Moves test::black_box to core::hint and fix black_box on wasm32 and asm.js

src/libcore/hint.rs

@@ -91,3 +91,25 @@ pub fn spin_loop() {
         }
     }
 }
+
+/// A function that is opaque to the optimizer, to allow benchmarks to
+/// pretend to use outputs to assist in avoiding dead-code
+/// elimination.
+///
+/// This function is a no-op, and does not even read from `dummy`.
+#[unstable(feature = "test", issue = "27812")]
+pub fn black_box<T>(dummy: T) -> T {
+    #[cfg(not(any(target_arch = "asmjs", target_arch = "wasm32")))] {
+        // we need to "use" the argument in some way LLVM can't
+        // introspect.
+        unsafe { asm!("" : : "r"(&dummy)) }
+        dummy
+    }
+    #[cfg(any(target_arch = "asmjs", target_arch = "wasm32"))] {
+        unsafe {
+            let ret = crate::ptr::read_volatile(&dummy);
+            crate::mem::forget(dummy);
+            ret
+        }
+   }
+}

src/libtest/lib.rs

@@ -27,23 +27,7 @@ pub use libtest::{
     TestResult, TrFailed, TrFailedMsg, TrIgnored, TrOk, stats::Summary
 };
 
-/// A function that is opaque to the optimizer, to allow benchmarks to
-/// pretend to use outputs to assist in avoiding dead-code
-/// elimination.
-///
-/// This function is a no-op, and does not even read from `dummy`.
-#[cfg(not(any(target_arch = "asmjs", target_arch = "wasm32")))]
-pub fn black_box<T>(dummy: T) -> T {
-    // we need to "use" the argument in some way LLVM can't
-    // introspect.
-    unsafe { asm!("" : : "r"(&dummy)) }
-    dummy
-}
-#[cfg(any(target_arch = "asmjs", target_arch = "wasm32"))]
-#[inline(never)]
-pub fn black_box<T>(dummy: T) -> T {
-    dummy
-}
+pub use std::hint::black_box;
 
 #[cfg(test)]
 mod tests {