From b4605f90ba9a64ccf893c7e2a583f4265dc2a5d1 Mon Sep 17 00:00:00 2001
From: Jiahao XU <Jiahao_XU@outlook.com>
Date: Tue, 1 Aug 2023 21:16:58 +1000
Subject: [PATCH] sync: Add const fn `OnceCell::from_value`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>
---
 tokio/src/sync/batch_semaphore.rs | 14 ++++++++++++++
 tokio/src/sync/once_cell.rs       | 32 +++++++++++++++++++++++++++++++
 tokio/src/sync/semaphore.rs       | 10 ++++++++++
 3 files changed, 56 insertions(+)

diff --git a/tokio/src/sync/batch_semaphore.rs b/tokio/src/sync/batch_semaphore.rs
index e0dafaa256e..d848b803656 100644
--- a/tokio/src/sync/batch_semaphore.rs
+++ b/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
diff --git a/tokio/src/sync/once_cell.rs b/tokio/src/sync/once_cell.rs
index 8a5463a3be7..1128bcfd81f 100644
--- a/tokio/src/sync/once_cell.rs
+++ b/tokio/src/sync/once_cell.rs
@@ -149,6 +149,38 @@ impl<T> OnceCell<T> {
         }
     }
 
+    /// Creates a new `OnceCell` that contains the provided value.
+    ///
+    /// This is equivalent to `<OnceCell<T> as From<T>>::from(value)`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use tokio::sync::OnceCell;
+    ///
+    /// static ONCE: OnceCell<u32> = OnceCell::from_value(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 from_value(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
diff --git a/tokio/src/sync/semaphore.rs b/tokio/src/sync/semaphore.rs
index cb770215488..96506bf17cc 100644
--- a/tokio/src/sync/semaphore.rs
+++ b/tokio/src/sync/semaphore.rs
@@ -190,6 +190,16 @@ 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()