move marking-locals-live out of push_stack_frame, so it happens with argument passing

this entirely avoids even creating unsized locals in Immediate::Uninitialized state

compiler/rustc_const_eval/messages.ftl

@@ -384,7 +384,7 @@ const_eval_unreachable_unwind =
 
 const_eval_unsigned_offset_from_overflow =
     `ptr_offset_from_unsigned` called when first pointer has smaller offset than second: {$a_offset} < {$b_offset}
-
+const_eval_unsized_local = unsized locals are not supported
 const_eval_unstable_const_fn = `{$def_path}` is not yet stable as a const fn
 
 const_eval_unstable_in_stable =

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -61,6 +61,7 @@ fn eval_body_using_ecx<'mir, 'tcx>(
         &ret.clone().into(),
         StackPopCleanup::Root { cleanup: false },
     )?;
+    ecx.storage_live_for_always_live_locals()?;
 
     // The main interpreter loop.
     while ecx.step()? {}

compiler/rustc_const_eval/src/errors.rs

@@ -795,6 +795,7 @@ impl ReportErrorExt for UnsupportedOpInfo {
         use crate::fluent_generated::*;
         match self {
             UnsupportedOpInfo::Unsupported(s) => s.clone().into(),
+            UnsupportedOpInfo::UnsizedLocal => const_eval_unsized_local,
             UnsupportedOpInfo::OverwritePartialPointer(_) => const_eval_partial_pointer_overwrite,
             UnsupportedOpInfo::ReadPartialPointer(_) => const_eval_partial_pointer_copy,
             UnsupportedOpInfo::ReadPointerAsInt(_) => const_eval_read_pointer_as_int,
@@ -814,7 +815,7 @@ impl ReportErrorExt for UnsupportedOpInfo {
             // `ReadPointerAsInt(Some(info))` is never printed anyway, it only serves as an error to
             // be further processed by validity checking which then turns it into something nice to
             // print. So it's not worth the effort of having diagnostics that can print the `info`.
-            Unsupported(_) | ReadPointerAsInt(_) => {}
+            UnsizedLocal | Unsupported(_) | ReadPointerAsInt(_) => {}
             OverwritePartialPointer(ptr) | ReadPartialPointer(ptr) => {
                 builder.set_arg("ptr", ptr);
             }

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -158,7 +158,9 @@ pub enum StackPopCleanup {
 #[derive(Clone, Debug)]
 pub struct LocalState<'tcx, Prov: Provenance = AllocId> {
     pub value: LocalValue<Prov>,
-    /// Don't modify if `Some`, this is only used to prevent computing the layout twice
+    /// Don't modify if `Some`, this is only used to prevent computing the layout twice.
+    /// Layout needs to be computed lazily because ConstProp wants to run on frames where we can't
+    /// compute the layout of all locals.
     pub layout: Cell<Option<TyAndLayout<'tcx>>>,
 }
 
@@ -483,7 +485,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     }
 
     #[inline(always)]
-    pub(super) fn body(&self) -> &'mir mir::Body<'tcx> {
+    pub fn body(&self) -> &'mir mir::Body<'tcx> {
         self.frame().body
     }
 
@@ -705,15 +707,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         return_to_block: StackPopCleanup,
     ) -> InterpResult<'tcx> {
         trace!("body: {:#?}", body);
+        let dead_local = LocalState { value: LocalValue::Dead, layout: Cell::new(None) };
+        let locals = IndexVec::from_elem(dead_local, &body.local_decls);
         // First push a stack frame so we have access to the local args
         let pre_frame = Frame {
             body,
             loc: Right(body.span), // Span used for errors caused during preamble.
             return_to_block,
             return_place: return_place.clone(),
-            // empty local array, we fill it in below, after we are inside the stack frame and
+            locals,
-            // all methods actually know about the frame
-            locals: IndexVec::new(),
             instance,
             tracing_span: SpanGuard::new(),
             extra: (),
@@ -728,19 +730,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             self.eval_mir_constant(&ct, Some(span), None)?;
         }
 
