1 files changed, 62 insertions, 6 deletions

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index 9ebb3176418..0f2665eba6f 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -958,6 +958,33 @@ impl<T: ?Sized> RefCell<T> {
         unsafe { &mut *self.value.get() }
     }
 
+    /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
+    ///
+    /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
+    /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
+    /// if some `Ref` or `RefMut` borrows have been leaked.
+    ///
+    /// [`get_mut`]: #method.get_mut
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cell_leak)]
+    /// use std::cell::RefCell;
+    ///
+    /// let mut c = RefCell::new(0);
+    /// std::mem::forget(c.borrow_mut());
+    ///
+    /// assert!(c.try_borrow().is_err());
+    /// c.undo_leak();
+    /// assert!(c.try_borrow().is_ok());
+    /// ```
+    #[unstable(feature = "cell_leak", issue = "69099")]
+    pub fn undo_leak(&mut self) -> &mut T {
+        *self.borrow.get_mut() = UNUSED;
+        self.get_mut()
+    }
+
     /// Immutably borrows the wrapped value, returning an error if the value is
     /// currently mutably borrowed.
     ///
@@ -996,6 +1023,31 @@ impl<T: ?Sized> RefCell<T> {
     }
 }
 
+impl<T: Default> RefCell<T> {
+    /// Takes the wrapped value, leaving `Default::default()` in its place.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the value is currently borrowed.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(refcell_take)]
+    /// use std::cell::RefCell;
+    ///
+    /// let c = RefCell::new(5);
+    /// let five = c.take();
+    ///
+    /// assert_eq!(five, 5);
+    /// assert_eq!(c.into_inner(), 0);
+    /// ```
+    #[unstable(feature = "refcell_take", issue = "71395")]
+    pub fn take(&self) -> T {
+        self.replace(Default::default())
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 
@@ -1272,8 +1324,10 @@ impl<'b, T: ?Sized> Ref<'b, T> {
     /// ```
     #[unstable(feature = "cell_leak", issue = "69099")]
     pub fn leak(orig: Ref<'b, T>) -> &'b T {
-        // By forgetting this Ref we ensure that the borrow counter in the RefCell never goes back
-        // to UNUSED again. No further mutable references can be created from the original cell.
+        // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
+        // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
+        // unique reference to the borrowed RefCell. No further mutable references can be created
+        // from the original cell.
         mem::forget(orig.borrow);
         orig.value
     }
@@ -1387,9 +1441,11 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
     /// ```
     #[unstable(feature = "cell_leak", issue = "69099")]
     pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
-        // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell never
-        // goes back to UNUSED again. No further references can be created from the original cell,
-        // making the current borrow the only reference for the remaining lifetime.
+        // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
+        // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
+        // require a unique reference to the borrowed RefCell. No further references can be created
+        // from the original cell within that lifetime, making the current borrow the only
+        // reference for the remaining lifetime.
         mem::forget(orig.borrow);
         orig.value
     }
@@ -1538,7 +1594,7 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 #[lang = "unsafe_cell"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[repr(transparent)]
-#[cfg_attr(not(bootstrap), repr(no_niche))] // rust-lang/rust#68303.
+#[repr(no_niche)] // rust-lang/rust#68303.
 pub struct UnsafeCell<T: ?Sized> {
     value: T,
 }