Auto merge of #100043 - RalfJung:scalar-always-init, r=RalfJung

interpret: remove support for uninitialized scalars

With Miri no longer supporting `-Zmiri-allow-uninit-numbers`, we no longer need to support storing uninit data in a `Scalar`. We anyway already only use this representation for types with *initialized* `Scalar` layout (and we have to, due to partial initialization), so let's get rid of the `ScalarMaybeUninit` type entirely.

I tried to stage this into meaningful commits, but the one that changes `read_immediate` to always trigger UB on uninit is the largest chunk of the PR and I don't see how it could be subdivided.

Fixes https://github.com/rust-lang/miri/issues/2187
r? `@oli-obk`

3 files changed, 9 insertions, 17 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index 5cfa63bd105..e13ad1c95bd 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -3,7 +3,7 @@ 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,
-    ScalarMaybeUninit, StackPopCleanup,
+    StackPopCleanup,
 };
 
 use rustc_hir::def::DefKind;
@@ -170,10 +170,7 @@ pub(super) fn op_to_const<'tcx>(
         // see comment on `let try_as_immediate` above
         Err(imm) => match *imm {
             _ if imm.layout.is_zst() => ConstValue::ZeroSized,
-            Immediate::Scalar(x) => match x {
-                ScalarMaybeUninit::Scalar(s) => ConstValue::Scalar(s),
-                ScalarMaybeUninit::Uninit => to_const_value(&op.assert_mem_place()),
-            },
+            Immediate::Scalar(x) => ConstValue::Scalar(x),
             Immediate::ScalarPair(a, b) => {
                 debug!("ScalarPair(a: {:?}, b: {:?})", a, b);
                 // We know `offset` is relative to the allocation, so we can use `into_parts`.
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index f24b19089c1..2a460c74b3d 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -347,8 +347,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         };
         match intrinsic_name {
             sym::ptr_guaranteed_eq | sym::ptr_guaranteed_ne => {
-                let a = ecx.read_immediate(&args[0])?.to_scalar()?;
-                let b = ecx.read_immediate(&args[1])?.to_scalar()?;
+                let a = ecx.read_scalar(&args[0])?;
+                let b = ecx.read_scalar(&args[1])?;
                 let cmp = if intrinsic_name == sym::ptr_guaranteed_eq {
                     ecx.guaranteed_eq(a, b)?
                 } else {
diff --git a/compiler/rustc_const_eval/src/const_eval/valtrees.rs b/compiler/rustc_const_eval/src/const_eval/valtrees.rs
index 8fff4571d12..373b139c86e 100644
--- a/compiler/rustc_const_eval/src/const_eval/valtrees.rs
+++ b/compiler/rustc_const_eval/src/const_eval/valtrees.rs
@@ -3,7 +3,7 @@ use super::machine::CompileTimeEvalContext;
 use super::{ValTreeCreationError, ValTreeCreationResult, VALTREE_MAX_NODES};
 use crate::interpret::{
     intern_const_alloc_recursive, ConstValue, ImmTy, Immediate, InternKind, MemPlaceMeta,
-    MemoryKind, PlaceTy, Scalar, ScalarMaybeUninit,
+    MemoryKind, PlaceTy, Scalar,
 };
 use crate::interpret::{MPlaceTy, Value};
 use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
@@ -90,7 +90,7 @@ pub(crate) fn const_to_valtree_inner<'tcx>(
             let Ok(val) = ecx.read_immediate(&place.into()) else {
                 return Err(ValTreeCreationError::Other);
             };
-            let val = val.to_scalar().unwrap();
+            let val = val.to_scalar();
             *num_nodes += 1;
 
             Ok(ty::ValTree::Leaf(val.assert_int()))
@@ -349,11 +349,7 @@ fn valtree_into_mplace<'tcx>(
         ty::Bool | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Char => {
             let scalar_int = valtree.unwrap_leaf();
             debug!("writing trivial valtree {:?} to place {:?}", scalar_int, place);
-            ecx.write_immediate(
-                Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar_int.into())),
-                &place.into(),
-            )
-            .unwrap();
+            ecx.write_immediate(Immediate::Scalar(scalar_int.into()), &place.into()).unwrap();
         }
         ty::Ref(_, inner_ty, _) => {
             let mut pointee_place = create_pointee_place(ecx, *inner_ty, valtree);
@@ -366,11 +362,10 @@ fn valtree_into_mplace<'tcx>(
             let imm = match inner_ty.kind() {
                 ty::Slice(_) | ty::Str => {
                     let len = valtree.unwrap_branch().len();
-                    let len_scalar =
-                        ScalarMaybeUninit::Scalar(Scalar::from_machine_usize(len as u64, &tcx));
+                    let len_scalar = Scalar::from_machine_usize(len as u64, &tcx);
 
                     Immediate::ScalarPair(
-                        ScalarMaybeUninit::from_maybe_pointer((*pointee_place).ptr, &tcx),
+                        Scalar::from_maybe_pointer((*pointee_place).ptr, &tcx),
                         len_scalar,
                     )
                 }