Rollup merge of rust-lang#52003 - Kerollmops:option-replace, r=Kimundi

Implement `Option::replace` in the core library

Here is the implementation of the `Option::replace` method. The first step of [the tracking issue rust-lang#51998](rust-lang#51998).

Author: Mark-Simulacrum

src/libcore/option.rs

@@ -845,6 +845,33 @@ impl<T> Option<T> {
     pub fn take(&mut self) -> Option<T> {
         mem::replace(self, None)
     }
+
+    /// Replaces the actual value in the option by the value given in parameter,
+    /// returning the old value if present,
+    /// leaving a [`Some`] in its place without deinitializing either one.
+    ///
+    /// [`Some`]: #variant.Some
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_replace)]
+    ///
+    /// let mut x = Some(2);
+    /// let old = x.replace(5);
+    /// assert_eq!(x, Some(5));
+    /// assert_eq!(old, Some(2));
+    ///
+    /// let mut x = None;
+    /// let old = x.replace(3);
+    /// assert_eq!(x, Some(3));
+    /// assert_eq!(old, None);
+    /// ```
+    #[inline]
+    #[unstable(feature = "option_replace", issue = "51998")]
+    pub fn replace(&mut self, value: T) -> Option<T> {
+        mem::replace(self, Some(value))
+    }
 }
 
 impl<'a, T: Clone> Option<&'a T> {

src/libcore/tests/lib.rs

@@ -44,6 +44,7 @@
 #![feature(reverse_bits)]
 #![feature(iterator_find_map)]
 #![feature(slice_internals)]
+#![feature(option_replace)]
 
 extern crate core;
 extern crate test;

src/libcore/tests/option.rs

@@ -297,3 +297,18 @@ fn test_try() {
     }
     assert_eq!(try_option_err(), Err(NoneError));
 }
+
+#[test]
+fn test_replace() {
+    let mut x = Some(2);
+    let old = x.replace(5);
+
+    assert_eq!(x, Some(5));
+    assert_eq!(old, Some(2));
+
+    let mut x = None;
+    let old = x.replace(3);
+
+    assert_eq!(x, Some(3));
+    assert_eq!(old, None);
+}