add simple downgrade implementations

std/src/sys/sync/rwlock/no_threads.rs

@@ -62,4 +62,9 @@ impl RwLock {
     pub unsafe fn write_unlock(&self) {
         assert_eq!(self.mode.replace(0), -1);
     }
+
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        assert_eq!(self.mode.replace(1), -1);
+    }
 }

std/src/sys/sync/rwlock/solid.rs

@@ -79,6 +79,12 @@ impl RwLock {
         let rwl = self.raw();
         expect_success_aborting(unsafe { abi::rwl_unl_rwl(rwl) }, &"rwl_unl_rwl");
     }
+
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        // The SOLID platform does not support the `downgrade` operation for reader writer locks, so
+        // this function is simply a no-op as only 1 reader can read: the original writer.
+    }
 }
 
 impl Drop for RwLock {

std/src/sys/sync/rwlock/teeos.rs

@@ -41,4 +41,10 @@ impl RwLock {
     pub unsafe fn write_unlock(&self) {
         unsafe { self.inner.unlock() };
     }
+
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        // Since there is no difference between read-locked and write-locked on this platform, this
+        // function is simply a no-op as only 1 reader can read: the original writer.
+    }
 }