Can't use generic_const_exprs

[yyy33]

I tried this code:

#![feature(generic_const_exprs)]
#![feature(portable_simd)]

use target_features::CURRENT_TARGET;
use std::simd::Simd;

fn main() {}

struct BitVec<const BIT_LEN: usize> 
where [(); { inline_unit_len(BIT_LEN) }]:
{
    inline: Simd<u8, { inline_unit_len(BIT_LEN) }>,
}

type Lane = usize;

const fn inline_unit_len(bit_len: usize) -> usize {
    8
}

I expected to see this happen: No compilation error

Instead, this happened: compilation error

❯ cargo run --
   Compiling test44 v0.1.0 (/home/yyy/test44)
warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
    --> src/main.rs:1766:12
     |
1766 | #![feature(generic_const_exprs)]
     |            ^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
     = note: `#[warn(incomplete_features)]` on by default

warning: unused import: `target_features::CURRENT_TARGET`
    --> src/main.rs:1769:5
     |
1769 | use target_features::CURRENT_TARGET;
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(unused_imports)]` on by default

warning: unnecessary braces around const expression
    --> src/main.rs:1775:12
     |
1775 | where [(); { inline_unit_len(BIT_LEN) }]:
     |            ^^                        ^^
     |
     = note: `#[warn(unused_braces)]` on by default
help: remove these braces
     |
1775 - where [(); { inline_unit_len(BIT_LEN) }]:
1775 + where [(); inline_unit_len(BIT_LEN)]:
     |

error[E0277]: the trait bound `LaneCount<{ inline_unit_len(BIT_LEN) }>: SupportedLaneCount` is not satisfied
    --> src/main.rs:1777:13
     |
1777 |     inline: Simd<u8, { inline_unit_len(BIT_LEN) }>,
     |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SupportedLaneCount` is not implemented for `LaneCount<{ inline_unit_len(BIT_LEN) }>`
     |
     = help: the following other types implement trait `SupportedLaneCount`:
               LaneCount<1>
               LaneCount<2>
               LaneCount<4>
               LaneCount<8>
               LaneCount<16>
               LaneCount<32>
               LaneCount<64>
note: required by a bound in `Simd`
    --> /rustc/381d69953bb7c3390cec0fee200f24529cb6320f/library/core/src/../../portable-simd/crates/core_simd/src/vector.rs:103:1

warning: unused variable: `bit_len`
    --> src/main.rs:1782:26
     |
1782 | const fn inline_unit_len(bit_len: usize) -> usize {
     |                          ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_bit_len`
     |
     = note: `#[warn(unused_variables)]` on by default

For more information about this error, try `rustc --explain E0277`.
warning: `test44` (bin "test44") generated 4 warnings
error: could not compile `test44` (bin "test44") due to 1 previous error; 4 warnings emitted

Meta

rustc --version --verbose:

❯ rustc --version --verbose
rustc 1.78.0-nightly (381d69953 2024-02-24)
binary: rustc
commit-hash: 381d69953bb7c3390cec0fee200f24529cb6320f
commit-date: 2024-02-24
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

[calebzulawski]

This is mostly a limitation of generic const exprs, but issues like this is one of the reasons I would like to change length validation to a post-mono check (see #364)