-        // Most locals are initially dead.
-        let dummy = LocalState { value: LocalValue::Dead, layout: Cell::new(None) };
-        let mut locals = IndexVec::from_elem(dummy, &body.local_decls);
-
-        // Now mark those locals as live that have no `Storage*` annotations.
-        let always_live = always_storage_live_locals(self.body());
-        for local in locals.indices() {
-            if always_live.contains(local) {
-                locals[local].value = LocalValue::Live(Operand::Immediate(Immediate::Uninit));
-            }
-        }
         // done
-        self.frame_mut().locals = locals;
         M::after_stack_push(self)?;
         self.frame_mut().loc = Left(mir::Location::START);
 
@@ -907,11 +897,29 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
+    /// In the current stack frame, mark all locals as live that are not arguments and don't have
+    /// `Storage*` annotations (this includes the return place).
+    pub fn storage_live_for_always_live_locals(&mut self) -> InterpResult<'tcx> {
+        self.storage_live(mir::RETURN_PLACE)?;
+
+        let body = self.body();
+        let always_live = always_storage_live_locals(body);
+        for local in body.vars_and_temps_iter() {
+            if always_live.contains(local) {
+                self.storage_live(local)?;
+            }
+        }
+        Ok(())
+    }
+
     /// Mark a storage as live, killing the previous content.
     pub fn storage_live(&mut self, local: mir::Local) -> InterpResult<'tcx> {
-        assert!(local != mir::RETURN_PLACE, "Cannot make return place live");
         trace!("{:?} is now live", local);
 
+        if self.layout_of_local(self.frame(), local, None)?.is_unsized() {
+            throw_unsup!(UnsizedLocal);
+        }
+
         let local_val = LocalValue::Live(Operand::Immediate(Immediate::Uninit));
         // StorageLive expects the local to be dead, and marks it live.
         let old = mem::replace(&mut self.frame_mut().locals[local].value, local_val);

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -33,7 +33,7 @@ pub enum Immediate<Prov: Provenance = AllocId> {
     /// A pair of two scalar value (must have `ScalarPair` ABI where both fields are
     /// `Scalar::Initialized`).
     ScalarPair(Scalar<Prov>, Scalar<Prov>),
-    /// A value of fully uninitialized memory. Can have arbitrary size and layout.
+    /// A value of fully uninitialized memory. Can have arbitrary size and layout, but must be sized.
     Uninit,
 }
 
@@ -190,16 +190,19 @@ impl<'tcx, Prov: Provenance> From<ImmTy<'tcx, Prov>> for OpTy<'tcx, Prov> {
 impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
     #[inline]
     pub fn from_scalar(val: Scalar<Prov>, layout: TyAndLayout<'tcx>) -> Self {
+        debug_assert!(layout.abi.is_scalar(), "`ImmTy::from_scalar` on non-scalar layout");
         ImmTy { imm: val.into(), layout }
     }
 
-    #[inline]
+    #[inline(always)]
     pub fn from_immediate(imm: Immediate<Prov>, layout: TyAndLayout<'tcx>) -> Self {
+        debug_assert!(layout.is_sized(), "immediates must be sized");
         ImmTy { imm, layout }
     }
 
     #[inline]
     pub fn uninit(layout: TyAndLayout<'tcx>) -> Self {
+        debug_assert!(layout.is_sized(), "immediates must be sized");
         ImmTy { imm: Immediate::Uninit, layout }
     }
 
@@ -322,15 +325,12 @@ impl<'tcx, Prov: Provenance + 'static> Projectable<'tcx, Prov> for OpTy<'tcx, Pr
         self.layout
     }
 
