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`

13 files changed, 185 insertions, 299 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,
                     )
                 }
diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index 14eb2a1537b..07dbd80e077 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -123,10 +123,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match src.layout.ty.kind() {
             // Floating point
             Float(FloatTy::F32) => {
-                return Ok(self.cast_from_float(src.to_scalar()?.to_f32()?, cast_ty).into());
+                return Ok(self.cast_from_float(src.to_scalar().to_f32()?, cast_ty).into());
             }
             Float(FloatTy::F64) => {
-                return Ok(self.cast_from_float(src.to_scalar()?.to_f64()?, cast_ty).into());
+                return Ok(self.cast_from_float(src.to_scalar().to_f64()?, cast_ty).into());
             }
             // The rest is integer/pointer-"like", including fn ptr casts
             _ => assert!(
@@ -153,7 +153,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 assert_eq!(dest_layout.size, self.pointer_size());
                 assert!(src.layout.ty.is_unsafe_ptr());
                 return match **src {
-                    Immediate::ScalarPair(data, _) => Ok(data.check_init()?.into()),
+                    Immediate::ScalarPair(data, _) => Ok(data.into()),
                     Immediate::Scalar(..) => span_bug!(
                         self.cur_span(),
                         "{:?} input to a fat-to-thin cast ({:?} -> {:?})",
@@ -167,7 +167,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
 
         // # The remaining source values are scalar and "int-like".
-        let scalar = src.to_scalar()?;
+        let scalar = src.to_scalar();
         Ok(self.cast_from_int_like(scalar, src.layout, cast_ty)?.into())
     }
 
@@ -179,7 +179,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         assert_matches!(src.layout.ty.kind(), ty::RawPtr(_) | ty::FnPtr(_));
         assert!(cast_ty.is_integral());
 
-        let scalar = src.to_scalar()?;
+        let scalar = src.to_scalar();
         let ptr = scalar.to_pointer(self)?;
         match ptr.into_pointer_or_addr() {
             Ok(ptr) => M::expose_ptr(self, ptr)?,
@@ -197,7 +197,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         assert_matches!(cast_ty.kind(), ty::RawPtr(_));
 
         // First cast to usize.
-        let scalar = src.to_scalar()?;
+        let scalar = src.to_scalar();
         let addr = self.cast_from_int_like(scalar, src.layout, self.tcx.types.usize)?;
         let addr = addr.to_machine_usize(self)?;
 
@@ -291,7 +291,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         match (&src_pointee_ty.kind(), &dest_pointee_ty.kind()) {
             (&ty::Array(_, length), &ty::Slice(_)) => {
-                let ptr = self.read_immediate(src)?.to_scalar()?;
+                let ptr = self.read_scalar(src)?;
                 // u64 cast is from usize to u64, which is always good
                 let val =
                     Immediate::new_slice(ptr, length.eval_usize(*self.tcx, self.param_env), self);
@@ -303,7 +303,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     // A NOP cast that doesn't actually change anything, should be allowed even with mismatching vtables.
                     return self.write_immediate(*val, dest);
                 }
-                let (old_data, old_vptr) = val.to_scalar_pair()?;
+                let (old_data, old_vptr) = val.to_scalar_pair();
                 let old_vptr = old_vptr.to_pointer(self)?;
                 let (ty, old_trait) = self.get_ptr_vtable(old_vptr)?;
                 if old_trait != data_a.principal() {
@@ -315,7 +315,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             (_, &ty::Dynamic(ref data, _)) => {
                 // Initial cast from sized to dyn trait
                 let vtable = self.get_vtable_ptr(src_pointee_ty, data.principal())?;
-                let ptr = self.read_immediate(src)?.to_scalar()?;
+                let ptr = self.read_scalar(src)?;
                 let val = Immediate::new_dyn_trait(ptr, vtable, &*self.tcx);
                 self.write_immediate(val, dest)
             }
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 150d6589b08..92596d059cd 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -21,7 +21,7 @@ use rustc_target::abi::{call::FnAbi, Align, HasDataLayout, Size, TargetDataLayou
 use super::{
     AllocId, GlobalId, Immediate, InterpErrorInfo, InterpResult, MPlaceTy, Machine, MemPlace,
     MemPlaceMeta, Memory, MemoryKind, Operand, Place, PlaceTy, PointerArithmetic, Provenance,
-    Scalar, ScalarMaybeUninit, StackPopJump,
+    Scalar, StackPopJump,
 };
 use crate::transform::validate::equal_up_to_regions;
 
