sync: add const fn OnceCell::from_value (#5903)

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

tokio/src/sync/batch_semaphore.rs

@@ -206,6 +206,20 @@ impl Semaphore {
         }
     }
 
+    /// Creates a new closed semaphore with 0 permits.
+    #[cfg(not(all(loom, test)))]
+    pub(crate) const fn const_new_closed() -> Self {
+        Self {
+            permits: AtomicUsize::new(Self::CLOSED),
+            waiters: Mutex::const_new(Waitlist {
+                queue: LinkedList::new(),
+                closed: true,
+            }),
+            #[cfg(all(tokio_unstable, feature = "tracing"))]
+            resource_span: tracing::Span::none(),
+        }
+    }
+
     /// Returns the current number of available permits.
     pub(crate) fn available_permits(&self) -> usize {
         self.permits.load(Acquire) >> Self::PERMIT_SHIFT

tokio/src/sync/once_cell.rs

@@ -149,6 +149,36 @@ impl<T> OnceCell<T> {
         }
     }
 
+    /// Creates a new `OnceCell` that contains the provided value.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use tokio::sync::OnceCell;
+    ///
+    /// static ONCE: OnceCell<u32> = OnceCell::const_new_with(1);
+    ///
+    /// async fn get_global_integer() -> &'static u32 {
+    ///     ONCE.get_or_init(|| async {
+    ///         1 + 1
+    ///     }).await
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let result = get_global_integer().await;
+    ///     assert_eq!(*result, 1);
+    /// }
+    /// ```
+    #[cfg(not(all(loom, test)))]
+    pub const fn const_new_with(value: T) -> Self {
+        OnceCell {
+            value_set: AtomicBool::new(true),
+            value: UnsafeCell::new(MaybeUninit::new(value)),
+            semaphore: Semaphore::const_new_closed(),
+        }
+    }
+
     /// Creates a new empty `OnceCell` instance.
     ///
     /// Equivalent to `OnceCell::new`, except that it can be used in static

tokio/src/sync/semaphore.rs

@@ -219,6 +219,16 @@ impl Semaphore {
         }
     }
 
+    /// Creates a new closed semaphore with 0 permits.
+    #[cfg(not(all(loom, test)))]
+    pub(crate) const fn const_new_closed() -> Self {
+        Self {
+            ll_sem: ll::Semaphore::const_new_closed(),
+            #[cfg(all(tokio_unstable, feature = "tracing"))]
+            resource_span: tracing::Span::none(),
+        }
+    }
+
     /// Returns the current number of available permits.
     pub fn available_permits(&self) -> usize {
         self.ll_sem.available_permits()