Rollup merge of rust-lang#46517 - notriddle:patch-2, r=BurntSushi

Stablize RefCell::{replace, swap}

RefCell::replace_with is not stablized in this PR, since it wasn't part of the RFC.

CC rust-lang#43570

src/libcore/cell.rs

@@ -584,15 +584,14 @@ impl<T> RefCell<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(refcell_replace_swap)]
     /// use std::cell::RefCell;
     /// let cell = RefCell::new(5);
     /// let old_value = cell.replace(6);
     /// assert_eq!(old_value, 5);
     /// assert_eq!(cell, RefCell::new(6));
     /// ```
     #[inline]
-    #[unstable(feature = "refcell_replace_swap", issue="43570")]
+    #[stable(feature = "refcell_replace", since="1.24.0")]
     pub fn replace(&self, t: T) -> T {
         mem::replace(&mut *self.borrow_mut(), t)
     }
@@ -636,7 +635,6 @@ impl<T> RefCell<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(refcell_replace_swap)]
     /// use std::cell::RefCell;
     /// let c = RefCell::new(5);
     /// let d = RefCell::new(6);
@@ -645,7 +643,7 @@ impl<T> RefCell<T> {
     /// assert_eq!(d, RefCell::new(5));
     /// ```
     #[inline]
-    #[unstable(feature = "refcell_replace_swap", issue="43570")]
+    #[stable(feature = "refcell_swap", since="1.24.0")]
     pub fn swap(&self, other: &Self) {
         mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
     }