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 

7 files changed, 52 insertions, 4 deletions

diff --git a/tokio/src/loom/std/atomic_u16.rs b/tokio/src/loom/std/atomic_u16.rs
index 70390972..c1c53120 100644
--- a/tokio/src/loom/std/atomic_u16.rs
+++ b/tokio/src/loom/std/atomic_u16.rs
@@ -11,7 +11,7 @@ unsafe impl Send for AtomicU16 {}
 unsafe impl Sync for AtomicU16 {}
 
 impl AtomicU16 {
-    pub(crate) fn new(val: u16) -> AtomicU16 {
+    pub(crate) const fn new(val: u16) -> AtomicU16 {
         let inner = UnsafeCell::new(std::sync::atomic::AtomicU16::new(val));
         AtomicU16 { inner }
     }
diff --git a/tokio/src/loom/std/atomic_u32.rs b/tokio/src/loom/std/atomic_u32.rs
index 6f786c51..61f95fb3 100644
--- a/tokio/src/loom/std/atomic_u32.rs
+++ b/tokio/src/loom/std/atomic_u32.rs
@@ -11,7 +11,7 @@ unsafe impl Send for AtomicU32 {}
 unsafe impl Sync for AtomicU32 {}
 
 impl AtomicU32 {
-    pub(crate) fn new(val: u32) -> AtomicU32 {
+    pub(crate) const fn new(val: u32) -> AtomicU32 {
         let inner = UnsafeCell::new(std::sync::atomic::AtomicU32::new(val));
         AtomicU32 { inner }
     }
diff --git a/tokio/src/loom/std/atomic_u8.rs b/tokio/src/loom/std/atomic_u8.rs
index 4fcd0df3..408aea33 100644
--- a/tokio/src/loom/std/atomic_u8.rs
+++ b/tokio/src/loom/std/atomic_u8.rs
@@ -11,7 +11,7 @@ unsafe impl Send for AtomicU8 {}
 unsafe impl Sync for AtomicU8 {}
 
 impl AtomicU8 {
-    pub(crate) fn new(val: u8) -> AtomicU8 {
+    pub(crate) const fn new(val: u8) -> AtomicU8 {
         let inner = UnsafeCell::new(std::sync::atomic::AtomicU8::new(val));
         AtomicU8 { inner }
     }
diff --git a/tokio/src/loom/std/unsafe_cell.rs b/tokio/src/loom/std/unsafe_cell.rs
index f2b03d8d..66c1d794 100644
--- a/tokio/src/loom/std/unsafe_cell.rs
+++ b/tokio/src/loom/std/unsafe_cell.rs
@@ -2,7 +2,7 @@
 pub(crate) struct UnsafeCell<T>(std::cell::UnsafeCell<T>);
 
 impl<T> UnsafeCell<T> {
-    pub(crate) fn new(data: T) -> UnsafeCell<T> {
+    pub(crate) const fn new(data: T) -> UnsafeCell<T> {
         UnsafeCell(std::cell::UnsafeCell::new(data))
     }
 
diff --git a/tokio/src/sync/notify.rs b/tokio/src/sync/notify.rs
index 74c69e50..c69e2b07 100644
--- a/tokio/src/sync/notify.rs
+++ b/tokio/src/sync/notify.rs
@@ -170,6 +170,24 @@ impl Notify {
         }
     }
 
+    /// Create a new `Notify`, initialized without a permit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::Notify;
+    ///
+    /// static NOTIFY: Notify = Notify::const_new();
+    /// ```
+    #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
+    pub const fn const_new() -> Notify {
+        Notify {
+            state: AtomicU8::new(0),
+            waiters: Mutex::const_new(LinkedList::new()),
+        }
+    }
+
     /// Wait for a notification.
     ///
     /// Equivalent to:
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.
     ///
diff --git a/tokio/src/sync/semaphore.rs b/tokio/src/sync/semaphore.rs
index 2489d34a..136f14e6 100644
--- a/tokio/src/sync/semaphore.rs
+++ b/tokio/src/sync/semaphore.rs
@@ -74,6 +74,15 @@ impl Semaphore {
         }
     }
 
+    /// Creates a new semaphore with the initial number of permits.
+    #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
+    pub const fn const_new(permits: usize) -> Self {
+        Self {
+            ll_sem: ll::Semaphore::const_new(permits),
+        }
+    }
+
     /// Returns the current number of available permits.
     pub fn available_permits(&self) -> usize {
         self.ll_sem.available_permits()