@@ -991,16 +991,16 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> std::fmt::Debug
                     }
                     LocalValue::Live(Operand::Immediate(Immediate::Scalar(val))) => {
                         write!(fmt, " {:?}", val)?;
-                        if let ScalarMaybeUninit::Scalar(Scalar::Ptr(ptr, _size)) = val {
+                        if let Scalar::Ptr(ptr, _size) = val {
                             allocs.push(ptr.provenance.get_alloc_id());
                         }
                     }
                     LocalValue::Live(Operand::Immediate(Immediate::ScalarPair(val1, val2))) => {
                         write!(fmt, " ({:?}, {:?})", val1, val2)?;
-                        if let ScalarMaybeUninit::Scalar(Scalar::Ptr(ptr, _size)) = val1 {
+                        if let Scalar::Ptr(ptr, _size) = val1 {
                             allocs.push(ptr.provenance.get_alloc_id());
                         }
-                        if let ScalarMaybeUninit::Scalar(Scalar::Ptr(ptr, _size)) = val2 {
+                        if let Scalar::Ptr(ptr, _size) = val2 {
                             allocs.push(ptr.provenance.get_alloc_id());
                         }
                     }
diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics.rs b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
index 08209eb7932..6f3bd3bf4c5 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
@@ -184,7 +184,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             | sym::bitreverse => {
                 let ty = substs.type_at(0);
                 let layout_of = self.layout_of(ty)?;
-                let val = self.read_scalar(&args[0])?.check_init()?;
+                let val = self.read_scalar(&args[0])?;
                 let bits = val.to_bits(layout_of.size)?;
                 let kind = match layout_of.abi {
                     Abi::Scalar(scalar) => scalar.primitive(),
@@ -256,7 +256,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let (val, overflowed, _ty) = self.overflowing_binary_op(bin_op, &l, &r)?;
                 if overflowed {
                     let layout = self.layout_of(substs.type_at(0))?;
-                    let r_val = r.to_scalar()?.to_bits(layout.size)?;
+                    let r_val = r.to_scalar().to_bits(layout.size)?;
                     if let sym::unchecked_shl | sym::unchecked_shr = intrinsic_name {
                         throw_ub_format!("overflowing shift by {} in `{}`", r_val, intrinsic_name);
                     } else {
@@ -269,9 +269,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // rotate_left: (X << (S % BW)) | (X >> ((BW - S) % BW))
                 // rotate_right: (X << ((BW - S) % BW)) | (X >> (S % BW))
                 let layout = self.layout_of(substs.type_at(0))?;
-                let val = self.read_scalar(&args[0])?.check_init()?;
+                let val = self.read_scalar(&args[0])?;
                 let val_bits = val.to_bits(layout.size)?;
-                let raw_shift = self.read_scalar(&args[1])?.check_init()?;
+                let raw_shift = self.read_scalar(&args[1])?;
                 let raw_shift_bits = raw_shift.to_bits(layout.size)?;
                 let width_bits = u128::from(layout.size.bits());
                 let shift_bits = raw_shift_bits % width_bits;
@@ -507,7 +507,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 self.copy_op(&args[0], dest, /*allow_transmute*/ false)?;
             }
             sym::assume => {
-                let cond = self.read_scalar(&args[0])?.check_init()?.to_bool()?;
+                let cond = self.read_scalar(&args[0])?.to_bool()?;
                 if !cond {
                     throw_ub_format!("`assume` intrinsic called with `false`");
                 }
@@ -570,7 +570,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // term since the sign of the second term can be inferred from this and
                 // the fact that the operation has overflowed (if either is 0 no
                 // overflow can occur)
-                let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?;
+                let first_term: u128 = l.to_scalar().to_bits(l.layout.size)?;
                 let first_term_positive = first_term & (1 << (num_bits - 1)) == 0;
                 if first_term_positive {
                     // Negative overflow not possible since the positive first term
diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
index dedbcf43755..6bed8a7a007 100644
--- a/compiler/rustc_const_eval/src/interpret/machine.rs
+++ b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -123,18 +123,15 @@ pub trait Machine<'mir, 'tcx>: Sized {
     /// Whether memory accesses should be alignment-checked.
     fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
 
-    /// Whether, when checking alignment, we should `force_int` and thus support
+    /// Whether, when checking alignment, we should look at the actual address and thus support
     /// custom alignment logic based on whatever the integer address happens to be.
     ///
-    /// Requires Provenance::OFFSET_IS_ADDR to be true.
-    fn force_int_for_alignment_check(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
+    /// If this returns true, Provenance::OFFSET_IS_ADDR must be true.
+    fn use_addr_for_alignment_check(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
 
     /// Whether to enforce the validity invariant
     fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
 
-    /// Whether to enforce integers and floats being initialized.
-    fn enforce_number_init(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
-
     /// Whether function calls should be [ABI](CallAbi)-checked.
     fn enforce_abi(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         true
@@ -437,17 +434,12 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     type FrameExtra = ();
 
     #[inline(always)]
-    fn force_int_for_alignment_check(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        // We do not support `force_int`.
+    fn use_addr_for_alignment_check(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
+        // We do not support `use_addr`.
         false
     }
 
     #[inline(always)]
-    fn enforce_number_init(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        true
-    }
-
-    #[inline(always)]
     fn checked_binop_checks_overflow(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         true
     }
diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
index 98e0c8cd78e..c4e93770292 100644
--- a/compiler/rustc_const_eval/src/interpret/memory.rs
+++ b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -21,7 +21,6 @@ use rustc_target::abi::{Align, HasDataLayout, Size};
 use super::{
     alloc_range, AllocId, AllocMap, AllocRange, Allocation, CheckInAllocMsg, GlobalAlloc, InterpCx,
     InterpResult, Machine, MayLeak, Pointer, PointerArithmetic, Provenance, Scalar,
-    ScalarMaybeUninit,
 };
 
 #[derive(Debug, PartialEq, Copy, Clone)]
@@ -445,8 +444,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // Test align. Check this last; if both bounds and alignment are violated
                 // we want the error to be about the bounds.
                 if let Some(align) = align {
-                    if M::force_int_for_alignment_check(self) {
-                        // `force_int_for_alignment_check` can only be true if `OFFSET_IS_ADDR` is true.
+                    if M::use_addr_for_alignment_check(self) {
+                        // `use_addr_for_alignment_check` can only be true if `OFFSET_IS_ADDR` is true.
                         check_offset_align(ptr.addr().bytes(), align)?;
                     } else {
                         // Check allocation alignment and offset alignment.
@@ -901,11 +900,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> std::fmt::Debug for DumpAllocs<'a,
 /// Reading and writing.
 impl<'tcx, 'a, Prov: Provenance, Extra> AllocRefMut<'a, 'tcx, Prov, Extra> {
     /// `range` is relative to this allocation reference, not the base of the allocation.
-    pub fn write_scalar(
-        &mut self,
-        range: AllocRange,
-        val: ScalarMaybeUninit<Prov>,
-    ) -> InterpResult<'tcx> {
+    pub fn write_scalar(&mut self, range: AllocRange, val: Scalar<Prov>) -> InterpResult<'tcx> {
         let range = self.range.subrange(range);
         debug!("write_scalar at {:?}{range:?}: {val:?}", self.alloc_id);
         Ok(self
@@ -915,11 +910,7 @@ impl<'tcx, 'a, Prov: Provenance, Extra> AllocRefMut<'a, 'tcx, Prov, Extra> {
     }
 
