Fix ice for feature-gated `cfg` attributes applied to the crate
This PR fixes two fixes:
1. When a feature gated option of the `cfg` attribute is applied to the crate, an ICE would occur because features are not yet available at that stage. This is fixed by ignoring the feature gate at that point, the attribute will later be re-checked (this was already done) when the feature gate is available. Fixes https://github.com/rust-lang/rust/issues/143977
2. Errors and lints on the `cfg` attribute applied to the crate would be produced twice, because of the re-checking. This is fixed by not producing any errors and lints during the first run.
The added regression test checks both problems.
r? ``@jdonszelmann``
Implement unstable trait impl
This PR allows marking impls of stable trait with stable type as unstable.
## Approach
In std/core, an impl can be marked as unstable by annotating it with ``#[unstable_feature_bound(feat_name)]``. This will add a ``ClauseKind::Unstable_Feature(feat_name)`` to the list of predicates in ``predicates_of`` .
When an unstable impl's function is called, we will first iterate through all the goals in ``param_env`` to check if there is any ``ClauseKind::UnstableFeature(feat_name)`` in ``param_env``.
The existence of ``ClauseKind::Unstable_Feature(feat_name)`` in ``param_env`` means an``#[unstable_feature_bound(feat_name)]`` is present at the call site of the function, so we allow the check to succeed in this case.
If ``ClauseKind::UnstableFeature(feat_name)`` does not exist in ``param_env``, we will still allow the check to succeed for either of the cases below:
1. The feature is enabled through ``#[feature(feat_name)]`` outside of std / core.
2. We are in codegen because we may be monomorphizing a body from an upstream crate which had an unstable feature enabled that the downstream crate do not.
For the rest of the case, it will fail with ambiguity.
## Limitation
In this PR, we do not support:
1. using items that need ``#[unstable_feature_bound]`` within stable APIs
2. annotate main function with ``#[unstable_feature_bound]``
3. annotate ``#[unstable_feature_bound]`` on items other than free function and impl
## Acknowledgement
The design and mentoring are done by `@BoxyUwU`
Port several trait/coherence-related attributes the new attribute system
Part of rust-lang/rust#131229
This ports:
- `#[const_trait]`
- `#[rustc_deny_explicit_impl]`
- `#[rustc_do_not_implement_via_object]`
- `#[rustc_coinductive]`
- `#[type_const]`
- `#[rustc_specialization_trait]`
- `#[rustc_unsafe_specialization_marker]`
- `#[marker]`
- `#[fundamental]`
- `#[rustc_paren_sugar]`
- `#[rustc_allow_incoherent_impl]`
- `#[rustc_coherence_is_core]`
This also changes `#[marker]` to error on duplicates instead of warning.
cc rust-lang/rust#142838, but I don't think it matters too much, since it's unstable.
r? ``@oli-obk``
Clarify the meaning of `AttributeOrder::KeepFirst` and `AttributeOrder::KeepLast`
Clarify the meaning of `KeepLast` and `KeepFirst` for https://github.com/rust-lang/rust/issues/131229#issuecomment-2971353197
Just a minor clarification, but me and ``@JonathanBrouwer`` have confused these two a few times so I think it's warranted.
r? ``@oli-obk``
cc ``@JonathanBrouwer``
