Reuse sign_extend helper

compiler/rustc_abi/src/layout.rs

@@ -769,7 +769,6 @@ struct TmpLayout<FieldIdx: Idx, VariantIdx: Idx> {
 
         let discr_type = repr.discr_type();
         let discr_int = Integer::from_attr(dl, discr_type);
-        let bits = discr_int.size().bits();
         // Because we can only represent one range of valid values, we'll look for the
         // largest range of invalid values and pick everything else as the range of valid
         // values.
@@ -780,7 +779,8 @@ struct TmpLayout<FieldIdx: Idx, VariantIdx: Idx> {
             .map(|(_, val)| {
                 if discr_type.is_signed() {
                     // sign extend the raw representation to be an i128
-                    (val << (128 - bits)) >> (128 - bits)
+                    // FIXME: do this at the discriminant iterator creation sites
+                    discr_int.size().sign_extend(val as u128)
                 } else {
                     val
                 }

compiler/rustc_middle/src/ty/util.rs

@@ -32,7 +32,7 @@
 
 #[derive(Copy, Clone, Debug)]
 pub struct Discr<'tcx> {
-    /// Bit representation of the discriminant (e.g., `-128i8` is `0xFF_u128`).
+    /// Bit representation of the discriminant (e.g., `-1i8` is `0xFF_u128`).
     pub val: u128,
     pub ty: Ty<'tcx>,
 }