add miri_spin_loop to make hint::spin_loop work consistently

Author: folkertdev

library/core/src/hint.rs

@@ -269,6 +269,16 @@ pub const unsafe fn assert_unchecked(cond: bool) {
 #[stable(feature = "renamed_spin_loop", since = "1.49.0")]
 pub fn spin_loop() {
     crate::cfg_select! {
+        miri => {
+            unsafe extern "Rust" {
+                safe fn miri_spin_loop();
+            }
+
+            // Miri does support some of the intrinsics that are called below, but to guarantee
+            // consistent behavior across targets, this custom function is used. To maximize the
+            // likelyhood of finding actual issues, it will attempt to find another thread to run.
+            miri_spin_loop();
+        }
         target_arch = "x86" => {
             // SAFETY: the `cfg` attr ensures that we only execute this on x86 targets.
             crate::arch::x86::_mm_pause()

src/tools/miri/src/shims/foreign_items.rs

@@ -435,6 +435,12 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 // Return value: 0 on success, otherwise the size it would have needed.
                 this.write_int(if success { 0 } else { needed_size }, dest)?;
             }
+            // Hint that a loop is spinning indefinitely.
+            "miri_spin_loop" => {
+                // To maximize the chance of finding actual bugs, try to find another thread to
+                // run.
+                this.yield_active_thread();
+            }
             // Obtains the size of a Miri backtrace. See the README for details.
             "miri_backtrace_size" => {
                 this.handle_miri_backtrace_size(abi, link_name, args, dest)?;