     /// `offset` is relative to this allocation reference, not the base of the allocation.
-    pub fn write_ptr_sized(
-        &mut self,
-        offset: Size,
-        val: ScalarMaybeUninit<Prov>,
-    ) -> InterpResult<'tcx> {
+    pub fn write_ptr_sized(&mut self, offset: Size, val: Scalar<Prov>) -> InterpResult<'tcx> {
         self.write_scalar(alloc_range(offset, self.tcx.data_layout().pointer_size), val)
     }
 
@@ -938,7 +929,7 @@ impl<'tcx, 'a, Prov: Provenance, Extra> AllocRef<'a, 'tcx, Prov, Extra> {
         &self,
         range: AllocRange,
         read_provenance: bool,
-    ) -> InterpResult<'tcx, ScalarMaybeUninit<Prov>> {
+    ) -> InterpResult<'tcx, Scalar<Prov>> {
         let range = self.range.subrange(range);
         let res = self
             .alloc
@@ -949,12 +940,12 @@ impl<'tcx, 'a, Prov: Provenance, Extra> AllocRef<'a, 'tcx, Prov, Extra> {
     }
 
     /// `range` is relative to this allocation reference, not the base of the allocation.
-    pub fn read_integer(&self, range: AllocRange) -> InterpResult<'tcx, ScalarMaybeUninit<Prov>> {
+    pub fn read_integer(&self, range: AllocRange) -> InterpResult<'tcx, Scalar<Prov>> {
         self.read_scalar(range, /*read_provenance*/ false)
     }
 
     /// `offset` is relative to this allocation reference, not the base of the allocation.
-    pub fn read_pointer(&self, offset: Size) -> InterpResult<'tcx, ScalarMaybeUninit<Prov>> {
+    pub fn read_pointer(&self, offset: Size) -> InterpResult<'tcx, Scalar<Prov>> {
         self.read_scalar(
             alloc_range(offset, self.tcx.data_layout().pointer_size),
             /*read_provenance*/ true,
@@ -962,15 +953,10 @@ impl<'tcx, 'a, Prov: Provenance, Extra> AllocRef<'a, 'tcx, Prov, Extra> {
     }
 
     /// `range` is relative to this allocation reference, not the base of the allocation.
-    pub fn check_bytes(
-        &self,
-        range: AllocRange,
-        allow_uninit: bool,
-        allow_ptr: bool,
-    ) -> InterpResult<'tcx> {
+    pub fn check_bytes(&self, range: AllocRange) -> InterpResult<'tcx> {
         Ok(self
             .alloc
-            .check_bytes(&self.tcx, self.range.subrange(range), allow_uninit, allow_ptr)
+            .check_bytes(&self.tcx, self.range.subrange(range))
             .map_err(|e| e.to_interp_error(self.alloc_id))?)
     }
 
diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index fe80a55dfd2..e80a82acd58 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -1,11 +1,9 @@
 //! Functions concerning immediate values and operands, and reading from operands.
 //! All high-level functions to read from memory work on operands as sources.
 
-use std::fmt::Write;
-
 use rustc_hir::def::Namespace;
 use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt, TyAndLayout};
-use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Printer};
+use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter};
 use rustc_middle::ty::{ConstInt, DelaySpanBugEmitted, Ty};
 use rustc_middle::{mir, ty};
 use rustc_target::abi::{self, Abi, Align, HasDataLayout, Size, TagEncoding};
@@ -14,7 +12,7 @@ use rustc_target::abi::{VariantIdx, Variants};
 use super::{
     alloc_range, from_known_layout, mir_assign_valid_types, AllocId, ConstValue, Frame, GlobalId,
     InterpCx, InterpResult, MPlaceTy, Machine, MemPlace, MemPlaceMeta, Place, PlaceTy, Pointer,
-    Provenance, Scalar, ScalarMaybeUninit,
+    Provenance, Scalar,
 };
 
 /// An `Immediate` represents a single immediate self-contained Rust value.
@@ -27,23 +25,14 @@ use super::{
 #[derive(Copy, Clone, Debug)]
 pub enum Immediate<Prov: Provenance = AllocId> {
     /// A single scalar value (must have *initialized* `Scalar` ABI).
-    /// FIXME: we also currently often use this for ZST.
-    /// `ScalarMaybeUninit` should reject ZST, and we should use `Uninit` for them instead.
-    Scalar(ScalarMaybeUninit<Prov>),
+    Scalar(Scalar<Prov>),
     /// A pair of two scalar value (must have `ScalarPair` ABI where both fields are
     /// `Scalar::Initialized`).
-    ScalarPair(ScalarMaybeUninit<Prov>, ScalarMaybeUninit<Prov>),
+    ScalarPair(Scalar<Prov>, Scalar<Prov>),
     /// A value of fully uninitialized memory. Can have and size and layout.
     Uninit,
 }
 
-impl<Prov: Provenance> From<ScalarMaybeUninit<Prov>> for Immediate<Prov> {
-    #[inline(always)]
-    fn from(val: ScalarMaybeUninit<Prov>) -> Self {
-        Immediate::Scalar(val)
-    }
-}
-
 impl<Prov: Provenance> From<Scalar<Prov>> for Immediate<Prov> {
     #[inline(always)]
     fn from(val: Scalar<Prov>) -> Self {
@@ -51,13 +40,13 @@ impl<Prov: Provenance> From<Scalar<Prov>> for Immediate<Prov> {
     }
 }
 
