don't allow ZST in ScalarInt

There are several indications that we should not ZST as a ScalarInt:
- We had two ways to have ZST valtrees, either an empty `Branch` or a `Leaf` with a ZST in it.
  `ValTree::zst()` used the former, but the latter could possibly arise as well.
- Likewise, the interpreter had `Immediate::Uninit` and `Immediate::Scalar(Scalar::ZST)`.
- LLVM codegen already had to special-case ZST ScalarInt.

So instead add new ZST variants to those types that did not have other variants
which could be used for this purpose.

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -2,7 +2,7 @@ use super::{CompileTimeEvalContext, CompileTimeInterpreter, ConstEvalErr};
 use crate::interpret::eval_nullary_intrinsic;
 use crate::interpret::{
     intern_const_alloc_recursive, Allocation, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId,
-    Immediate, InternKind, InterpCx, InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking, Scalar,
+    Immediate, InternKind, InterpCx, InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking,
     ScalarMaybeUninit, StackPopCleanup,
 };
 
@@ -157,7 +157,7 @@ pub(super) fn op_to_const<'tcx>(
                     "this MPlaceTy must come from a validated constant, thus we can assume the \
                 alignment is correct",
                 );
-                ConstValue::Scalar(Scalar::ZST)
+                ConstValue::ZST
             }
         }
     };

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -272,7 +272,7 @@ pub fn valtree_to_const_value<'tcx>(
     match ty.kind() {
         ty::FnDef(..) => {
             assert!(valtree.unwrap_branch().is_empty());
-            ConstValue::Scalar(Scalar::ZST)
+            ConstValue::ZST
         }
         ty::Bool | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Char => match valtree {
             ty::ValTree::Leaf(scalar_int) => ConstValue::Scalar(Scalar::Int(scalar_int)),
@@ -344,11 +344,7 @@ fn valtree_into_mplace<'tcx>(
 
     match ty.kind() {
         ty::FnDef(_, _) => {
-            ecx.write_immediate(
-                Immediate::Scalar(ScalarMaybeUninit::Scalar(Scalar::ZST)),
-                &place.into(),
-            )
-            .unwrap();
+            ecx.write_immediate(Immediate::Uninit, &place.into()).unwrap();
         }
         ty::Bool | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Char => {
             let scalar_int = valtree.unwrap_leaf();

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -297,8 +297,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         let Some(alloc) = self.get_place_alloc(mplace)? else {
             return Ok(Some(ImmTy {
-                // zero-sized type
-                imm: Scalar::ZST.into(),
+                // zero-sized type can be left uninit
+                imm: Immediate::Uninit,
                 layout: mplace.layout,
             }));
         };
@@ -441,8 +441,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // This makes several assumptions about what layouts we will encounter; we match what
         // codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
         let field_val: Immediate<_> = match (*base, base.layout.abi) {
-            // the field contains no information
-            _ if field_layout.is_zst() => Scalar::ZST.into(),
+            // the field contains no information, can be left uninit
+            _ if field_layout.is_zst() => Immediate::Uninit,
             // the field covers the entire type
             _ if field_layout.size == base.layout.size => {
                 assert!(match (base.layout.abi, field_layout.abi) {
@@ -553,8 +553,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
         let layout = self.layout_of_local(frame, local, layout)?;
         let op = if layout.is_zst() {
-            // Do not read from ZST, they might not be initialized
-            Operand::Immediate(Scalar::ZST.into())
+            // Bypass `access_local` (helps in ConstProp)
+            Operand::Immediate(Immediate::Uninit)
         } else {
             *M::access_local(frame, local)?
         };
@@ -709,6 +709,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 Operand::Indirect(MemPlace::from_ptr(ptr.into()))
             }
             ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x)?.into()),
+            ConstValue::ZST => Operand::Immediate(Immediate::Uninit),
             ConstValue::Slice { data, start, end } => {
                 // We rely on mutability being set correctly in `data` to prevent writes
                 // where none should happen.