Stabilize const_black_box

This has been unstably const since [1], but a tracking issue was never
created. Per discussion on Zulip [2], there should not be any blockers
to making this const-stable. The function does not provide any
functionality at compile time but does allow code reuse between const-
and non-const functions, so stabilize it here.

[1]: rust-lang#92226
[2]: https://rust-lang.zulipchat.com/#narrow/channel/146212-t-compiler.2Fconst-eval/topic/const_black_box

Author: tgross35

core/src/hint.rs

@@ -468,9 +468,11 @@ pub fn spin_loop() {
 /// // No assumptions can be made about either operand, so the multiplication is not optimized out.
 /// let y = black_box(5) * black_box(10);
 /// ```
+///
+/// During constant evaluation, `black_box` is treated as a no-op.
 #[inline]
 #[stable(feature = "bench_black_box", since = "1.66.0")]
-#[rustc_const_unstable(feature = "const_black_box", issue = "none")]
+#[rustc_const_stable(feature = "const_black_box", since = "CURRENT_RUSTC_VERSION")]
 pub const fn black_box<T>(dummy: T) -> T {
     crate::intrinsics::black_box(dummy)
 }

core/src/intrinsics/mod.rs

@@ -3725,6 +3725,7 @@ pub const unsafe fn compare_bytes(_left: *const u8, _right: *const u8, _bytes: u
 #[rustc_nounwind]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
+#[rustc_intrinsic_const_stable_indirect]
 pub const fn black_box<T>(_dummy: T) -> T {
     unimplemented!()
 }

coretests/tests/lib.rs

@@ -14,7 +14,6 @@
 #![feature(bstr)]
 #![feature(cell_update)]
 #![feature(clone_to_uninit)]
-#![feature(const_black_box)]
 #![feature(const_eval_select)]
 #![feature(const_swap_nonoverlapping)]
 #![feature(const_trait_impl)]