regression: can't compare `X` with `_` in const contexts

[Mark-Simulacrum]

Found in crater:

• https://crater-reports.s3.amazonaws.com/beta-1.64-2/beta-2022-08-09/gh/aharisu.navi/log.txt
  • Looking briefly into the code it's doing something pretty weird (e.g., transmutes here), but at the very least this diagnostic seems unfortunate.

[compiler-errors]

searched nightlies: from nightly-2022-04-01 to nightly-2022-08-11
regressed nightly: nightly-2022-07-28
searched commit range: 4d6d601...2643b16
regressed commit: 2a22093

bisected with cargo-bisect-rustc v0.6.3

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

cargo bisect-rustc --start=2022-04-01 -- build 

This regressed in #99704 cc @fee1-dead

[compiler-errors]

Maybe "regressed" is better here, maybe it's a legitimate reason to fail. Haven't looked into it.

[fee1-dead]

I'm going to look into this later, but it seems like a case of wf checks requiring const bounds.

[fee1-dead]

Repro:

#[derive(PartialEq)]
struct S;

const FOO: fn(&S, &S) -> bool = S::eq;

[apiraino]

Just to be sure I understood: is the regression about code that should compile in the next Rust 1.64 or is that code now invalid code and the regression is just a bad diagnostic? thanks!

[fee1-dead]

@apiraino The code is valid. It should compile in the next Rust 1.64. Hope that helps.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium