Add String::split_off.

src/libcollections/string.rs

@@ -1260,6 +1260,39 @@ impl String {
         self.len() == 0
     }
 
+    /// Divide one string into two at an index.
+    ///
+    /// The argument, `mid`, should be a byte offset from the start of the string. It must also
+    /// be on the boundary of a UTF-8 code point.
+    ///
+    /// The two strings returned go from the start of the string to `mid`, and from `mid` to the end
+    /// of the string.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `mid` is not on a `UTF-8` code point boundary, or if it is beyond the last
+    /// code point of the string.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(string_split_off)]
+    /// # fn main() {
+    /// let mut hello = String::from("Hello, World!");
+    /// let world = hello.split_off(7);
+    /// assert_eq!(hello, "Hello, ");
+    /// assert_eq!(world, "World!");
+    /// # }
+    /// ```
+    #[inline]
+    #[unstable(feature = "string_split_off", issue = "38080")]
+    pub fn split_off(&mut self, mid: usize) -> String {
+        assert!(self.is_char_boundary(mid));
+        assert!(mid <= self.len());
+        let other = self.vec.split_off(mid);
+        unsafe { String::from_utf8_unchecked(other) }
+    }
+
     /// Truncates this `String`, removing all contents.
     ///
     /// While this means the `String` will have a length of zero, it does not

src/libcollectionstest/lib.rs

@@ -25,6 +25,7 @@
 #![feature(step_by)]
 #![feature(str_escape)]
 #![feature(str_replacen)]
+#![feature(string_split_off)]
 #![feature(test)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]

src/libcollectionstest/string.rs

@@ -231,6 +231,45 @@ fn test_pop() {
     assert_eq!(data, "ประเทศไทย中");
 }
 
+#[test]
+fn test_split_off_empty() {
+    let orig = "Hello, world!";
+    let mut split = String::from(orig);
+    let empty: String = split.split_off(orig.len());
+    assert!(empty.is_empty());
+}
+
+#[test]
+#[should_panic]
+fn test_split_off_past_end() {
+    let orig = "Hello, world!";
+    let mut split = String::from(orig);
+    split.split_off(orig.len() + 1);
+}
+
+#[test]
+#[should_panic]
+fn test_split_off_mid_char() {
+    let mut orig = String::from("山");
+    orig.split_off(1);
+}
+
+#[test]
+fn test_split_off_ascii() {
+    let mut ab = String::from("ABCD");
+    let cd = ab.split_off(2);
+    assert_eq!(ab, "AB");
+    assert_eq!(cd, "CD");
+}
+
+#[test]
+fn test_split_off_unicode() {
+    let mut nihon = String::from("日本語");
+    let go = nihon.split_off("日本".len());
+    assert_eq!(nihon, "日本");
+    assert_eq!(go, "語");
+}
+
 #[test]
 fn test_str_truncate() {
     let mut s = String::from("12345");