arrays as const parameters result in bad error messages

[workingjubilee]

This is a much abbreviated version of the code from my experiments with adding a shuffle API to the stdsimd repo, it probably needs further reduction but I wanted to file this before I forgot:

#![feature(const_generics, repr_simd, platform_intrinsics)]

#[repr(simd)]
pub struct SimdU32<const LANES: usize>([u32; LANES]);

impl<const LANES: usize> SimdU32<LANES> {
     pub fn shuffle2<const M: [u32; 2]>(self, other: Self) -> SimdU32<2> {
         // this is just the rustc platform intrinsic, don't worry too much about what it's doing
         unsafe { crate::intrinsics::simd_shuffle2(self, other, M) }
     }
}
// and now in another file, over in the tests... 
use core_simd::SimdU32;

#[test]
fn attempt_shuffle() {
    let a = SimdU32::from_array([2, 4, 1, 9]);
    let b = a;
    let c = a.shuffle2::<[3, 1]>(b);
}

I expected to see this happen: it works, provides a useful error, or even just ICEs.

Instead, this happened: I got a request to input a type instead!

error: unmatched angle bracket
  --> crates/core_simd/tests/permute.rs:11:25
   |
11 |     let c = a.shuffle2::<[3, 1]>(b);
   |                         ^ help: remove extra angle bracket

error: expected type, found `3`
  --> crates/core_simd/tests/permute.rs:11:27
   |
11 |     let c = a.shuffle2::<[3, 1]>(b);
   |                           ^ expected type

And no, that doesn't work either if you put it in, because instead you get:

error[E0747]: type provided when a constant was expected
 --> crates/core_simd/tests/permute.rs:9:26
  |
9 |     let c = a.shuffle2::<[u32; 2]>(b);
  |                          ^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0747`.

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (a2f8f6281 2021-01-27)
binary: rustc
commit-hash: a2f8f6281817d430e20726128b739d3c6708561c
commit-date: 2021-01-27
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

[petrochenkov]

The fix is

let c = a.shuffle2::<{ [3, 1] }>(b);

Due to parsing ambiguities generic arguments currently need to be put into braces, unless they are really simple (like 1 or A), [3, 1] is not simple enough.

[workingjubilee]

Oh, cool! That does compile! I knew about that but wasn't sure if that was the problem because I thought that was just for expressions? I guess that's technically an array expression?

That is none of the errors I was presented with, though, which makes this a diagnostics problem I suppose?

[pickfire]

error: unmatched angle bracket
 --> src/lib.rs:8:10
  |
8 |     foo::<(20 * 100 + 20 * 10 + 1)>(); // ok: const expression contains no generic parameters
  |          ^ help: remove extra angle bracket

I believe diagnostic for const generic (example taken from https://blog.rust-lang.org/2021/02/26/const-generics-mvp-beta) needs to be improved too before release, at least I thought that ( can be used instead, if one didn't know about { then they need to search it up.

[lcnr]

const generics triage

Arrays as const parameters still have bad error messages.

#![feature(adt_const_params)]

pub struct SimdU32<const LANES: usize>([u32; LANES]);

impl<const LANES: usize> SimdU32<LANES> {
    pub fn shuffle2<const M: [u32; 2]>(self) -> SimdU32<2> {
        todo!()
    }
}

fn main() {
    let a = SimdU32([2, 4, 1, 9]);
    let c = a.shuffle2::<[3, 1]>();
}

same with tuples

#![feature(adt_const_params)]

pub struct SimdU32<const LANES: usize>([u32; LANES]);

impl<const LANES: usize> SimdU32<LANES> {
    pub fn shuffle2<const M: (u32, u32)>(self) -> SimdU32<2> {
        todo!()
    }
}

fn main() {
    let a = SimdU32([2, 4, 1, 9]);
    let c = a.shuffle2::<(3, 1)>();
}