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>

2 files changed, 42 insertions, 0 deletions

diff --git a/tokio/src/sync/batch_semaphore.rs b/tokio/src/sync/batch_semaphore.rs
index c48a911c..a1048ca3 100644
--- a/tokio/src/sync/batch_semaphore.rs
+++ b/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
diff --git a/tokio/src/sync/mutex.rs b/tokio/src/sync/mutex.rs
index df348457..71f0136f 100644
--- a/tokio/src/sync/mutex.rs
+++ b/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`].