add the bool::not method.

library/core/src/bool.rs

@@ -31,4 +31,23 @@ impl bool {
     pub fn then<T, F: FnOnce() -> T>(self, f: F) -> Option<T> {
         if self { Some(f()) } else { None }
     }
+
+    /// Returns the opposite value.
+    ///
+    /// This is identical in functionality to usage of the `!` operator, it's
+    /// simply an alternate way to write it. Because it's post-fix instead of
+    /// pre-fix, it is often more readable within chained function call
+    /// expressions.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(!true, true.not());
+    /// assert_eq!(!false, false.not());
+    /// ```
+    #[unstable(feature = "bool_not_method", issue = "none")]
+    #[inline]
+    pub const fn not(self) -> Self {
+        !self
+    }
 }