4 files changed, 34 insertions, 0 deletions
diff --git a/src/libcollections/str.rs b/src/libcollections/str.rs
index 711e2887865..306a8ba3e09 100644
--- a/src/libcollections/str.rs
+++ b/src/libcollections/str.rs
@@ -793,6 +793,12 @@ impl str {
         core_str::StrExt::split_at(self, mid)
     }
 
+    /// Divide one mutable string slice into two at an index.
+    #[inline]
+    pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
+        core_str::StrExt::split_at_mut(self, mid)
+    }
+
     /// An iterator over the codepoints of `self`.
     ///
     /// # Examples
diff --git a/src/libcollectionstest/lib.rs b/src/libcollectionstest/lib.rs
index 6bf7380fdeb..233623b8a5e 100644
--- a/src/libcollectionstest/lib.rs
+++ b/src/libcollectionstest/lib.rs
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(ascii)]
 #![feature(append)]
 #![feature(bitset)]
 #![feature(bitvec)]
diff --git a/src/libcollectionstest/str.rs b/src/libcollectionstest/str.rs
index 4eee99f2bc9..66d95697102 100644
--- a/src/libcollectionstest/str.rs
+++ b/src/libcollectionstest/str.rs
@@ -702,6 +702,18 @@ fn test_split_at() {
 }
 
 #[test]
+fn test_split_at_mut() {
+    use std::ascii::AsciiExt;
+    let mut s = "Hello World".to_string();
+    {
+        let (a, b) = s.split_at_mut(5);
+        a.make_ascii_uppercase();
+        b.make_ascii_lowercase();
+    }
+    assert_eq!(s, "HELLO world");
+}
+
+#[test]
 #[should_panic]
 fn test_split_at_boundscheck() {
     let s = "ศไทย中华Việt Nam";
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index c8237720b0e..7e4c2ba3be8 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -1279,6 +1279,7 @@ pub trait StrExt {
         where P::Searcher: ReverseSearcher<'a>;
     fn find_str<'a, P: Pattern<'a>>(&'a self, pat: P) -> Option<usize>;
     fn split_at(&self, mid: usize) -> (&str, &str);
+    fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);
     fn slice_shift_char<'a>(&'a self) -> Option<(char, &'a str)>;
     fn subslice_offset(&self, inner: &str) -> usize;
     fn as_ptr(&self) -> *const u8;
@@ -1591,6 +1592,20 @@ impl StrExt for str {
         }
     }
 
+    fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
+        // is_char_boundary checks that the index is in [0, .len()]
+        if self.is_char_boundary(mid) {
+            let len = self.len();
+            unsafe {
+                let self2: &mut str = mem::transmute_copy(&self);
+                (self.slice_mut_unchecked(0, mid),
+                 self2.slice_mut_unchecked(mid, len))
+            }
+        } else {
+            slice_error_fail(self, 0, mid)
+        }
+    }
+
     #[inline]
     fn slice_shift_char(&self) -> Option<(char, &str)> {
         if self.is_empty() {