Add skeleton downgrade internal methods

Author: connortsui20

library/std/src/sys/sync/rwlock/futex.rs

@@ -164,6 +164,11 @@ impl RwLock {
         }
     }
 
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        todo!()
+    }
+
     #[cold]
     fn write_contended(&self) {
         let mut state = self.spin_write();

library/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) {
+        todo!()
+    }
 }

library/std/src/sys/sync/rwlock/queue.rs

@@ -449,6 +449,11 @@ impl RwLock {
         }
     }
 
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        todo!()
+    }
+
     /// # Safety
     /// * The lock must be exclusively owned by this thread.
     /// * There must be threads queued on the lock.

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

@@ -79,6 +79,11 @@ impl RwLock {
         let rwl = self.raw();
         expect_success_aborting(unsafe { abi::rwl_unl_rwl(rwl) }, &"rwl_unl_rwl");
     }
+
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        todo!()
+    }
 }
 
 impl Drop for RwLock {

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

@@ -41,4 +41,9 @@ impl RwLock {
     pub unsafe fn write_unlock(&self) {
         unsafe { self.inner.unlock() };
     }
+
+    #[inline]
+    pub unsafe fn downgrade(&self) {
+        todo!()
+    }
 }