Add String::extend_from_within

This patch adds `String::extend_from_within` function under the
`string_extend_from_within` feature gate similar to the
`Vec::extend_from_within` function.

Author: WaffleLapkin

library/alloc/src/lib.rs

@@ -144,6 +144,7 @@
 #![feature(associated_type_bounds)]
 #![feature(slice_group_by)]
 #![feature(decl_macro)]
+#![feature(bindings_after_at)]
 // Allow testing this library
 
 #[cfg(test)]

library/alloc/src/string.rs

@@ -843,6 +843,42 @@ impl String {
         self.vec.extend_from_slice(string.as_bytes())
     }
 
+    /// Copies elements from `src` range to the end of the string.
+    ///
+    /// ## Panics
+    ///
+    /// Panics if the starting point or end point do not lie on a [`char`]
+    /// boundary, or if they're out of bounds.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// #![feature(string_extend_from_within)]
+    /// let mut string = String::from("abcde");
+    ///
+    /// string.extend_from_within(2..);
+    /// assert_eq!(string, "abcdecde");
+    ///
+    /// string.extend_from_within(..2);
+    /// assert_eq!(string, "abcdecdeab");
+    ///
+    /// string.extend_from_within(4..8);
+    /// assert_eq!(string, "abcdecdeabecde");
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[unstable(feature = "string_extend_from_within", issue = "none")]
+    pub fn extend_from_within<R>(&mut self, src: R)
+    where
+        R: RangeBounds<usize>,
+    {
+        let src @ Range { start, end } = slice::range(src, ..self.len());
+
+        assert!(self.is_char_boundary(start));
+        assert!(self.is_char_boundary(end));
+
+        self.vec.extend_from_within(src);
+    }
+
     /// Returns this `String`'s capacity, in bytes.
     ///
     /// # Examples