Possible degradation in const generics

[drskalman]

The following code compiles with rustc 1.56.0-nightly (4e28279 2021-07-31) but not rustc 1.56.0-nightly (5d68044 2021-08-30) or 1.57.0

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]
//#![feature(generic_const_exprs)]

pub trait AnotherTrait{
    const ARRAY_SIZE: usize;
}
pub trait Shard<T: AnotherTrait>:
    AsMut<[[u8; T::ARRAY_SIZE]]>
where
    [(); T::ARRAY_SIZE]: Sized
{
}

fn main() {
    println!("Hello, world!");
}

As you see it compiles with the earlier compiler but not with the new one:

Meta

Build outputs

[user@machine as_mut_bug]$ rustup default nightly-2021-08-01-x86_64-unknown-linux-gnu info: using existing install for 'nightly-2021-08-01-x86_64-unknown-linux-gnu' info: default toolchain set to 'nightly-2021-08-01-x86_64-unknown-linux-gnu'

nightly-2021-08-01-x86_64-unknown-linux-gnu unchanged - rustc 1.56.0-nightly (4e28279 2021-07-31)

[user@machine as_mut_bug]$ cargo build
Compiling as_mut_bug v0.1.0 (/home/user/lazyhole/code/playground/rust-playground/as_mut_bug)
warning: the feature const_generics is incomplete and may not be safe to use and/or cause compiler crashes
--> src/main.rs:1:12
|
1 | #![feature(const_generics)]
| ^^^^^^^^^^^^^^
|
= note: #[warn(incomplete_features)] on by default
= note: see issue #44580 #44580 for more information

warning: the feature const_evaluatable_checked is incomplete and may not be safe to use and/or cause compiler crashes
--> src/main.rs:2:12
|
2 | #![feature(const_evaluatable_checked)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #76560 #76560 for more information

warning: as_mut_bug (bin "as_mut_bug") generated 2 warnings
Finished dev [unoptimized + debuginfo] target(s) in 0.90s
[user@machine as_mut_bug]$ rustup default nightly-2021-08-31-x86_64-unknown-linux-gnu
info: using existing install for 'nightly-2021-08-31-x86_64-unknown-linux-gnu'
info: default toolchain set to 'nightly-2021-08-31-x86_64-unknown-linux-gnu'

nightly-2021-08-31-x86_64-unknown-linux-gnu unchanged - rustc 1.56.0-nightly (5d68044 2021-08-30)

[user@machine as_mut_bug]$ cargo build
Compiling as_mut_bug v0.1.0 (/home/user/lazyhole/code/playground/rust-playground/as_mut_bug)
warning: the feature const_generics is incomplete and may not be safe to use and/or cause compiler crashes
--> src/main.rs:1:12
|
1 | #![feature(const_generics)]
| ^^^^^^^^^^^^^^
|
= note: #[warn(incomplete_features)] on by default
= note: see issue #44580 #44580 for more information

warning: the feature const_evaluatable_checked is incomplete and may not be safe to use and/or cause compiler crashes
--> src/main.rs:2:12
|
2 | #![feature(const_evaluatable_checked)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #76560 #76560 for more information

error[E0283]: type annotations needed
--> src/main.rs:9:5
|
9 | AsMut<[[u8; T::ARRAY_SIZE]]>
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter Self
|
= note: cannot satisfy Self: AsMut<[[u8; _]]>
note: required by a bound in AsMut

For more information about this error, try rustc --explain E0283.
warning: as_mut_bug (bin "as_mut_bug") generated 2 warnings
error: could not compile as_mut_bug due to previous error; 2 warnings emitted
[user@machine as_mut_bug]$

[BoxyUwU]

more minimal:

#![feature(generic_const_exprs)]
#![allow(unused_braces, incomplete_features)]

pub trait Foo<const N: usize> {}
pub trait Bar: Foo<{ 1 }> { }

[b-naber]

@rustbot claim