From a0215d8e46aab41219dea0bb1cbaaf97dafe2f89 Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sun, 6 Oct 2024 19:59:19 +0200 Subject: 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. --- library/std/src/thread/local.rs | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'library/std/src/thread') diff --git a/library/std/src/thread/local.rs b/library/std/src/thread/local.rs index 88bf186700f..9edb3fa4193 100644 --- a/library/std/src/thread/local.rs +++ b/library/std/src/thread/local.rs @@ -237,7 +237,7 @@ impl LocalKey { reason = "recently added to create a key", issue = "none" )] - #[rustc_const_unstable(feature = "thread_local_internals", issue = "none")] + #[cfg_attr(bootstrap, rustc_const_unstable(feature = "thread_local_internals", issue = "none"))] pub const unsafe fn new(inner: fn(Option<&mut Option>) -> *const T) -> LocalKey { LocalKey { inner } } -- cgit 1.4.1-3-g733a5