Implement MVP for opaque generic const arguments

[camelid]

This is meant to be the interim successor to generic const expressions.
Essentially, const item RHS's will be allowed to do arbitrary const
operations using generics. The limitation is that these const items will
be treated opaquely, like ADTs in nominal typing, such that uses of them
will only be equal if the same const item is referenced. In other words,
two const items with the exact same RHS will not be considered equal.

I also added some logic to check feature gates that depend on others
being enabled (like oGCA depending on mGCA).

Coherence

During coherence, OGCA consts should be normalized ambiguously because
they are opaque but eventually resolved to a real value. We don't want
two OGCAs that have the same value to be treated as distinct for
coherence purposes. (Just like opaque types.)

This actually doesn't work yet because there are pre-existing
fundamental issues with equate relations involving consts that need to
be normalized. The problem is that we normalize only one layer of the
const item and don't actually process the resulting anon const. Normally
the created inference variable should be handled, which in this case
would cause us to hit the anon const, but that's not happening.
Specifically, visit_const on Generalizer should be updated to be
similar to visit_ty.

r? @BoxyUwU

[camelid]

TODO:

• only use AnonConstKind::OGCA for const item RHS's
• add tests
• (less urgent:) don't normalize opaque const items (those with OGCA RHS)
• handle coherence correctly (ambiguous if not definitionally equal)

[BoxyUwU]

When we avoid normalizing type consts to opaque anon consts we need to make sure that the normalization is ambiguous in coherence instead of NoSolution:

#![feature(min_generic_const_args, opaque_generic_const_args)]

#[type_const]
const FOO<const N: usize>: usize = const { N + 1 };

#[type_const]
const BAR<const N: usize>: usize = const { N + 1 };

trait Trait {}

impl Trait for [(); FOO::<1>] {}
impl Trait for [(); BAR::<1>] {}

ie we don't want this code to pass due to the two opaque anon consts being unequal.

[camelid]

Ooh good point 👍

[rustbot]

HIR ty lowering was modified

cc @fmease

[BoxyUwU]

also need to add the coherence test :3

[BoxyUwU]

@rustbot author

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[camelid]

@rustbot ready

[BoxyUwU]

lgtm but I think you forgot to bless a test and had a git whoopsie with a fluent file

View changes since this review

[BoxyUwU]

(blessed and removed the fluent file for you, see the diff here: https://triagebot.infra.rust-lang.org/gh-changes-since/rust-lang/rust/150823/c7f5f3e0d5defe632d44743cbaed56272e2b67f0..5f63932e140feac21c5764ea3c72a5211407a44f)

[BoxyUwU]

@bors r+

[rust-bors]

📌 Commit 9a30ec8 has been approved by BoxyUwU

It is now in the queue for this repository.