Remove dead broken code from const zst handling in backends

Author: oli-obk

compiler/rustc_codegen_gcc/src/common.rs

@@ -158,10 +158,6 @@ 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 {

compiler/rustc_codegen_llvm/src/common.rs

@@ -226,10 +226,6 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         })
     }
 
-    fn zst_to_backend(&self, _llty: &'ll Type) -> &'ll Value {
-        self.const_undef(self.type_ix(0))
-    }
-
     fn scalar_to_backend(&self, cv: Scalar, layout: abi::Scalar, llty: &'ll Type) -> &'ll Value {
         let bitsize = if layout.is_bool() { 1 } else { layout.size(self).bits() };
         match cv {

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -72,10 +72,6 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
     ) -> Self {
         let layout = bx.layout_of(ty);
 
-        if layout.is_zst() {
-            return OperandRef::new_zst(bx, layout);
-        }
-
         let val = match val {
             ConstValue::Scalar(x) => {
                 let Abi::Scalar(scalar) = layout.abi else {
@@ -84,10 +80,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 let llval = bx.scalar_to_backend(x, scalar, bx.immediate_backend_type(layout));
                 OperandValue::Immediate(llval)
             }
-            ConstValue::ZeroSized => {
-                let llval = bx.zst_to_backend(bx.immediate_backend_type(layout));
-                OperandValue::Immediate(llval)
-            }
+            ConstValue::ZeroSized => return OperandRef::new_zst(bx, layout),
             ConstValue::Slice { data, start, end } => {
                 let Abi::ScalarPair(a_scalar, _) = layout.abi else {
                     bug!("from_const: invalid ScalarPair layout: {:#?}", layout);

compiler/rustc_codegen_ssa/src/traits/consts.rs

@@ -29,7 +29,6 @@ pub trait ConstMethods<'tcx>: BackendTypes {
     fn const_data_from_alloc(&self, alloc: ConstAllocation<'tcx>) -> Self::Value;
 
     fn scalar_to_backend(&self, cv: Scalar, layout: abi::Scalar, llty: Self::Type) -> Self::Value;
-    fn zst_to_backend(&self, llty: Self::Type) -> Self::Value;
     fn from_const_alloc(
         &self,
         layout: TyAndLayout<'tcx>,