Auto merge of #98957 - RalfJung:zst-are-different, r=lcnr,oli-obk

 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 I propose we stop using ScalarInt to represent ZST (which are clearly not integers). Instead, we can add new ZST variants to those types that did not have other variants which could be used for this purpose.

Based on https://github.com/rust-lang/rust/pull/98831. Only the commits starting from "don't allow ZST in ScalarInt" are new.

r? `@oli-obk`

1 files changed, 7 insertions, 6 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index 576958a855d..d11ae7b4925 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/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::ZeroSized => 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.