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 #98831. Only the commits starting from "don't allow ZST in ScalarInt" are new.

r? `@oli-obk`

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -167,6 +167,7 @@ pub(crate) fn codegen_const_value<'tcx>(
     }
 
     match const_val {
+        ConstValue::ZeroSized => unreachable!(), // we already handles ZST above
         ConstValue::Scalar(x) => match x {
             Scalar::Int(int) => {
                 if fx.clif_type(layout.ty).is_some() {

compiler/rustc_codegen_gcc/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));
 

compiler/rustc_codegen_llvm/src/common.rs

@@ -13,7 +13,6 @@ use rustc_codegen_ssa::traits::*;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::{ConstAllocation, GlobalAlloc, Scalar};
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
-use rustc_middle::ty::ScalarInt;
 use rustc_target::abi::{self, AddressSpace, HasDataLayout, Pointer, Size};
 
 use libc::{c_char, c_uint};
@@ -223,13 +222,13 @@ 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 {
-            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));
                 let llval = self.const_uint_big(self.type_ix(bitsize), data);

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -84,6 +84,10 @@ 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::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

@@ -29,6 +29,7 @@ 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>,

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::ZeroSized
             }
         }
     };

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::ZeroSized
         }
         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::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.

compiler/rustc_middle/src/lib.rs

@@ -59,6 +59,7 @@
 #![feature(drain_filter)]
 #![feature(intra_doc_pointers)]
 #![feature(yeet_expr)]
+#![feature(const_option)]
 #![recursion_limit = "512"]
 #![allow(rustc::potential_query_instability)]
 

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -29,11 +29,14 @@ pub struct ConstAlloc<'tcx> {
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Hash)]
 #[derive(HashStable)]
 pub enum ConstValue<'tcx> {
-    /// Used only for types with `layout::abi::Scalar` ABI and ZSTs.
+    /// Used only for types with `layout::abi::Scalar` ABI.
     ///
     /// Not using the enum `Value` to encode that this must not be `Uninit`.
     Scalar(Scalar),
 
+    /// Only used for ZSTs.
+    ZeroSized,
+
     /// Used only for `&[u8]` and `&str`
     Slice { data: ConstAllocation<'tcx>, start: usize, end: usize },
 
@@ -55,6 +58,7 @@ impl<'a, 'tcx> Lift<'tcx> for ConstValue<'a> {
     fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ConstValue<'tcx>> {
         Some(match self {
             ConstValue::Scalar(s) => ConstValue::Scalar(s),
+            ConstValue::ZeroSized => ConstValue::ZeroSized,
             ConstValue::Slice { data, start, end } => {
                 ConstValue::Slice { data: tcx.lift(data)?, start, end }
             }
@@ -69,7 +73,7 @@ impl<'tcx> ConstValue<'tcx> {
     #[inline]
     pub fn try_to_scalar(&self) -> Option<Scalar<AllocId>> {
         match *self {
-            ConstValue::ByRef { .. } | ConstValue::Slice { .. } => None,
+            ConstValue::ByRef { .. } | ConstValue::Slice { .. } | ConstValue::ZeroSized => None,
             ConstValue::Scalar(val) => Some(val),
         }
     }
@@ -111,10 +115,6 @@ impl<'tcx> ConstValue<'tcx> {
     pub fn from_machine_usize(i: u64, cx: &impl HasDataLayout) -> Self {
         ConstValue::Scalar(Scalar::from_machine_usize(i, cx))
     }
-
-    pub fn zst() -> Self {
-        Self::Scalar(Scalar::ZST)
-    }
 }
 
 /// A `Scalar` represents an immediate, primitive value existing outside of a
@@ -194,8 +194,6 @@ impl<Tag> From<ScalarInt> for Scalar<Tag> {
 }
 
 impl<Tag> Scalar<Tag> {
-    pub const ZST: Self = Scalar::Int(ScalarInt::ZST);
-
     #[inline(always)]
     pub fn from_pointer(ptr: Pointer<Tag>, cx: &impl HasDataLayout) -> Self {
         Scalar::Ptr(ptr, u8::try_from(cx.pointer_size().bytes()).unwrap())

compiler/rustc_middle/src/mir/mod.rs

@@ -1711,7 +1711,7 @@ impl<'tcx> Operand<'tcx> {
         Operand::Constant(Box::new(Constant {
             span,
             user_ty: None,
-            literal: ConstantKind::Val(ConstValue::zst(), ty),
+            literal: ConstantKind::Val(ConstValue::ZeroSized, ty),
         }))
     }
 