-impl<'tcx, Prov: Provenance> Immediate<Prov> {
+impl<Prov: Provenance> Immediate<Prov> {
     pub fn from_pointer(p: Pointer<Prov>, cx: &impl HasDataLayout) -> Self {
-        Immediate::Scalar(ScalarMaybeUninit::from_pointer(p, cx))
+        Immediate::Scalar(Scalar::from_pointer(p, cx))
     }
 
     pub fn from_maybe_pointer(p: Pointer<Option<Prov>>, cx: &impl HasDataLayout) -> Self {
-        Immediate::Scalar(ScalarMaybeUninit::from_maybe_pointer(p, cx))
+        Immediate::Scalar(Scalar::from_maybe_pointer(p, cx))
     }
 
     pub fn new_slice(val: Scalar<Prov>, len: u64, cx: &impl HasDataLayout) -> Self {
@@ -69,41 +58,28 @@ impl<'tcx, Prov: Provenance> Immediate<Prov> {
         vtable: Pointer<Option<Prov>>,
         cx: &impl HasDataLayout,
     ) -> Self {
-        Immediate::ScalarPair(val.into(), ScalarMaybeUninit::from_maybe_pointer(vtable, cx))
+        Immediate::ScalarPair(val.into(), Scalar::from_maybe_pointer(vtable, cx))
     }
 
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
-    pub fn to_scalar_or_uninit(self) -> ScalarMaybeUninit<Prov> {
+    pub fn to_scalar(self) -> Scalar<Prov> {
         match self {
             Immediate::Scalar(val) => val,
             Immediate::ScalarPair(..) => bug!("Got a scalar pair where a scalar was expected"),
-            Immediate::Uninit => ScalarMaybeUninit::Uninit,
+            Immediate::Uninit => bug!("Got uninit where a scalar was expected"),
         }
     }
 
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
-    pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Prov>> {
-        self.to_scalar_or_uninit().check_init()
-    }
-
-    #[inline]
-    #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
-    pub fn to_scalar_or_uninit_pair(self) -> (ScalarMaybeUninit<Prov>, ScalarMaybeUninit<Prov>) {
+    pub fn to_scalar_pair(self) -> (Scalar<Prov>, Scalar<Prov>) {
         match self {
             Immediate::ScalarPair(val1, val2) => (val1, val2),
             Immediate::Scalar(..) => bug!("Got a scalar where a scalar pair was expected"),
-            Immediate::Uninit => (ScalarMaybeUninit::Uninit, ScalarMaybeUninit::Uninit),
+            Immediate::Uninit => bug!("Got uninit where a scalar pair was expected"),
         }
     }
-
-    #[inline]
-    #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
-    pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Prov>, Scalar<Prov>)> {
-        let (val1, val2) = self.to_scalar_or_uninit_pair();
-        Ok((val1.check_init()?, val2.check_init()?))
-    }
 }
 
 // ScalarPair needs a type to interpret, so we often have an immediate and a type together
@@ -119,27 +95,17 @@ impl<Prov: Provenance> std::fmt::Display for ImmTy<'_, Prov> {
         /// Helper function for printing a scalar to a FmtPrinter
         fn p<'a, 'tcx, Prov: Provenance>(
             cx: FmtPrinter<'a, 'tcx>,
-            s: ScalarMaybeUninit<Prov>,
+            s: Scalar<Prov>,
             ty: Ty<'tcx>,
         ) -> Result<FmtPrinter<'a, 'tcx>, std::fmt::Error> {
             match s {
-                ScalarMaybeUninit::Scalar(Scalar::Int(int)) => {
-                    cx.pretty_print_const_scalar_int(int, ty, true)
-                }
-                ScalarMaybeUninit::Scalar(Scalar::Ptr(ptr, _sz)) => {
+                Scalar::Int(int) => cx.pretty_print_const_scalar_int(int, ty, true),
+                Scalar::Ptr(ptr, _sz) => {
                     // Just print the ptr value. `pretty_print_const_scalar_ptr` would also try to
                     // print what is points to, which would fail since it has no access to the local
                     // memory.
                     cx.pretty_print_const_pointer(ptr, ty, true)
                 }
-                ScalarMaybeUninit::Uninit => cx.typed_value(
-                    |mut this| {
-                        this.write_str("uninit ")?;
-                        Ok(this)
-                    },
-                    |this| this.print_type(ty),
-                    " ",
-                ),
             }
         }
         ty::tls::with(|tcx| {
@@ -269,7 +235,7 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
     #[inline]
     pub fn to_const_int(self) -> ConstInt {
         assert!(self.layout.ty.is_integral());
-        let int = self.to_scalar().expect("to_const_int doesn't work on scalar pairs").assert_int();
+        let int = self.to_scalar().assert_int();
         ConstInt::new(int, self.layout.ty.is_signed(), self.layout.ty.is_ptr_sized_integral())
     }
 }
@@ -327,7 +293,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     fn read_immediate_from_mplace_raw(
         &self,
         mplace: &MPlaceTy<'tcx, M::Provenance>,
-        force: bool,
     ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::Provenance>>> {
         if mplace.layout.is_unsized() {
             // Don't touch unsized
@@ -345,47 +310,44 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // case where some of the bytes are initialized and others are not. So, we need an extra
         // check that walks over the type of `mplace` to make sure it is truly correct to treat this
         // like a `Scalar` (or `ScalarPair`).
-        let scalar_layout = match mplace.layout.abi {
-            // `if` does not work nested inside patterns, making this a bit awkward to express.
-            Abi::Scalar(abi::Scalar::Initialized { value: s, .. }) => Some(s),
-            Abi::Scalar(s) if force => Some(s.primitive()),
-            _ => None,
-        };
-        if let Some(s) = scalar_layout {
-            let size = s.size(self);
-            assert_eq!(size, mplace.layout.size, "abi::Scalar size does not match layout size");
-            let scalar = alloc
-                .read_scalar(alloc_range(Size::ZERO, size), /*read_provenance*/ s.is_ptr())?;
-            return Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }));
-        }
-        let scalar_pair_layout = match mplace.layout.abi {
+        Ok(match mplace.layout.abi {
+            Abi::Scalar(abi::Scalar::Initialized { value: s, .. }) => {
+                let size = s.size(self);
+                assert_eq!(size, mplace.layout.size, "abi::Scalar size does not match layout size");
+                let scalar = alloc.read_scalar(
+                    alloc_range(Size::ZERO, size),
+                    /*read_provenance*/ s.is_ptr(),
+                )?;
+                Some(ImmTy { imm: scalar.into(), layout: mplace.layout })
+            }
             Abi::ScalarPair(
                 abi::Scalar::Initialized { value: a, .. },
                 abi::Scalar::Initialized { value: b, .. },
-            ) => Some((a, b)),
-            Abi::ScalarPair(a, b) if force => Some((a.primitive(), b.primitive())),
-            _ => None,
-        };
-        if let Some((a, b)) = scalar_pair_layout {
-            // We checked `ptr_align` above, so all fields will have the alignment they need.
-            // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
-            // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
-            let (a_size, b_size) = (a.size(self), b.size(self));
-            let b_offset = a_size.align_to(b.align(self).abi);
-            assert!(b_offset.bytes() > 0); // in `operand_field` we use the offset to tell apart the fields
-            let a_val = alloc.read_scalar(
-                alloc_range(Size::ZERO, a_size),
-                /*read_provenance*/ a.is_ptr(),
-            )?;
-            let b_val = alloc
-                .read_scalar(alloc_range(b_offset, b_size), /*read_provenance*/ b.is_ptr())?;
-            return Ok(Some(ImmTy {
-                imm: Immediate::ScalarPair(a_val, b_val),
-                layout: mplace.layout,
-            }));
-        }
-        // Neither a scalar nor scalar pair.
-        return Ok(None);
+            ) => {
+                // We checked `ptr_align` above, so all fields will have the alignment they need.
+                // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
+                // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
+                let (a_size, b_size) = (a.size(self), b.size(self));
+                let b_offset = a_size.align_to(b.align(self).abi);
+                assert!(b_offset.bytes() > 0); // in `operand_field` we use the offset to tell apart the fields
+                let a_val = alloc.read_scalar(
+                    alloc_range(Size::ZERO, a_size),
+                    /*read_provenance*/ a.is_ptr(),
+                )?;
+                let b_val = alloc.read_scalar(
+                    alloc_range(b_offset, b_size),
+                    /*read_provenance*/ b.is_ptr(),
+                )?;
+                Some(ImmTy {
+                    imm: Immediate::ScalarPair(a_val.into(), b_val.into()),
+                    layout: mplace.layout,
+                })
+            }
+            _ => {
+                // Neither a scalar nor scalar pair.
+                None
+            }
+        })
     }
 
