Added get_mut_unchecked

src/imp_cs.rs

@@ -66,6 +66,20 @@ impl<T> OnceCell<T> {
         self.value.borrow(CriticalSection::new()).get().unwrap_unchecked()
     }
 
+    /// Gets the mutable reference to the underlying value, without checking if the cell
+    /// is initialized.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that the cell is in initialized state, and that
+    /// the contents are acquired by (synchronized to) this thread.
+    pub(crate) unsafe fn get_mut_unchecked(&mut self) -> &mut T {
+        debug_assert!(self.is_initialized());
+        // SAFETY: The caller ensures that the value is initialized and access synchronized.
+        let slot = &mut *self.value.get();
+        slot.as_mut().unwrap_unchecked()
+    }
+
     #[inline]
     pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
         self.value.get_mut().get_mut()

src/imp_pl.rs

@@ -104,6 +104,19 @@ impl<T> OnceCell<T> {
         slot.as_ref().unwrap_unchecked()
     }
 
+    /// Gets the mutable reference to the underlying value, without checking if the cell
+    /// is initialized.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that the cell is in initialized state, and that
+    /// the contents are acquired by (synchronized to) this thread.
+    pub(crate) unsafe fn get_mut_unchecked(&mut self) -> &mut T {
+        debug_assert!(self.is_initialized());
+        let slot = &mut *self.value.get();
+        slot.as_mut().unwrap_unchecked()
+    }
+
     /// Gets the mutable reference to the underlying value.
     /// Returns `None` if the cell is empty.
     pub(crate) fn get_mut(&mut self) -> Option<&mut T> {

src/imp_std.rs

@@ -102,6 +102,19 @@ impl<T> OnceCell<T> {
         slot.as_ref().unwrap_unchecked()
     }
 
+    /// Gets the reference to the underlying value, without checking if the cell
+    /// is initialized.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that the cell is in initialized state, and that
+    /// the contents are acquired by (synchronized to) this thread.
+    pub(crate) unsafe fn get_mut_unchecked(&mut self) -> &mut T {
+        debug_assert!(self.is_initialized());
+        let slot = &mut *self.value.get();
+        slot.as_mut().unwrap_unchecked()
+    }
+
     /// Gets the mutable reference to the underlying value.
     /// Returns `None` if the cell is empty.
     pub(crate) fn get_mut(&mut self) -> Option<&mut T> {

src/lib.rs

@@ -1034,6 +1034,18 @@ pub mod sync {
             self.0.get_unchecked()
         }
 
+        /// Get the mutable reference to the underlying value, without checking if the
+        /// cell is initialized.
+        ///
+        /// # Safety
+        ///
+        /// Caller must ensure that the cell is in initialized state, and that
+        /// the contents are acquired by (synchronized to) this thread.
+        #[inline]
+        pub unsafe fn get_mut_unchecked(&mut self) -> &mut T {
+            self.0.get_mut_unchecked()
+        }
+
         /// Sets the contents of this cell to `value`.
         ///
         /// Returns `Ok(())` if the cell was empty and `Err(value)` if it was