fix cranelift and gcc backends

Author: RalfJung

src/common.rs

@@ -9,7 +9,6 @@ use rustc_codegen_ssa::traits::{
     StaticMethods,
 };
 use rustc_middle::mir::Mutability;
-use rustc_middle::ty::ScalarInt;
 use rustc_middle::ty::layout::{TyAndLayout, LayoutOf};
 use rustc_middle::mir::interpret::{ConstAllocation, GlobalAlloc, Scalar};
 use rustc_target::abi::{self, HasDataLayout, Pointer, Size};
@@ -159,13 +158,13 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         None
     }
 
+    fn zst_to_backend(&self, _ty: Type<'gcc>) -> RValue<'gcc> {
+        self.const_undef(self.type_ix(0))
+    }
+
     fn scalar_to_backend(&self, cv: Scalar, layout: abi::Scalar, ty: Type<'gcc>) -> RValue<'gcc> {
         let bitsize = if layout.is_bool() { 1 } else { layout.size(self).bits() };
         match cv {
-            Scalar::Int(ScalarInt::ZST) => {
-                assert_eq!(0, layout.size(self).bytes());
-                self.const_undef(self.type_ix(0))
-            }
             Scalar::Int(int) => {
                 let data = int.assert_bits(layout.size(self));