     /// Try returning an immediate for the operand. If the layout does not permit loading this as an
@@ -394,20 +356,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// succeed!  Whether it succeeds depends on whether the layout can be represented
     /// in an `Immediate`, not on which data is stored there currently.
     ///
-    /// If `force` is `true`, then even scalars with fields that can be ununit will be
-    /// read. This means the load is lossy and should not be written back!
-    /// This flag exists only for validity checking.
-    ///
     /// This is an internal function that should not usually be used; call `read_immediate` instead.
     /// ConstProp needs it, though.
     pub fn read_immediate_raw(
         &self,
         src: &OpTy<'tcx, M::Provenance>,
-        force: bool,
     ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::Provenance>, MPlaceTy<'tcx, M::Provenance>>> {
         Ok(match src.try_as_mplace() {
             Ok(ref mplace) => {
-                if let Some(val) = self.read_immediate_from_mplace_raw(mplace, force)? {
+                if let Some(val) = self.read_immediate_from_mplace_raw(mplace)? {
                     Ok(val)
                 } else {
                     Err(*mplace)
@@ -418,24 +375,33 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     }
 
     /// Read an immediate from a place, asserting that that is possible with the given layout.
+    ///
+    /// If this suceeds, the `ImmTy` is never `Uninit`.
     #[inline(always)]
     pub fn read_immediate(
         &self,
         op: &OpTy<'tcx, M::Provenance>,
     ) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
-        if let Ok(imm) = self.read_immediate_raw(op, /*force*/ false)? {
-            Ok(imm)
-        } else {
-            span_bug!(self.cur_span(), "primitive read failed for type: {:?}", op.layout.ty);
+        if !matches!(
+            op.layout.abi,
+            Abi::Scalar(abi::Scalar::Initialized { .. })
+                | Abi::ScalarPair(abi::Scalar::Initialized { .. }, abi::Scalar::Initialized { .. })
+        ) {
+            span_bug!(self.cur_span(), "primitive read not possible for type: {:?}", op.layout.ty);
+        }
+        let imm = self.read_immediate_raw(op)?.unwrap();
+        if matches!(*imm, Immediate::Uninit) {
+            throw_ub!(InvalidUninitBytes(None));
         }
+        Ok(imm)
     }
 
     /// Read a scalar from a place
     pub fn read_scalar(
         &self,
         op: &OpTy<'tcx, M::Provenance>,
-    ) -> InterpResult<'tcx, ScalarMaybeUninit<M::Provenance>> {
-        Ok(self.read_immediate(op)?.to_scalar_or_uninit())
+    ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
+        Ok(self.read_immediate(op)?.to_scalar())
     }
 