@@ -2196,7 +2196,7 @@ impl<'tcx> ConstantKind<'tcx> {
 
     #[inline]
     pub fn zero_sized(ty: Ty<'tcx>) -> Self {
-        let cv = ConstValue::Scalar(Scalar::ZST);
+        let cv = ConstValue::ZeroSized;
         Self::Val(cv, ty)
     }
 
@@ -2772,6 +2772,13 @@ fn pretty_print_const_value<'tcx>(
                 fmt.write_str(&cx.into_buffer())?;
                 return Ok(());
             }
+            (ConstValue::ZeroSized, ty::FnDef(d, s)) => {
+                let mut cx = FmtPrinter::new(tcx, Namespace::ValueNS);
+                cx.print_alloc_ids = true;
+                let cx = cx.print_value_path(*d, s)?;
+                fmt.write_str(&cx.into_buffer())?;
+                return Ok(());
+            }
             // FIXME(oli-obk): also pretty print arrays and other aggregate constants by reading
             // their fields instead of just dumping the memory.
             _ => {}

compiler/rustc_middle/src/mir/pretty.rs

@@ -448,7 +448,9 @@ impl<'tcx> Visitor<'tcx> for ExtraComments<'tcx> {
                 self.push(&format!("+ user_ty: {:?}", user_ty));
             }
 
+            // FIXME: this is a poor version of `pretty_print_const_value`.
             let fmt_val = |val: &ConstValue<'tcx>| match val {
+                ConstValue::ZeroSized => format!("<ZST>"),
                 ConstValue::Scalar(s) => format!("Scalar({:?})", s),
                 ConstValue::Slice { .. } => format!("Slice(..)"),
                 ConstValue::ByRef { .. } => format!("ByRef(..)"),
@@ -679,6 +681,7 @@ pub fn write_allocations<'tcx>(
             ConstValue::Scalar(interpret::Scalar::Int { .. }) => {
                 Either::Left(Either::Right(std::iter::empty()))
             }
+            ConstValue::ZeroSized => Either::Left(Either::Right(std::iter::empty())),
             ConstValue::ByRef { alloc, .. } | ConstValue::Slice { data: alloc, .. } => {
                 Either::Right(alloc_ids_from_alloc(alloc))
             }

compiler/rustc_middle/src/thir.rs

@@ -419,6 +419,10 @@ pub enum ExprKind<'tcx> {
         lit: ty::ScalarInt,
         user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
     },
+    /// A literal of a ZST type.
+    ZstLiteral {
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
     /// Associated constants and named constants
     NamedConst {
         def_id: DefId,
@@ -454,12 +458,6 @@ pub enum ExprKind<'tcx> {
     },
 }
 
-impl<'tcx> ExprKind<'tcx> {
-    pub fn zero_sized_literal(user_ty: Option<Canonical<'tcx, UserType<'tcx>>>) -> Self {
-        ExprKind::NonHirLiteral { lit: ty::ScalarInt::ZST, user_ty }
-    }
-}
-
 /// Represents the association of a field identifier and an expression.
 ///
 /// This is used in struct constructors.

compiler/rustc_middle/src/thir/visit.rs

@@ -129,6 +129,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
         Closure { closure_id: _, substs: _, upvars: _, movability: _, fake_reads: _ } => {}
         Literal { lit: _, neg: _ } => {}
         NonHirLiteral { lit: _, user_ty: _ } => {}
+        ZstLiteral { user_ty: _ } => {}
         NamedConst { def_id: _, substs: _, user_ty: _ } => {}
         ConstParam { param: _, def_id: _ } => {}
         StaticRef { alloc_id: _, ty: _, def_id: _ } => {}