Handle &mut reference in more places
diff --git a/src/rust_string.rs b/src/rust_string.rs
index a923ced..f0d1df1 100644
--- a/src/rust_string.rs
+++ b/src/rust_string.rs
@@ -14,6 +14,10 @@
         unsafe { &*(s as *const String as *const RustString) }
     }
 
+    pub fn from_mut(s: &mut String) -> &mut Self {
+        unsafe { &mut *(s as *mut String as *mut RustString) }
+    }
+
     pub fn into_string(self) -> String {
         self.repr
     }