Rollup merge of #76434 - RalfJung:black-box, r=Mark-Simulacrum

do not inline black_box when building for Miri

We cannot do the assembly trick in Miri, but let's at least make sure MIR inlining does not circumvent the black_box.

Also use black_box instead of local optimization barriers in a few const tests.

Author: RalfJung

library/core/src/hint.rs

@@ -108,7 +108,8 @@ pub fn spin_loop() {
 /// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
 /// extent to which it can block optimisations may vary depending upon the platform and code-gen
 /// backend used. Programs cannot rely on `black_box` for *correctness* in any way.
-#[inline]
+#[cfg_attr(not(miri), inline)]
+#[cfg_attr(miri, inline(never))]
 #[unstable(feature = "test", issue = "50297")]
 #[allow(unreachable_code)] // this makes #[cfg] a bit easier below.
 pub fn black_box<T>(mut dummy: T) -> T {

src/test/ui/consts/cast-discriminant-zst-enum.rs

@@ -1,5 +1,7 @@
 // run-pass
 // Test a ZST enum whose dicriminant is ~0i128. This caused an ICE when casting to a i32.
+#![feature(test)]
+use std::hint::black_box;
 
 #[derive(Copy, Clone)]
 enum Nums {
@@ -12,9 +14,6 @@ const NEG_ONE_I32: i32 = Nums::NegOne as i32;
 const NEG_ONE_I64: i64 = Nums::NegOne as i64;
 const NEG_ONE_I128: i128 = Nums::NegOne as i128;
 
-#[inline(never)]
-fn identity<T>(t: T) -> T { t }
-
 fn test_as_arg(n: Nums) {
     assert_eq!(-1i8, n as i8);
     assert_eq!(-1i16, n as i16);
@@ -31,11 +30,11 @@ fn main() {
     assert_eq!(-1i64, kind as i64);
     assert_eq!(-1i128, kind as i128);
 
-    assert_eq!(-1i8, identity(kind) as i8);
-    assert_eq!(-1i16, identity(kind) as i16);
-    assert_eq!(-1i32, identity(kind) as i32);
-    assert_eq!(-1i64, identity(kind) as i64);
-    assert_eq!(-1i128, identity(kind) as i128);
+    assert_eq!(-1i8, black_box(kind) as i8);
+    assert_eq!(-1i16, black_box(kind) as i16);
+    assert_eq!(-1i32, black_box(kind) as i32);
+    assert_eq!(-1i64, black_box(kind) as i64);
+    assert_eq!(-1i128, black_box(kind) as i128);
 
     test_as_arg(Nums::NegOne);
 

src/test/ui/consts/const_discriminant.rs

@@ -1,14 +1,10 @@
 // run-pass
 #![feature(const_discriminant)]
+#![feature(test)]
 #![allow(dead_code)]
 
 use std::mem::{discriminant, Discriminant};
-
-// `discriminant(const_expr)` may get const-propagated.
-// As we want to check that const-eval is equal to ordinary exection,
-// we wrap `const_expr` with a function which is not const to prevent this.
-#[inline(never)]
-fn identity<T>(x: T) -> T { x }
+use std::hint::black_box;
 
 enum Test {
     A(u8),
@@ -31,10 +27,10 @@ const TEST_V: Discriminant<SingleVariant> = discriminant(&SingleVariant::V);
 
 fn main() {
     assert_eq!(TEST_A, TEST_A_OTHER);
-    assert_eq!(TEST_A, discriminant(identity(&Test::A(17))));
-    assert_eq!(TEST_B, discriminant(identity(&Test::B)));
+    assert_eq!(TEST_A, discriminant(black_box(&Test::A(17))));
+    assert_eq!(TEST_B, discriminant(black_box(&Test::B)));
     assert_ne!(TEST_A, TEST_B);
-    assert_ne!(TEST_B, discriminant(identity(&Test::C { a: 42, b: 7 })));
+    assert_ne!(TEST_B, discriminant(black_box(&Test::C { a: 42, b: 7 })));
 
-    assert_eq!(TEST_V, discriminant(identity(&SingleVariant::V)));
+    assert_eq!(TEST_V, discriminant(black_box(&SingleVariant::V)));
 }