u128 truncation and sign extension are not just interpreter related

compiler/rustc_target/src/abi/mod.rs

@@ -306,6 +306,35 @@ impl Size {
         let bytes = self.bytes().checked_mul(count)?;
         if bytes < dl.obj_size_bound() { Some(Size::from_bytes(bytes)) } else { None }
     }
+
+    /// Truncates `value` to `self` bits and then sign-extend it to 128 bits
+    /// (i.e., if it is negative, fill with 1's on the left).
+    #[inline]
+    pub fn sign_extend(self, value: u128) -> u128 {
+        let size = self.bits();
+        if size == 0 {
+            // Truncated until nothing is left.
+            return 0;
+        }
+        // Sign-extend it.
+        let shift = 128 - size;
+        // Shift the unsigned value to the left, then shift back to the right as signed
+        // (essentially fills with FF on the left).
+        (((value << shift) as i128) >> shift) as u128
+    }
+
+    /// Truncates `value` to `self` bits.
+    #[inline]
+    pub fn truncate(self, value: u128) -> u128 {
+        let size = self.bits();
+        if size == 0 {
+            // Truncated until nothing is left.
+            return 0;
+        }
+        let shift = 128 - size;
+        // Truncate (shift left to drop out leftover values, shift right to fill with zeroes).
+        (value << shift) >> shift
+    }
 }
 
 // Panicking addition, subtraction and multiplication for convenience.