stability break in rust 1.54: generic `Self` types are currently not permitted in anonymous constants

[eduardosm]

I tried this code:

use std::arch::x86_64::*;

struct Test<const X: u8>;

impl<const X: u8> Test<X> {
    const Y: i32 = (X | (X << 2) | (X << 4) | (X << 6)) as i32;

    unsafe fn f(v: __m128i) -> __m128i {
        _mm_shufflelo_epi16(v, Self::Y)
    }
}

I expected to see this happen:

This code was accepted in Rust 1.53, so newer versions should also accept it.

Instead, this happened:

error: generic `Self` types are currently not permitted in anonymous constants
 --> <source>:9:32
  |
9 |         _mm_shufflelo_epi16(v, Self::Y)
  |                                ^^^^
  |
note: not a concrete type
 --> <source>:5:19
  |
5 | impl<const X: u8> Test<X> {
  |                   ^^^^^^^

error: aborting due to previous error

Meta

Tested using rustc 1.53 and 1.54 (and newer) from Compiler Explorer.

[Mark-Simulacrum]

I suspect this might actually be down to #83167, but not sure.

[eduardosm]

This example, which does not use const generics, also compiled in 1.53 and fails in newer versions:

use std::arch::x86_64::*;

trait GenU8 {
    const X: i32;
}

unsafe fn f<X: GenU8>(v: __m128i) -> __m128i {
    _mm_shufflelo_epi16(v, X::X)
}

[Mark-Simulacrum]

That example is similarly using const generics; just implicitly. From 1.54 onwards, the immediate parameter in _mm_shufflelo_epi16 is defined using const generics, with desugaring from the function form implicitly done by the compiler.

[eduardosm]

That example is similarly using const generics; just implicitly. From 1.54 onwards, the immediate parameter in _mm_shufflelo_epi16 is defined using const generics, with desugaring from the function form implicitly done by the compiler.

My point is that it breaks code that compiled much before (explicit) const generics were introduced.

[steffahn]

Bisection on the second example

searched nightlies: from nightly-2020-04-01 to nightly-2021-09-11
regressed nightly: nightly-2021-05-09
searched commits: from 770792f to 881c1ac
regressed commit: 881c1ac

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2020-04-01 --access github --regress error 

---

Which is #83278 which indeed belongs to #83167.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-high

[pnkfelix]

Visited during P-high review

@rust-lang/project-const-generics , can you all spend some time trying to brainstorm best way to resolve this?