Workaround for const_evaluatable_checked on stable?

help
1

I'm in a situation where I'd really like to use arithmetic with const generics to allow something like this:

fn foo<const N: usize>(input: [f32; N]) -> [f32; N/4] {
    ... 
}

But this requires the const_generics and const_evaluatable_checked features and I don't want to be relying on anything that triggers the incomplete_features lint. min_const_generics is in beta though, so I'm happy to use that.

Are there any workarounds on stable Rust where I can take a fixed-length array and return a new array which is 1/4 the length?

I tried adding another layer of indirection using traits and an associated const, but that didn't get me very far.

Example using associated constants 
trait MultiplyBy4 {
    const VALUE: usize;
    type Array;
}

impl<const N: usize> MultiplyBy4 for [f32; N]
{
    const VALUE: usize = N*4;
    type Array = [f32; Self::VALUE];
}

Fails to compile with

error: generic `Self` types are currently not permitted in anonymous constants
  --> src/lib.rs:16:24
   |
16 |     type Array = [f32; Self::VALUE];
   |                        ^^^^^^^^^^^
   |
note: not a concrete type
  --> src/lib.rs:13:38
   |
13 | impl<const N: usize> MultiplyBy4 for [f32; N]
   |                                      ^^^^^^^^

Variations on this seem to fail with similar errors.

2

Would a signature like

fn foo<const N: usize>(input: [[f32; 4]; N]) -> [f32; N]

(or with [[f32; N]; 4] instead of [[f32; 4]; N] if that makes more sense) do the job?

(That won't work on stable because even min_const_generics has not hit stable yet. But if you're willing to use beta, or nightly without feature gates, it should be okay.)

3

Hmm... I didn't think of using a multi-dimensional array like that

The actual use case is in a "pipeline" where each stage will do some processing on the data, with each step looking something like this:

trait Foo<Input> {
    type Output;

    fn bar(&mut self, input: Input) -> Self::Output;
}

The previous step returns a [f32; 32] that this step needs to "condense" into a [f32; 4], but I wanted to make this operation work with more than just [f32; 32] because it'll pop up elsewhere.

I guess I could make the previous step return a multidimensional array instead. We're already using FFI to populate the array, so an extra transmute() won't cause much harm.

4

You can't transmute from or into an array of generic length unfortunately, see

I don't know what a good workaround for this would be in your case, maybe some additional raw pointer shenanigans.

5

FWIW, another option is not to use const_generics yet, and keep using the poor man's equivalent that most crypto libs use:

fn main ()
{
    use ::generic_array::{arr, typenum::consts::*};

    let _: [f32; 1] = lib::foo::<U4>([1., 2., 3., 4.].into()).into();

    let _: [f32; 1] = lib::foo(arr![f32; 1., 2., 3., 4.]).into();
}

mod lib {
    pub
    fn foo<N: ConstUsize> (input: Array_f32<N>)
      -> Array_f32<op!(N / U4)>
    where
        // [(); N / 4]:,
        N : ::core::ops::Div<U4>, op!(N / U4) : ConstUsize,
    {
        let fst = input[0];
        todo!()
    }

    …
}