3 files changed, 116 insertions, 0 deletions
diff --git a/library/std/src/sync/once.rs b/library/std/src/sync/once.rs
index 5e0694e8de2..bf595fdea2d 100644
--- a/library/std/src/sync/once.rs
+++ b/library/std/src/sync/once.rs
@@ -264,6 +264,47 @@ impl Once {
         self.inner.is_completed()
     }
 
+    /// Blocks the current thread until initialization has completed.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// #![feature(once_wait)]
+    ///
+    /// use std::sync::Once;
+    /// use std::thread;
+    ///
+    /// static READY: Once = Once::new();
+    ///
+    /// let thread = thread::spawn(|| {
+    ///     READY.wait();
+    ///     println!("everything is ready");
+    /// });
+    ///
+    /// READY.call_once(|| println!("performing setup"));
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// If this [`Once`] has been poisoned because an initialization closure has
+    /// panicked, this method will also panic. Use [`wait_force`](Self::wait_force)
+    /// if this behaviour is not desired.
+    #[unstable(feature = "once_wait", issue = "127527")]
+    pub fn wait(&self) {
+        if !self.inner.is_completed() {
+            self.inner.wait(false);
+        }
+    }
+
+    /// Blocks the current thread until initialization has completed, ignoring
+    /// poisoning.
+    #[unstable(feature = "once_wait", issue = "127527")]
+    pub fn wait_force(&self) {
+        if !self.inner.is_completed() {
+            self.inner.wait(true);
+        }
+    }
+
     /// Returns the current state of the `Once` instance.
     ///
     /// Since this takes a mutable reference, no initialization can currently
diff --git a/library/std/src/sync/once/tests.rs b/library/std/src/sync/once/tests.rs
index d43dabc1cf1..ce96468aeb6 100644
--- a/library/std/src/sync/once/tests.rs
+++ b/library/std/src/sync/once/tests.rs
@@ -1,5 +1,8 @@
 use super::Once;
+use crate::sync::atomic::AtomicBool;
+use crate::sync::atomic::Ordering::Relaxed;
 use crate::sync::mpsc::channel;
+use crate::time::Duration;
 use crate::{panic, thread};
 
 #[test]
@@ -113,3 +116,47 @@ fn wait_for_force_to_finish() {
     assert!(t1.join().is_ok());
     assert!(t2.join().is_ok());
 }
+
+#[test]
+fn wait() {
+    for _ in 0..50 {
+        let val = AtomicBool::new(false);
+        let once = Once::new();
+
+        thread::scope(|s| {
+            for _ in 0..4 {
+                s.spawn(|| {
+                    once.wait();
+                    assert!(val.load(Relaxed));
+                });
+            }
+
+            once.call_once(|| val.store(true, Relaxed));
+        });
+    }
+}
+
+#[test]
+fn wait_on_poisoned() {
+    let once = Once::new();
+
+    panic::catch_unwind(|| once.call_once(|| panic!())).unwrap_err();
+    panic::catch_unwind(|| once.wait()).unwrap_err();
+}
+
+#[test]
+fn wait_force_on_poisoned() {
+    let once = Once::new();
+
+    thread::scope(|s| {
+        panic::catch_unwind(|| once.call_once(|| panic!())).unwrap_err();
+
+        s.spawn(|| {
+            thread::sleep(Duration::from_millis(100));
+
+            once.call_once_force(|_| {});
+        });
+
+        once.wait_force();
+    })
+}
diff --git a/library/std/src/sync/once_lock.rs b/library/std/src/sync/once_lock.rs
index 60e43a1cde1..efc1f415edf 100644
--- a/library/std/src/sync/once_lock.rs
+++ b/library/std/src/sync/once_lock.rs
@@ -167,6 +167,34 @@ impl<T> OnceLock<T> {
         }
     }
 
+    /// Blocks the current thread until the cell is initialized.
+    ///
+    /// # Example
+    ///
+    /// Waiting for a computation on another thread to finish:
+    /// ```rust
+    /// #![feature(once_wait)]
+    ///
+    /// use std::thread;
+    /// use std::sync::OnceLock;
+    ///
+    /// let value = OnceLock::new();
+    ///
+    /// thread::scope(|s| {
+    ///     s.spawn(|| value.set(1 + 1));
+    ///
+    ///     let result = value.wait();
+    ///     assert_eq!(result, &2);
+    /// })
+    /// ```
+    #[inline]
+    #[unstable(feature = "once_wait", issue = "127527")]
+    pub fn wait(&self) -> &T {
+        self.once.wait_force();
+
+        unsafe { self.get_unchecked() }
+    }
+
     /// Sets the contents of this cell to `value`.
     ///
     /// May block if another thread is currently attempting to initialize the cell. The cell is