by this bound in `foo`

[GrigorenkoPV]

I expected there to be a blanket impl, but

Ideally, AsMut would be reflexive, i.e. there would be an impl<T: ?Sized> AsMut<T> for T with as_mut simply returning its argument unchanged. Such a blanket implementation is currently not provided due to technical restrictions of Rust’s type system (it would be overlapping with another existing blanket implementation for &mut T where T: AsMut<U> which allows AsMut to auto-dereference, see “Generic Implementations” above).

A trivial implementation of AsMut<T> for T must be added explicitly for a particular type T where needed or desired. Note, however, that not all types from std contain such an implementation, and those cannot be added by external code due to orphan rules.

https://doc.rust-lang.org/stable/core/convert/trait.AsMut.html#reflexivity

Sad.

Anyways, you can try to submit a PR if you want, or, alternatively:

Note, however, that APIs don’t need to be generic. In many cases taking a &mut [u8] or &mut Vec, for example, is the better choice (callers need to pass the correct type then).

But I think I'll try to submit a PR anyways, unless you want to do it yourself.

[alexkazik]

Since I haven't done a PR (or even compiled the compiler) I'd appreciate you submitting the PR - but I can give it a try.

I use a generic slice since it's used as a buffer of exactly that size.
And I tried AsMut as as abstraction so that the pure reference (on no alloc systems) or Box::<[u8; N]>::try_from(vec![0u8; N]).unwrap() for alloc/testing can be used.

[GrigorenkoPV]

Oh wow, this is worse than I thought. Many places (even in compiler itself) use .as_ref() as a substitute for .as_slice(), because array.as_ref() currently gets desugared to effectively { let tmp: &[_] = array.deref(); tmp.as_ref() }. This is more or less the same situation as with https://doc.rust-lang.org/edition-guide/rust-2021/IntoIterator-for-arrays.html, so I think I can try to push this through in time for the 2024 edition.

@rustbot claim

[Enselic]

Triage: Are you still working on this @GrigorenkoPV ?

[GrigorenkoPV]

Triage: Are you still working on this @GrigorenkoPV ?

Not currently. But planning to, eventually. I can release the assignment, if you want me to.

[Enselic]

No worries, feel free to keep the assignment.

[GrigorenkoPV]

Status update: I've created an ACP: rust-lang/libs-team#605