Rollup merge of rust-lang#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

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 {