+    #[inline]
     fn meta(&self) -> InterpResult<'tcx, MemPlaceMeta<Prov>> {
         Ok(match self.as_mplace_or_imm() {
             Left(mplace) => mplace.meta,
             Right(_) => {
-                if self.layout.is_unsized() {
+                debug_assert!(self.layout.is_sized(), "unsized immediates are not a thing");
-                    // Unsized immediate OpTy cannot occur. We create a MemPlace for all unsized locals during argument passing.
-                    // However, ConstProp doesn't do that, so we can run into this nonsense situation.
-                    throw_inval!(ConstPropNonsense);
-                }
                 MemPlaceMeta::None
             }
         })
@@ -346,9 +346,10 @@ impl<'tcx, Prov: Provenance + 'static> Projectable<'tcx, Prov> for OpTy<'tcx, Pr
         match self.as_mplace_or_imm() {
             Left(mplace) => Ok(mplace.offset_with_meta(offset, meta, layout, ecx)?.into()),
             Right(imm) => {
-                assert!(!meta.has_meta()); // no place to store metadata here
+                debug_assert!(layout.is_sized(), "unsized immediates are not a thing");
+                assert_matches!(meta, MemPlaceMeta::None); // no place to store metadata here
                 // Every part of an uninit is uninit.
-                Ok(imm.offset(offset, layout, ecx)?.into())
+                Ok(imm.offset_(offset, layout, ecx).into())
             }
         }
     }
@@ -576,6 +577,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
         let layout = self.layout_of_local(frame, local, layout)?;
         let op = *frame.locals[local].access()?;
+        if matches!(op, Operand::Immediate(_)) {
+            if layout.is_unsized() {
+                // ConstProp marks *all* locals as `Immediate::Uninit` since it cannot
+                // efficiently check whether they are sized. We have to catch that case here.
+                throw_inval!(ConstPropNonsense);
+            }
+        }
         Ok(OpTy { op, layout, align: Some(layout.align.abi) })
     }
 
@@ -589,16 +597,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match place.as_mplace_or_local() {
             Left(mplace) => Ok(mplace.into()),
             Right((frame, local, offset)) => {
+                debug_assert!(place.layout.is_sized()); // only sized locals can ever be `Place::Local`.
                 let base = self.local_to_op(&self.stack()[frame], local, None)?;
-                let mut field = if let Some(offset) = offset {
+                let mut field = match offset {
-                    // This got offset. We can be sure that the field is sized.
+                    Some(offset) => base.offset(offset, place.layout, self)?,
-                    base.offset(offset, place.layout, self)?
+                    None => {
-                } else {
+                        // In the common case this hasn't been projected.
-                    assert_eq!(place.layout, base.layout);
+                        debug_assert_eq!(place.layout, base.layout);
-                    // Unsized cases are possible here since an unsized local will be a
-                    // `Place::Local` until the first projection calls `place_to_op` to extract the
-                    // underlying mplace.
                         base
+                    }
                 };
                 field.align = Some(place.align);
                 Ok(field)

compiler/rustc_const_eval/src/interpret/place.rs

@@ -41,6 +41,7 @@ impl<Prov: Provenance> MemPlaceMeta<Prov> {
         }
     }
 
+    #[inline(always)]
     pub fn has_meta(self) -> bool {
         match self {
             Self::Meta(_) => true,
@@ -255,15 +256,12 @@ impl<'tcx, Prov: Provenance + 'static> Projectable<'tcx, Prov> for PlaceTy<'tcx,
         self.layout
     }
 
+    #[inline]
     fn meta(&self) -> InterpResult<'tcx, MemPlaceMeta<Prov>> {
         Ok(match self.as_mplace_or_local() {
             Left(mplace) => mplace.meta,
             Right(_) => {
-                if self.layout.is_unsized() {
+                debug_assert!(self.layout.is_sized(), "unsized locals should live in memory");
-                    // Unsized `Place::Local` cannot occur. We create a MemPlace for all unsized locals during argument passing.
-                    // However, ConstProp doesn't do that, so we can run into this nonsense situation.
-                    throw_inval!(ConstPropNonsense);
-                }
                 MemPlaceMeta::None
             }
         })
@@ -331,7 +329,7 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
 
