move core::hint::black_box under its own feature gate

Author: RalfJung

core/benches/fmt.rs

@@ -112,28 +112,28 @@ fn write_str_macro_debug(bh: &mut Bencher) {
 #[bench]
 fn write_u128_max(bh: &mut Bencher) {
     bh.iter(|| {
-        std::hint::black_box(format!("{}", u128::MAX));
+        test::black_box(format!("{}", u128::MAX));
     });
 }
 
 #[bench]
 fn write_u128_min(bh: &mut Bencher) {
     bh.iter(|| {
         let s = format!("{}", 0u128);
-        std::hint::black_box(s);
+        test::black_box(s);
     });
 }
 
 #[bench]
 fn write_u64_max(bh: &mut Bencher) {
     bh.iter(|| {
-        std::hint::black_box(format!("{}", u64::MAX));
+        test::black_box(format!("{}", u64::MAX));
     });
 }
 
 #[bench]
 fn write_u64_min(bh: &mut Bencher) {
     bh.iter(|| {
-        std::hint::black_box(format!("{}", 0u64));
+        test::black_box(format!("{}", 0u64));
     });
 }

core/src/hint.rs

@@ -154,7 +154,7 @@ pub fn spin_loop() {
 /// [`std::convert::identity`]: crate::convert::identity
 #[cfg_attr(not(miri), inline)]
 #[cfg_attr(miri, inline(never))]
-#[unstable(feature = "test", issue = "50297")]
+#[unstable(feature = "bench_black_box", issue = "64102")]
 #[cfg_attr(miri, allow(unused_mut))]
 pub fn black_box<T>(mut dummy: T) -> T {
     // We need to "use" the argument in some way LLVM can't introspect, and on

std/src/lib.rs

@@ -233,6 +233,7 @@
 #![feature(assert_matches)]
 #![feature(associated_type_bounds)]
 #![feature(atomic_mut_ptr)]
+#![feature(bench_black_box)]
 #![feature(box_syntax)]
 #![feature(c_variadic)]
 #![feature(cfg_accessible)]

test/src/bench.rs

@@ -1,6 +1,4 @@
 //! Benchmarking module.
-pub use std::hint::black_box;
-
 use super::{
     event::CompletedTest,
     options::BenchMode,
@@ -16,6 +14,15 @@ use std::panic::{catch_unwind, AssertUnwindSafe};
 use std::sync::{Arc, Mutex};
 use std::time::{Duration, Instant};
 
+/// An identity function that *__hints__* to the compiler to be maximally pessimistic about what
+/// `black_box` could do.
+///
+/// See [`std::hint::black_box`] for details.
+#[inline(always)]
+pub fn black_box<T>(dummy: T) -> T {
+    std::hint::black_box(dummy)
+}
+
 /// Manager of the benchmarking runs.
 ///
 /// This is fed into functions marked with `#[bench]` to allow for

test/src/lib.rs

@@ -24,6 +24,7 @@
 #![feature(rustc_private)]
 #![feature(nll)]
 #![feature(available_concurrency)]
+#![feature(bench_black_box)]
 #![feature(internal_output_capture)]
 #![feature(panic_unwind)]
 #![feature(staged_api)]