     /// Read a pointer from a place.
@@ -727,7 +693,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // Figure out which discriminant and variant this corresponds to.
         Ok(match *tag_encoding {
             TagEncoding::Direct => {
-                let scalar = tag_val.to_scalar()?;
+                let scalar = tag_val.to_scalar();
                 // Generate a specific error if `tag_val` is not an integer.
                 // (`tag_bits` itself is only used for error messages below.)
                 let tag_bits = scalar
@@ -758,7 +724,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 (discr_val, index.0)
             }
             TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start } => {
-                let tag_val = tag_val.to_scalar()?;
+                let tag_val = tag_val.to_scalar();
                 // Compute the variant this niche value/"tag" corresponds to. With niche layout,
                 // discriminant (encoded in niche/tag) and variant index are the same.
                 let variants_start = niche_variants.start().as_u32();
@@ -785,9 +751,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
                         let variant_index_relative_val =
                             self.binary_op(mir::BinOp::Sub, &tag_val, &niche_start_val)?;
-                        let variant_index_relative = variant_index_relative_val
-                            .to_scalar()?
-                            .assert_bits(tag_val.layout.size);
+                        let variant_index_relative =
+                            variant_index_relative_val.to_scalar().assert_bits(tag_val.layout.size);
                         // Check if this is in the range that indicates an actual discriminant.
                         if variant_index_relative <= u128::from(variants_end - variants_start) {
                             let variant_index_relative = u32::try_from(variant_index_relative)
diff --git a/compiler/rustc_const_eval/src/interpret/operator.rs b/compiler/rustc_const_eval/src/interpret/operator.rs
index f9912d706fb..1f1d0665139 100644
--- a/compiler/rustc_const_eval/src/interpret/operator.rs
+++ b/compiler/rustc_const_eval/src/interpret/operator.rs
@@ -329,21 +329,21 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match left.layout.ty.kind() {
             ty::Char => {
                 assert_eq!(left.layout.ty, right.layout.ty);
-                let left = left.to_scalar()?;
-                let right = right.to_scalar()?;
+                let left = left.to_scalar();
+                let right = right.to_scalar();
                 Ok(self.binary_char_op(bin_op, left.to_char()?, right.to_char()?))
             }
             ty::Bool => {
                 assert_eq!(left.layout.ty, right.layout.ty);
-                let left = left.to_scalar()?;
-                let right = right.to_scalar()?;
+                let left = left.to_scalar();
+                let right = right.to_scalar();
                 Ok(self.binary_bool_op(bin_op, left.to_bool()?, right.to_bool()?))
             }
             ty::Float(fty) => {
                 assert_eq!(left.layout.ty, right.layout.ty);
                 let ty = left.layout.ty;
-                let left = left.to_scalar()?;
-                let right = right.to_scalar()?;
+                let left = left.to_scalar();
+                let right = right.to_scalar();
                 Ok(match fty {
                     FloatTy::F32 => {
                         self.binary_float_op(bin_op, ty, left.to_f32()?, right.to_f32()?)
@@ -363,8 +363,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     right.layout.ty
                 );
 
-                let l = left.to_scalar()?.to_bits(left.layout.size)?;
-                let r = right.to_scalar()?.to_bits(right.layout.size)?;
+                let l = left.to_scalar().to_bits(left.layout.size)?;
+                let r = right.to_scalar().to_bits(right.layout.size)?;
                 self.binary_int_op(bin_op, l, left.layout, r, right.layout)
             }
             _ if left.layout.ty.is_any_ptr() => {
@@ -410,7 +410,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         use rustc_middle::mir::UnOp::*;
 
         let layout = val.layout;
-        let val = val.to_scalar()?;
+        let val = val.to_scalar();
         trace!("Running unary op {:?}: {:?} ({:?})", un_op, val, layout.ty);
 
         match layout.ty.kind() {
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 97fe23cb5bc..7aa76fe1dae 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -13,7 +13,7 @@ use rustc_target::abi::{self, Abi, Align, HasDataLayout, Size, TagEncoding, Vari
 use super::{
     alloc_range, mir_assign_valid_types, AllocId, AllocRef, AllocRefMut, CheckInAllocMsg,
     ConstAlloc, ImmTy, Immediate, InterpCx, InterpResult, Machine, MemoryKind, OpTy, Operand,
-    Pointer, Provenance, Scalar, ScalarMaybeUninit,
+    Pointer, Provenance, Scalar,
 };
 
 #[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
@@ -254,8 +254,6 @@ impl<'tcx, Prov: Provenance> MPlaceTy<'tcx, Prov> {
 // These are defined here because they produce a place.
 impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
     #[inline(always)]
-    /// Note: do not call `as_ref` on the resulting place. This function should only be used to
-    /// read from the resulting mplace, not to get its address back.
     pub fn try_as_mplace(&self) -> Result<MPlaceTy<'tcx, Prov>, ImmTy<'tcx, Prov>> {
         match **self {
             Operand::Indirect(mplace) => {
@@ -267,8 +265,6 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
 
     #[inline(always)]
     #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
-    /// Note: do not call `as_ref` on the resulting place. This function should only be used to
-    /// read from the resulting mplace, not to get its address back.
     pub fn assert_mem_place(&self) -> MPlaceTy<'tcx, Prov> {
         self.try_as_mplace().unwrap()
     }
@@ -312,7 +308,7 @@ where
         let layout = self.layout_of(pointee_type)?;
         let (ptr, meta) = match **val {
             Immediate::Scalar(ptr) => (ptr, MemPlaceMeta::None),
-            Immediate::ScalarPair(ptr, meta) => (ptr, MemPlaceMeta::Meta(meta.check_init()?)),
+            Immediate::ScalarPair(ptr, meta) => (ptr, MemPlaceMeta::Meta(meta)),
             Immediate::Uninit => throw_ub!(InvalidUninitBytes(None)),
         };
 
@@ -467,7 +463,7 @@ where
     #[inline(always)]
     pub fn write_scalar(
         &mut self,
-        val: impl Into<ScalarMaybeUninit<M::Provenance>>,
+        val: impl Into<Scalar<M::Provenance>>,
         dest: &PlaceTy<'tcx, M::Provenance>,
     ) -> InterpResult<'tcx> {
         self.write_immediate(Immediate::Scalar(val.into()), dest)
@@ -644,7 +640,7 @@ where
 
         // Let us see if the layout is simple so we take a shortcut,
         // avoid force_allocation.
-        let src = match self.read_immediate_raw(src, /*force*/ false)? {
+        let src = match self.read_immediate_raw(src)? {
             Ok(src_val) => {
                 assert!(!src.layout.is_unsized(), "cannot have unsized immediates");
                 assert!(
diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index c8557d172ed..f23fa573100 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -129,8 +129,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Assert { ref cond, expected, ref msg, target, cleanup } => {
-                let cond_val =
-                    self.read_immediate(&self.eval_operand(cond, None)?)?.to_scalar()?.to_bool()?;
+                let cond_val = self.read_scalar(&self.eval_operand(cond, None)?)?.to_bool()?;
                 if expected == cond_val {
                     self.go_to_block(target);
                 } else {
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index f1b1855c3ec..e1555f68737 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -20,8 +20,8 @@ use rustc_target::abi::{Abi, Scalar as ScalarAbi, Size, VariantIdx, Variants, Wr
 use std::hash::Hash;
 
 use super::{
-    alloc_range, CheckInAllocMsg, GlobalAlloc, Immediate, InterpCx, InterpResult, MPlaceTy,
-    Machine, MemPlaceMeta, OpTy, Scalar, ScalarMaybeUninit, ValueVisitor,
+    alloc_range, CheckInAllocMsg, GlobalAlloc, ImmTy, Immediate, InterpCx, InterpResult, MPlaceTy,
+    Machine, MemPlaceMeta, OpTy, Scalar, ValueVisitor,
 };
 
 macro_rules! throw_validation_failure {
@@ -304,6 +304,27 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
         Ok(r)
     }
 
