ptr: Add mut_offset, to be able to calculate an offset on mutable pointers.

src/lib/ptr.rs

@@ -34,6 +34,16 @@ fn offset<T>(ptr: *T, count: uint) -> *T {
     ret rusti::ptr_offset(ptr, count);
 }
 
+/*
+Function: mut_offset
+
+Calculate the offset from a mutable pointer
+*/
+fn mut_offset<T>(ptr: *mutable T, count: uint) -> *mutable T {
+    ret rusti::ptr_offset(ptr as *T, count) as *mutable T;
+}
+
+
 /*
 Function: null