Rollup merge of #102232 - Urgau:stabilize-bench_black_box, r=TaKO8Ki

Stabilize bench_black_box

This PR stabilize `feature(bench_black_box)`.

```rust
pub fn black_box<T>(dummy: T) -> T;
```

The FCP was completed in rust-lang/rust#64102.

`@rustbot` label +T-libs-api -T-libs

Author: JohnTitor

alloc/tests/lib.rs

@@ -41,7 +41,6 @@
 #![feature(pointer_is_aligned)]
 #![feature(slice_flatten)]
 #![feature(thin_box)]
-#![feature(bench_black_box)]
 #![feature(strict_provenance)]
 #![feature(once_cell)]
 #![feature(drain_keep_rest)]

core/src/hint.rs

@@ -217,7 +217,7 @@ pub fn spin_loop() {
 ///
 /// [`std::convert::identity`]: crate::convert::identity
 #[inline]
-#[unstable(feature = "bench_black_box", issue = "64102")]
+#[stable(feature = "bench_black_box", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_const_unstable(feature = "const_black_box", issue = "none")]
 pub const fn black_box<T>(dummy: T) -> T {
     crate::intrinsics::black_box(dummy)

core/tests/lib.rs

@@ -2,7 +2,6 @@
 #![feature(array_chunks)]
 #![feature(array_methods)]
 #![feature(array_windows)]
-#![feature(bench_black_box)]
 #![feature(bigint_helper_methods)]
 #![feature(cell_update)]
 #![feature(const_assume)]

std/src/lib.rs

@@ -348,7 +348,6 @@
 #![feature(trace_macros)]
 //
 // Only used in tests/benchmarks:
-#![feature(bench_black_box)]
 //
 // Only for const-ness:
 #![feature(const_io_structs)]

test/src/lib.rs

@@ -15,7 +15,6 @@
 
 #![unstable(feature = "test", issue = "50297")]
 #![doc(test(attr(deny(warnings))))]
-#![feature(bench_black_box)]
 #![feature(internal_output_capture)]
 #![feature(staged_api)]
 #![feature(process_exitcode_internals)]