+    fn read_immediate(
+        &self,
+        op: &OpTy<'tcx, M::Provenance>,
+        expected: &str,
+    ) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
+        Ok(try_validation!(
+            self.ecx.read_immediate(op),
+            self.path,
+            err_unsup!(ReadPointerAsBytes) => { "(potentially part of) a pointer" } expected { "{expected}" },
+            err_ub!(InvalidUninitBytes(None)) => { "uninitialized memory" } expected { "{expected}" }
+        ))
+    }
+
+    fn read_scalar(
+        &self,
+        op: &OpTy<'tcx, M::Provenance>,
+        expected: &str,
+    ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
+        Ok(self.read_immediate(op, expected)?.to_scalar())
+    }
+
     fn check_wide_ptr_meta(
         &mut self,
         meta: MemPlaceMeta<M::Provenance>,
@@ -348,18 +369,9 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
         value: &OpTy<'tcx, M::Provenance>,
         kind: &str,
     ) -> InterpResult<'tcx> {
-        let value = try_validation!(
-            self.ecx.read_immediate(value),
-            self.path,
-            err_unsup!(ReadPointerAsBytes) => { "part of a pointer" } expected { "a proper pointer or integer value" },
-        );
+        let place = self.ecx.ref_to_mplace(&self.read_immediate(value, &format!("a {kind}"))?)?;
         // Handle wide pointers.
         // Check metadata early, for better diagnostics
-        let place = try_validation!(
-            self.ecx.ref_to_mplace(&value),
-            self.path,
-            err_ub!(InvalidUninitBytes(None)) => { "uninitialized {}", kind },
-        );
         if place.layout.is_unsized() {
             self.check_wide_ptr_meta(place.meta, place.layout)?;
         }
@@ -454,28 +466,6 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
         Ok(())
     }
 
-    fn read_scalar(
-        &self,
-        op: &OpTy<'tcx, M::Provenance>,
-    ) -> InterpResult<'tcx, ScalarMaybeUninit<M::Provenance>> {
-        Ok(try_validation!(
-            self.ecx.read_scalar(op),
-            self.path,
-            err_unsup!(ReadPointerAsBytes) => { "(potentially part of) a pointer" } expected { "plain (non-pointer) bytes" },
-        ))
-    }
-
-    fn read_immediate_forced(
-        &self,
-        op: &OpTy<'tcx, M::Provenance>,
-    ) -> InterpResult<'tcx, Immediate<M::Provenance>> {
-        Ok(*try_validation!(
-            self.ecx.read_immediate_raw(op, /*force*/ true),
-            self.path,
-            err_unsup!(ReadPointerAsBytes) => { "(potentially part of) a pointer" } expected { "plain (non-pointer) bytes" },
-        ).unwrap())
-    }
-
     /// Check if this is a value of primitive type, and if yes check the validity of the value
     /// at that type.  Return `true` if the type is indeed primitive.
     fn try_visit_primitive(
@@ -486,41 +476,39 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
         let ty = value.layout.ty;
         match ty.kind() {
             ty::Bool => {
-                let value = self.read_scalar(value)?;
+                let value = self.read_scalar(value, "a boolean")?;
                 try_validation!(
                     value.to_bool(),
                     self.path,
-                    err_ub!(InvalidBool(..)) | err_ub!(InvalidUninitBytes(None)) =>
+                    err_ub!(InvalidBool(..)) =>
                         { "{:x}", value } expected { "a boolean" },
                 );
                 Ok(true)
             }
             ty::Char => {
-                let value = self.read_scalar(value)?;
+                let value = self.read_scalar(value, "a unicode scalar value")?;
                 try_validation!(
                     value.to_char(),
                     self.path,
-                    err_ub!(InvalidChar(..)) | err_ub!(InvalidUninitBytes(None)) =>
+                    err_ub!(InvalidChar(..)) =>
                         { "{:x}", value } expected { "a valid unicode scalar value (in `0..=0x10FFFF` but not in `0xD800..=0xDFFF`)" },
                 );
                 Ok(true)
             }
             ty::Float(_) | ty::Int(_) | ty::Uint(_) => {
-                let value = self.read_scalar(value)?;
                 // NOTE: Keep this in sync with the array optimization for int/float
                 // types below!
-                if M::enforce_number_init(self.ecx) {
-                    try_validation!(
-                        value.check_init(),
-                        self.path,
-                        err_ub!(InvalidUninitBytes(..)) =>
-                            { "{:x}", value } expected { "initialized bytes" }
-                    );
-                }
+                let value = self.read_scalar(
+                    value,
+                    if matches!(ty.kind(), ty::Float(..)) {
+                        "a floating point number"
+                    } else {
+                        "an integer"
+                    },
+                )?;
                 // As a special exception we *do* match on a `Scalar` here, since we truly want
                 // to know its underlying representation (and *not* cast it to an integer).
-                let is_ptr = value.check_init().map_or(false, |v| matches!(v, Scalar::Ptr(..)));
-                if is_ptr {
+                if matches!(value, Scalar::Ptr(..)) {
                     throw_validation_failure!(self.path,
                         { "{:x}", value } expected { "plain (non-pointer) bytes" }
                     )
@@ -531,12 +519,8 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 // We are conservative with uninit for integers, but try to
                 // actually enforce the strict rules for raw pointers (mostly because
                 // that lets us re-use `ref_to_mplace`).
-                let place = try_validation!(
-                    self.ecx.read_immediate(value).and_then(|ref i| self.ecx.ref_to_mplace(i)),
-                    self.path,
-                    err_ub!(InvalidUninitBytes(None)) => { "uninitialized raw pointer" },
-                    err_unsup!(ReadPointerAsBytes) => { "part of a pointer" } expected { "a proper pointer or integer value" },
-                );
+                let place =
+                    self.ecx.ref_to_mplace(&self.read_immediate(value, "a raw pointer")?)?;
                 if place.layout.is_unsized() {
                     self.check_wide_ptr_meta(place.meta, place.layout)?;
                 }
@@ -557,12 +541,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 Ok(true)
             }
             ty::FnPtr(_sig) => {
-                let value = try_validation!(
-                    self.ecx.read_scalar(value).and_then(|v| v.check_init()),
-                    self.path,
-                    err_unsup!(ReadPointerAsBytes) => { "part of a pointer" } expected { "a proper pointer or integer value" },
-                    err_ub!(InvalidUninitBytes(None)) => { "uninitialized bytes" } expected { "a proper pointer or integer value" },
-                );
+                let value = self.read_scalar(value, "a function pointer")?;
 
