Add DerefMut for ArrayString

src/array_string.rs

@@ -1,7 +1,8 @@
 use std::borrow::Borrow;
 use std::fmt;
 use std::hash::{Hash, Hasher};
-use std::ops::Deref;
+use std::mem;
+use std::ops::{Deref, DerefMut};
 use std::str;
 use std::slice;
 
@@ -142,6 +143,17 @@ impl<A: Array<Item=u8>> Deref for ArrayString<A> {
     }
 }
 
+impl<A: Array<Item=u8>> DerefMut for ArrayString<A> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut str {
+        unsafe {
+            let sl = slice::from_raw_parts_mut(self.xs.as_mut_ptr(), self.len.to_usize());
+            // FIXME: Nothing but transmute to do this right now
+            mem::transmute(sl)
+        }
+    }
+}
+
 impl<A: Array<Item=u8>> PartialEq for ArrayString<A> {
     fn eq(&self, rhs: &Self) -> bool {
         **self == **rhs

tests/tests.rs

@@ -283,6 +283,12 @@ fn test_string() {
     assert!(t.push_str(text).is_err());
     assert_eq!(&t, "");
 
+    t.push_str("ab").unwrap();
+    // DerefMut
+    let tmut: &mut str = &mut t;
+    assert_eq!(tmut, "ab");
+
+
     // Test Error trait / try
     let t = || -> Result<(), Box<Error>> {
         let mut t = ArrayString::<[_; 2]>::new();