fizyk20/generic-array

More generic From<[T; N]> and Into<[T; N]> implementations

Closed this issue · 2 comments

Qqwy commented

Currently, these implementations are hard-coded for certain array lengths.

This was necessary on old Rust versions in which

  • const generics were not possible, and
  • array traits were restricted to max lengths of 32.
    But since v1.47 this is no longer the case.

By enabling the consts feature on typenum, we get access to the U<const N: usize> type. And this is what lets us write a single implementation for From and for Into that will work for all lengths!

Qqwy commented

Currently I am working around this in my own code by transmuting directly (as the current hard-coded implementations of From/Into do as well) which works, but is hacky as it makes my code deeply dependent on the internals of this crate.

novacrazy commented

This is now implemented in the 1.0 branch. However, to ease use with const-generics in user libraries, I'm considering something like:

pub unsafe trait IntoArrayLength {
    type ArrayLength: ArrayLength;
}

unsafe impl<const N: usize> IntoArrayLength for Const<N>
where
    Const<N>: ToUInt,
    U<N>: ArrayLength,
{
    type ArrayLength = U<N>;
}

pub type ConstArrayLength<const N: usize> = <Const<N> as IntoArrayLength>::ArrayLength;

impl<T, const N: usize> From<[T; N]> for GenericArray<T, ConstArrayLength<N>>
where
    Const<N>: IntoArrayLength,

Cuts down on one where bound.

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