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).

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)
• (once we don't normalize:) 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

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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fmt: checked 6650 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (compiler)]: /checkout/compiler/rustc_feature/src/unstable.rs:603: TODO is used for tasks that should be done before merging a PR; If you want to leave a message in the codebase use FIXME
tidy [style (compiler)]: FAIL
tidy [features]: /checkout/compiler/rustc_feature/src/unstable.rs:604: no tracking issue for feature opaque_generic_const_args
tidy [features]: FAIL
tidy: The following checks failed: features, style (compiler)
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 yarn` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1357:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:51
  local time: Fri Jan 16 03:42:39 UTC 2026
  network time: Fri, 16 Jan 2026 03:42:40 GMT
##[error]Process completed with exit code 1.

[BoxyUwU]

in theory this means we'll suggest the feature gate on stable right? as we use this codepath also for forbidding uses of generic parameters on stable for cases that nameres cant catch

[camelid]

I changed it locally to require mgca but forgot to push apparently haha

[BoxyUwU]

also need to add the coherence test :3