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Add unborrow to reset RefCell borrow state
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This method is complementary for the feature refcell_leak added in an
earlier PR. It allows reverting the effects of leaking a borrow guard by
statically proving that such a guard could not longer exist. This was
not added to the existing `get_mut` out of concern of impacting the
complexity of the otherwise pure pointer cast and because the name
`get_mut` poorly communicates the intent of resetting remaining borrows.
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HeroicKatora committed Mar 4, 2020
1 parent 329022d commit 51b9396
Showing 1 changed file with 36 additions and 5 deletions.
41 changes: 36 additions & 5 deletions src/libcore/cell.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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.
///
Expand Down Expand Up @@ -1272,8 +1299,10 @@ 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
}
Expand Down Expand Up @@ -1387,9 +1416,11 @@ 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
}
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