sync: add const constructors to RwLock, Notify, and Semaphore (#2833)

* Add const constructors to `RwLock`, `Notify`, and `Semaphore`.

Referring to the types in `tokio::sync`.
Also add `const` to `new` for the remaining atomic integers in `src/loom` and `UnsafeCell`.

Builds upon previous work in #2790
Closes #2756 

1 files changed, 21 insertions, 0 deletions

diff --git a/tokio/src/sync/rwlock.rs b/tokio/src/sync/rwlock.rs
index 650a7cf6..840889ba 100644
--- a/tokio/src/sync/rwlock.rs
+++ b/tokio/src/sync/rwlock.rs
@@ -485,6 +485,27 @@ impl<T: ?Sized> RwLock<T> {
         }
     }
 
+    /// Creates a new instance of an `RwLock<T>` which is unlocked.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::RwLock;
+    ///
+    /// static LOCK: RwLock<i32> = RwLock::const_new(5);
+    /// ```
+    #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
+    pub const fn const_new(value: T) -> RwLock<T>
+    where
+        T: Sized,
+    {
+        RwLock {
+            c: UnsafeCell::new(value),
+            s: Semaphore::const_new(MAX_READS),
+        }
+    }
+
     /// Locks this rwlock with shared read access, causing the current task
     /// to yield until the lock has been acquired.
     ///