removed the definition of mask

src/librustc/mir/interpret/mod.rs

@@ -434,9 +434,3 @@ pub fn truncate(value: u128, size: Size) -> u128 {
     // truncate (shift left to drop out leftover values, shift right to fill with zeroes)
     (value << shift) >> shift
 }
-
-pub fn mask(size: Size) -> u128 {
-    let size = size.bits();
-    let shift = 128 - size;
-    !0u128 >> shift
-}