Re-do recursive const stability checks
Fundamentally, we have *three* disjoint categories of functions: 1. const-stable functions 2. private/unstable functions that are meant to be callable from const-stable functions 3. functions that can make use of unstable const features This PR implements the following system: - `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions. - `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category. - `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls. Also, several holes in recursive const stability checking are being closed. There's still one potential hole that is hard to avoid, which is when MIR building automatically inserts calls to a particular function in stable functions -- which happens in the panic machinery. Those need to *not* be `rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be sure they follow recursive const stability. But that's a fairly rare and special case so IMO it's fine. The net effect of this is that a `#[unstable]` or unmarked function can be constified simply by marking it as `const fn`, and it will then be const-callable from stable `const fn` and subject to recursive const stability requirements. If it is publicly reachable (which implies it cannot be unmarked), it will be const-unstable under the same feature gate. Only if the function ever becomes `#[stable]` does it need a `#[rustc_const_unstable]` or `#[rustc_const_stable]` marker to decide if this should also imply const-stability. Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to use unstable const lang features (including intrinsics), or (b) `#[stable]` functions that are not yet intended to be const-stable. Adding `#[rustc_const_stable]` is only needed for functions that are actually meant to be directly callable from stable const code. `#[rustc_const_stable_indirect]` is used to mark intrinsics as const-callable and for `#[rustc_const_unstable]` functions that are actually called from other, exposed-on-stable `const fn`. No other attributes are required.
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@ -617,11 +617,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
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DuplicatesOk, EncodeCrossCrate::Yes,
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"allow_internal_unstable side-steps feature gating and stability checks",
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),
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gated!(
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rustc_allow_const_fn_unstable, Normal,
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template!(Word, List: "feat1, feat2, ..."), DuplicatesOk, EncodeCrossCrate::No,
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"rustc_allow_const_fn_unstable side-steps feature gating and stability checks"
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),
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gated!(
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allow_internal_unsafe, Normal, template!(Word), WarnFollowing,
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EncodeCrossCrate::No, "allow_internal_unsafe side-steps the unsafe_code lint",
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@ -838,6 +833,15 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
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rustc_const_panic_str, Normal, template!(Word), WarnFollowing,
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EncodeCrossCrate::Yes, INTERNAL_UNSTABLE
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),
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rustc_attr!(
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rustc_const_stable_indirect, Normal,
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template!(Word), WarnFollowing, EncodeCrossCrate::No, IMPL_DETAIL,
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),
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gated!(
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rustc_allow_const_fn_unstable, Normal,
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template!(Word, List: "feat1, feat2, ..."), DuplicatesOk, EncodeCrossCrate::No,
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"rustc_allow_const_fn_unstable side-steps feature gating and stability checks"
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),
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// ==========================================================================
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// Internal attributes, Layout related:
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