Offset from (#705)

src/lib.rs

@@ -148,3 +148,18 @@ fn panic_does_not_fit(size: usize, nbytes: usize) -> ! {
         size, nbytes
     );
 }
+
+/// Precondition: dst >= original
+///
+/// The following line is equivalent to:
+///
+/// ```rust,ignore
+/// self.ptr.as_ptr().offset_from(ptr) as usize;
+/// ```
+///
+/// But due to min rust is 1.39 and it is only stabilized
+/// in 1.47, we cannot use it.
+#[inline]
+fn offset_from(dst: *const u8, original: *const u8) -> usize {
+    dst as usize - original as usize
+}