Add support for std:🧵:yield_now

src/shims/foreign_items/posix.rs

@@ -312,6 +312,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 // We do not support forking, so there is nothing to do here.
                 this.write_null(dest)?;
             }
+            "sched_yield" => {
+                this.write_null(dest)?;
+            }
 
             // Incomplete shims that we "stub out" just to get pre-main initialization code to work.
             // These shims are enabled only when the caller is in the standard library.

src/shims/foreign_items/windows.rs

@@ -201,6 +201,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 // FIXME: we should set last_error, but to what?
                 this.write_null(dest)?;
             }
+            "SwitchToThread" => {
+                // Note that once Miri supports concurrency, this will need to return a nonzero
+                // value if this call does result in switching to another thread.
+                this.write_null(dest)?;
+            }
 
             // Better error for attempts to create a thread
             "CreateThread" => {

tests/run-pass/sync.rs

@@ -10,6 +10,7 @@ fn main() {
         test_rwlock_stdlib();
     }
     test_spin_loop_hint();
+    test_thread_yield_now();
 }
 
 fn test_mutex_stdlib() {
@@ -56,3 +57,7 @@ impl<T> TryLockErrorExt<T> for TryLockError<T> {
 fn test_spin_loop_hint() {
     atomic::spin_loop_hint();
 }
+
+fn test_thread_yield_now() {
+    std::thread::yield_now();
+}