sync: add const-constructors for some sync primitives (#2790)

Co-authored-by: Mikail Bagishov <bagishov.mikail@yandex.ru>
Co-authored-by: Eliza Weisman <eliza@buoyant.io>
Co-authored-by: Alice Ryhl <alice@ryhl.io>

tokio/src/loom/std/atomic_usize.rs

@@ -11,7 +11,7 @@ unsafe impl Send for AtomicUsize {}
 unsafe impl Sync for AtomicUsize {}
 
 impl AtomicUsize {
-    pub(crate) fn new(val: usize) -> AtomicUsize {
+    pub(crate) const fn new(val: usize) -> AtomicUsize {
         let inner = UnsafeCell::new(std::sync::atomic::AtomicUsize::new(val));
         AtomicUsize { inner }
     }

tokio/src/loom/std/parking_lot.rs

@@ -26,6 +26,13 @@ impl<T> Mutex<T> {
         Mutex(parking_lot::Mutex::new(t))
     }
 
+    #[inline]
+    #[cfg(all(feature = "parking_lot", not(all(loom, test)),))]
+    #[cfg_attr(docsrs, doc(cfg(all(feature = "parking_lot",))))]
+    pub(crate) const fn const_new(t: T) -> Mutex<T> {
+        Mutex(parking_lot::const_mutex(t))
+    }
+
     #[inline]
     pub(crate) fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
         Ok(self.0.lock())

tokio/src/sync/batch_semaphore.rs

@@ -123,6 +123,27 @@ impl Semaphore {
         }
     }
 
+    /// Creates a new semaphore with the initial number of permits
+    ///
+    /// Maximum number of permits on 32-bit platforms is `1<<29`.
+    ///
+    /// If the specified number of permits exceeds the maximum permit amount
+    /// Then the value will get clamped to the maximum number of permits.
+    #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
+    pub(crate) const fn const_new(mut permits: usize) -> Self {
+        // NOTE: assertions and by extension panics are still being worked on: https://github.com/rust-lang/rust/issues/74925
+        // currently we just clamp the permit count when it exceeds the max
+        permits = permits & Self::MAX_PERMITS;
+
+        Self {
+            permits: AtomicUsize::new(permits << Self::PERMIT_SHIFT),
+            waiters: Mutex::const_new(Waitlist {
+                queue: LinkedList::new(),
+                closed: false,
+            }),
+        }
+    }
+
     /// Returns the current number of available permits
     pub(crate) fn available_permits(&self) -> usize {
         self.permits.load(Acquire) >> Self::PERMIT_SHIFT

tokio/src/sync/mutex.rs

@@ -219,6 +219,27 @@ impl<T: ?Sized> Mutex<T> {
         }
     }
 
+    /// Creates a new lock in an unlocked state ready for use.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::Mutex;
+    ///
+    /// static LOCK: Mutex<i32> = Mutex::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(t: T) -> Self
+    where
+        T: Sized,
+    {
+        Self {
+            c: UnsafeCell::new(t),
+            s: semaphore::Semaphore::const_new(1),
+        }
+    }
+
     /// Locks this mutex, causing the current task
     /// to yield until the lock has been acquired.
     /// When the lock has been acquired, function returns a [`MutexGuard`].

tokio/src/util/linked_list.rs

@@ -72,7 +72,6 @@ unsafe impl<T: Sync> Sync for Pointers<T> {}
 
 impl<L, T> LinkedList<L, T> {
     /// Creates an empty linked list.
-    #[allow(dead_code)] // NOTE: This will get removed with: https://github.com/tokio-rs/tokio/pull/2790
     pub(crate) const fn new() -> LinkedList<L, T> {
         LinkedList {
             head: None,