From 03acea646c025846e9421e33d77a5b19782762db Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Mon, 31 Jul 2017 17:14:16 +0000
Subject: [PATCH 1/3] Implement `RefCell::replace` and `RefCell::swap`

---
 src/libcore/cell.rs       | 53 +++++++++++++++++++++++++++++++++++++++
 src/libcore/tests/cell.rs | 17 +++++++++++++
 src/libcore/tests/lib.rs  |  1 +
 3 files changed, 71 insertions(+)

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index 35744f3f16b..b250bf49e0c 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -543,6 +543,59 @@ 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).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![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).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![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 the `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> {
diff --git a/src/libcore/tests/cell.rs b/src/libcore/tests/cell.rs
index 8585f2f0871..cc0ef6a6f17 100644
--- a/src/libcore/tests/cell.rs
+++ b/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);
+}
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index a85c347146b..84a3be99c27 100644
--- a/src/libcore/tests/lib.rs
+++ b/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)]

From dc76247a0a7778a0f38d63bcc3ae5428327bfcf1 Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Wed, 2 Aug 2017 10:58:27 -0700
Subject: [PATCH 2/3] Change "Example" to "Examples" in the doc comments

---
 src/libcore/cell.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index b250bf49e0c..675fcef5e5a 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -549,7 +549,7 @@ impl<T> RefCell<T> {
     ///
     /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
     ///
-    /// # Example
+    /// # Examples
     ///
     /// ```
     /// #![feature(refcell_replace_swap)]
@@ -575,7 +575,7 @@ impl<T> RefCell<T> {
     ///
     /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
     ///
-    /// # Example
+    /// # Examples
     ///
     /// ```
     /// #![feature(refcell_replace_swap)]

From 846d373ddf31aeb76dabedb81a4ae9200d9da1cd Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Tue, 8 Aug 2017 09:53:51 -0700
Subject: [PATCH 3/3] Clarify the language around `RefCell::swap`

---
 src/libcore/cell.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index 675fcef5e5a..f0a94a634f5 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -589,7 +589,7 @@ impl<T> RefCell<T> {
     ///
     /// # Panics
     ///
-    /// This function will panic if the `RefCell` has any outstanding borrows,
+    /// 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")]