Auto merge of rust-lang#43574 - notriddle:master, r=sfackler

Implement `RefCell::replace` and `RefCell::swap`

Tracking issue: rust-lang#43570

Author: bors

src/libcore/cell.rs

@@ -571,6 +571,59 @@ impl<T> RefCell<T> {
         debug_assert!(self.borrow.get() == UNUSED);
         unsafe { self.value.into_inner() }
     }
+
+    /// Replaces the wrapped value with a new one, returning the old value,
+    /// without deinitializing either one.
+    ///
+    /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(refcell_replace_swap)]
+    /// use std::cell::RefCell;
+    /// let c = RefCell::new(5);
+    /// let u = c.replace(6);
+    /// assert_eq!(u, 5);
+    /// assert_eq!(c, RefCell::new(6));
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the `RefCell` has any outstanding borrows,
+    /// whether or not they are full mutable borrows.
+    #[inline]
+    #[unstable(feature = "refcell_replace_swap", issue="43570")]
+    pub fn replace(&self, t: T) -> T {
+        mem::replace(&mut *self.borrow_mut(), t)
+    }
+
+    /// Swaps the wrapped value of `self` with the wrapped value of `other`,
+    /// without deinitializing either one.
+    ///
+    /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(refcell_replace_swap)]
+    /// use std::cell::RefCell;
+    /// let c = RefCell::new(5);
+    /// let d = RefCell::new(6);
+    /// c.swap(&d);
+    /// assert_eq!(c, RefCell::new(6));
+    /// assert_eq!(d, RefCell::new(5));
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if either `RefCell` has any outstanding borrows,
+    /// whether or not they are full mutable borrows.
+    #[inline]
+    #[unstable(feature = "refcell_replace_swap", issue="43570")]
+    pub fn swap(&self, other: &Self) {
+        mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
+    }
 }
 
 impl<T: ?Sized> RefCell<T> {

src/libcore/tests/cell.rs

@@ -287,3 +287,20 @@ fn refcell_ref_coercion() {
         assert_eq!(&*coerced, comp);
     }
 }
+
+#[test]
+#[should_panic]
+fn refcell_swap_borrows() {
+    let x = RefCell::new(0);
+    let _b = x.borrow();
+    let y = RefCell::new(1);
+    x.swap(&y);
+}
+
+#[test]
+#[should_panic]
+fn refcell_replace_borrows() {
+    let x = RefCell::new(0);
+    let _b = x.borrow();
+    x.replace(1);
+}

src/libcore/tests/lib.rs

@@ -31,6 +31,7 @@
 #![feature(ord_max_min)]
 #![feature(rand)]
 #![feature(raw)]
+#![feature(refcell_replace_swap)]
 #![feature(sip_hash_13)]
 #![feature(slice_patterns)]
 #![feature(slice_rotate)]