                 // If we check references recursively, also check that this points to a function.
                 if let Some(_) = self.ref_tracking {
@@ -613,40 +592,15 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
 
     fn visit_scalar(
         &mut self,
-        scalar: ScalarMaybeUninit<M::Provenance>,
+        scalar: Scalar<M::Provenance>,
         scalar_layout: ScalarAbi,
     ) -> InterpResult<'tcx> {
-        // We check `is_full_range` in a slightly complicated way because *if* we are checking
-        // number validity, then we want to ensure that `Scalar::Initialized` is indeed initialized,
-        // i.e. that we go over the `check_init` below.
         let size = scalar_layout.size(self.ecx);
-        let is_full_range = match scalar_layout {
-            ScalarAbi::Initialized { .. } => {
-                if M::enforce_number_init(self.ecx) {
-                    false // not "full" since uninit is not accepted
-                } else {
-                    scalar_layout.is_always_valid(self.ecx)
-                }
-            }
-            ScalarAbi::Union { .. } => true,
-        };
-        if is_full_range {
-            // Nothing to check. Cruciall we don't even `read_scalar` until here, since that would
-            // fail for `Union` scalars!
-            return Ok(());
-        }
-        // We have something to check: it must at least be initialized.
         let valid_range = scalar_layout.valid_range(self.ecx);
         let WrappingRange { start, end } = valid_range;
         let max_value = size.unsigned_int_max();
         assert!(end <= max_value);
-        let value = try_validation!(
-            scalar.check_init(),
-            self.path,
-            err_ub!(InvalidUninitBytes(None)) => { "{:x}", scalar }
-                expected { "something {}", wrapping_range_format(valid_range, max_value) },
-        );
-        let bits = match value.try_to_int() {
+        let bits = match scalar.try_to_int() {
             Ok(int) => int.assert_bits(size),
             Err(_) => {
                 // So this is a pointer then, and casting to an int failed.
@@ -654,7 +608,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 // We support 2 kinds of ranges here: full range, and excluding zero.
                 if start == 1 && end == max_value {
                     // Only null is the niche.  So make sure the ptr is NOT null.
-                    if self.ecx.scalar_may_be_null(value)? {
+                    if self.ecx.scalar_may_be_null(scalar)? {
                         throw_validation_failure!(self.path,
                             { "a potentially null pointer" }
                             expected {
@@ -808,10 +762,11 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                 );
             }
             Abi::Scalar(scalar_layout) => {
-                // We use a 'forced' read because we always need a `Immediate` here
-                // and treating "partially uninit" as "fully uninit" is fine for us.
-                let scalar = self.read_immediate_forced(op)?.to_scalar_or_uninit();
-                self.visit_scalar(scalar, scalar_layout)?;
+                if !scalar_layout.is_uninit_valid() {
+                    // There is something to check here.
+                    let scalar = self.read_scalar(op, "initiailized scalar value")?;
+                    self.visit_scalar(scalar, scalar_layout)?;
+                }
             }
             Abi::ScalarPair(a_layout, b_layout) => {
                 // There is no `rustc_layout_scalar_valid_range_start` for pairs, so
@@ -819,10 +774,15 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                 // but that can miss bugs in layout computation. Layout computation
                 // is subtle due to enums having ScalarPair layout, where one field
                 // is the discriminant.
-                if cfg!(debug_assertions) {
-                    // We use a 'forced' read because we always need a `Immediate` here
-                    // and treating "partially uninit" as "fully uninit" is fine for us.
-                    let (a, b) = self.read_immediate_forced(op)?.to_scalar_or_uninit_pair();
+                if cfg!(debug_assertions)
+                    && !a_layout.is_uninit_valid()
+                    && !b_layout.is_uninit_valid()
+                {
+                    // We can only proceed if *both* scalars need to be initialized.
+                    // FIXME: find a way to also check ScalarPair when one side can be uninit but
+                    // the other must be init.
+                    let (a, b) =
+                        self.read_immediate(op, "initiailized scalar value")?.to_scalar_pair();
                     self.visit_scalar(a, a_layout)?;
                     self.visit_scalar(b, b_layout)?;
                 }
@@ -901,11 +861,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                 // We also accept uninit, for consistency with the slow path.
                 let alloc = self.ecx.get_ptr_alloc(mplace.ptr, size, mplace.align)?.expect("we already excluded size 0");
 
-                match alloc.check_bytes(
-                    alloc_range(Size::ZERO, size),
-                    /*allow_uninit*/ !M::enforce_number_init(self.ecx),
-                    /*allow_ptr*/ false,
-                ) {
+                match alloc.check_bytes(alloc_range(Size::ZERO, size)) {
                     // In the happy case, we needn't check anything else.
                     Ok(()) => {}
                     // Some error happened, try to provide a more detailed description.