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a0215d8e46
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.
74 lines
2.1 KiB
Rust
74 lines
2.1 KiB
Rust
//@ revisions: aarch64 x86-64
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//@ [aarch64] compile-flags: -Ctarget-feature=+neon,+fp16,+fhm --target=aarch64-unknown-linux-gnu
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//@ [aarch64] needs-llvm-components: aarch64
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//@ [x86-64] compile-flags: -Ctarget-feature=+sse4.2,+rdrand --target=x86_64-unknown-linux-gnu
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//@ [x86-64] needs-llvm-components: x86
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//@ build-pass
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#![no_core]
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#![crate_type = "rlib"]
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#![feature(intrinsics, rustc_attrs, no_core, lang_items, staged_api)]
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#![stable(feature = "test", since = "1.0.0")]
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// Supporting minimal rust core code
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#[lang = "sized"]
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trait Sized {}
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#[lang = "copy"]
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trait Copy {}
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impl Copy for bool {}
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extern "rust-intrinsic" {
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#[stable(feature = "test", since = "1.0.0")]
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#[rustc_const_stable(feature = "test", since = "1.0.0")]
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fn unreachable() -> !;
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}
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#[rustc_builtin_macro]
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macro_rules! cfg {
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($($cfg:tt)*) => {};
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}
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// Test code
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const fn do_or_die(cond: bool) {
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if cond {
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} else {
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unsafe { unreachable() }
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}
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}
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macro_rules! assert {
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($x:expr $(,)?) => {
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const _: () = do_or_die($x);
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};
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}
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#[cfg(target_arch = "aarch64")]
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fn check_aarch64() {
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// These checks that the rustc feature name is used, not the LLVM feature.
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assert!(cfg!(target_feature = "neon"));
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// #[expect(unexpected_cfgs)] except that 32-bit arm actually use fp-armv8
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{ assert!(cfg!(not(target_feature = "fp-armv8"))); }
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assert!(cfg!(target_feature = "fhm"));
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#[expect(unexpected_cfgs)]
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{ assert!(cfg!(not(target_feature = "fp16fml"))); }
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assert!(cfg!(target_feature = "fp16"));
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#[expect(unexpected_cfgs)]
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{ assert!(cfg!(not(target_feature = "fullfp16"))); }
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}
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#[cfg(target_arch = "x86_64")]
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fn check_x86_64() {
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// This checks that the rustc feature name is used, not the LLVM feature.
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assert!(cfg!(target_feature = "rdrand"));
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#[expect(unexpected_cfgs)]
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{ assert!(cfg!(not(target_feature = "rdrnd"))); }
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// Likewise: We enable LLVM's crc32 feature with SSE4.2, but Rust says it's just SSE4.2
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assert!(cfg!(target_feature = "sse4.2"));
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#[expect(unexpected_cfgs)]
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{ assert!(cfg!(not(target_feature = "crc32"))); }
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}
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