Implement ToOwned::clone_into for CStr

It can try to keep its allocation by converting the inner `Box` to
`Vec`, using `clone_into` on the bytes, then convert back to `Box`.

src/libstd/ffi/c_str.rs

@@ -1329,6 +1329,12 @@ impl ToOwned for CStr {
     fn to_owned(&self) -> CString {
         CString { inner: self.to_bytes_with_nul().into() }
     }
+
+    fn clone_into(&self, target: &mut CString) {
+        let mut b = Vec::from(mem::take(&mut target.inner));
+        self.to_bytes_with_nul().clone_into(&mut b);
+        target.inner = b.into_boxed_slice();
+    }
 }
 
 #[stable(feature = "cstring_asref", since = "1.7.0")]
@@ -1510,6 +1516,17 @@ mod tests {
         assert_eq!(boxed.to_bytes_with_nul(), &[0]);
     }
 
+    #[test]
+    fn test_c_str_clone_into() {
+        let mut c_string = CString::new("lorem").unwrap();
+        let c_ptr = c_string.as_ptr();
+        let c_str = CStr::from_bytes_with_nul(b"ipsum\0").unwrap();
+        c_str.clone_into(&mut c_string);
+        assert_eq!(c_str, c_string.as_c_str());
+        // The exact same size shouldn't have needed to move its allocation
+        assert_eq!(c_ptr, c_string.as_ptr());
+    }
+
     #[test]
     fn into_rc() {
         let orig: &[u8] = b"Hello, world!\0";