Auto merge of #92226 - woppopo:const_black_box, r=joshtriplett

Constify `core::intrinsics::black_box` and `core::hint::black_box`.

`core::intrinsics::black_box` is already constified, but it wasn't marked as const (see: https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/interpret/intrinsics.rs#L471).

Tracking issue: None

library/core/src/hint.rs

@@ -159,6 +159,7 @@ pub fn spin_loop() {
 /// [`std::convert::identity`]: crate::convert::identity
 #[inline]
 #[unstable(feature = "bench_black_box", issue = "64102")]
-pub fn black_box<T>(dummy: T) -> T {
+#[rustc_const_unstable(feature = "const_black_box", issue = "none")]
+pub const fn black_box<T>(dummy: T) -> T {
     crate::intrinsics::black_box(dummy)
 }

library/core/src/intrinsics.rs

@@ -1940,6 +1940,7 @@ extern "rust-intrinsic" {
     /// See documentation of [`std::hint::black_box`] for details.
     ///
     /// [`std::hint::black_box`]: crate::hint::black_box
+    #[rustc_const_unstable(feature = "const_black_box", issue = "none")]
     pub fn black_box<T>(dummy: T) -> T;
 }
 

library/core/src/lib.rs

@@ -103,6 +103,7 @@
 #![feature(const_arguments_as_str)]
 #![feature(const_array_into_iter_constructors)]
 #![feature(const_bigint_helper_methods)]
+#![feature(const_black_box)]
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_char_convert)]

library/core/tests/intrinsics.rs

@@ -65,3 +65,18 @@ const fn test_write_bytes_in_const_contexts() {
     assert!(TEST2[1] == 16843009);
     assert!(TEST2[2] == 3);
 }
+
+#[test]
+fn test_hints_in_const_contexts() {
+    use core::intrinsics::{likely, unlikely};
+
+    // In const contexts, they just return their argument.
+    const {
+        assert!(true == likely(true));
+        assert!(false == likely(false));
+        assert!(true == unlikely(true));
+        assert!(false == unlikely(false));
+        assert!(42u32 == core::intrinsics::black_box(42u32));
+        assert!(42u32 == core::hint::black_box(42u32));
+    }
+}

library/core/tests/lib.rs

@@ -2,12 +2,14 @@
 #![feature(array_chunks)]
 #![feature(array_methods)]
 #![feature(array_windows)]
+#![feature(bench_black_box)]
 #![feature(bool_to_option)]
 #![feature(box_syntax)]
 #![feature(cell_update)]
 #![feature(cfg_panic)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_assume)]
+#![feature(const_black_box)]
 #![feature(const_bool_to_option)]
 #![feature(const_cell_into_inner)]
 #![feature(const_convert)]
@@ -18,6 +20,7 @@
 #![feature(const_ptr_write)]
 #![feature(const_ptr_offset)]
 #![feature(const_trait_impl)]
+#![feature(const_likely)]
 #![feature(core_intrinsics)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]
@@ -35,6 +38,7 @@
 #![feature(array_from_fn)]
 #![feature(hashmap_internals)]
 #![feature(try_find)]
+#![feature(inline_const)]
 #![feature(is_sorted)]
 #![feature(pattern)]
 #![feature(sort_internals)]