 impl<'tcx, Prov: Provenance + 'static> PlaceTy<'tcx, Prov> {
     /// A place is either an mplace or some local.
-    #[inline]
+    #[inline(always)]
     pub fn as_mplace_or_local(
         &self,
     ) -> Either<MPlaceTy<'tcx, Prov>, (usize, mir::Local, Option<Size>)> {
@@ -535,9 +533,19 @@ where
         // So we eagerly check here if this local has an MPlace, and if yes we use it.
         let frame_ref = &self.stack()[frame];
         let layout = self.layout_of_local(frame_ref, local, None)?;
-        let place = match frame_ref.locals[local].access()? {
+        let place = if layout.is_sized() {
-            Operand::Immediate(_) => Place::Local { frame, local, offset: None },
+            // We can just always use the `Local` for sized values.
+            Place::Local { frame, local, offset: None }
+        } else {
+            // Unsized `Local` isn't okay (we cannot store the metadata).
+            match frame_ref.locals[local].access()? {
+                Operand::Immediate(_) => {
+                    // ConstProp marks *all* locals as `Immediate::Uninit` since it cannot
+                    // efficiently check whether they are sized. We have to catch that case here.
+                    throw_inval!(ConstPropNonsense);
+                }
                 Operand::Indirect(mplace) => Place::Ptr(*mplace),
+            }
         };
         Ok(PlaceTy { place, layout, align: layout.align.abi })
     }
@@ -896,9 +904,7 @@ where
                         // that has different alignment than the outer field.
                         let local_layout =
                             self.layout_of_local(&self.stack()[frame], local, None)?;
-                        if local_layout.is_unsized() {
+                        assert!(local_layout.is_sized(), "unsized locals cannot be immediate");
-                            throw_unsup_format!("unsized locals are not supported");
-                        }
                         let mplace = self.allocate(local_layout, MemoryKind::Stack)?;
                         // Preserve old value. (As an optimization, we can skip this if it was uninit.)
                         if !matches!(local_val, Immediate::Uninit) {

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -1,19 +1,21 @@
 use std::borrow::Cow;
+use std::mem;
 
 use either::Either;
 use rustc_ast::ast::InlineAsmOptions;
+use rustc_middle::mir::ProjectionElem;
 use rustc_middle::ty::layout::{FnAbiOf, LayoutOf, TyAndLayout};
 use rustc_middle::ty::Instance;
 use rustc_middle::{
     mir,
     ty::{self, Ty},
 };
-use rustc_target::abi;
 use rustc_target::abi::call::{ArgAbi, ArgAttribute, ArgAttributes, FnAbi, PassMode};
+use rustc_target::abi::{self, FieldIdx};
 use rustc_target::spec::abi::Abi;
 
 use super::{
-    AllocId, FnVal, ImmTy, Immediate, InterpCx, InterpResult, MPlaceTy, Machine, MemoryKind, OpTy,
+    AllocId, FnVal, ImmTy, InterpCx, InterpResult, LocalValue, MPlaceTy, Machine, MemoryKind, OpTy,
     Operand, PlaceTy, Provenance, Scalar, StackPopCleanup,
 };
 use crate::fluent_generated as fluent;
@@ -358,23 +360,28 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Item = (&'x FnArg<'tcx, M::Provenance>, &'y ArgAbi<'tcx, Ty<'tcx>>),
         >,
         callee_abi: &ArgAbi<'tcx, Ty<'tcx>>,
-        callee_arg: &PlaceTy<'tcx, M::Provenance>,
+        callee_arg: &mir::Place<'tcx>,
+        callee_ty: Ty<'tcx>,
+        already_live: bool,
     ) -> InterpResult<'tcx>
     where
         'tcx: 'x,
         'tcx: 'y,
     {
         if matches!(callee_abi.mode, PassMode::Ignore) {
-            // This one is skipped.
+            // This one is skipped. Still must be made live though!
+            if !already_live {
+                self.storage_live(callee_arg.as_local().unwrap())?;
+            }
             return Ok(());
         }
         // Find next caller arg.
         let Some((caller_arg, caller_abi)) = caller_args.next() else {
             throw_ub_custom!(fluent::const_eval_not_enough_caller_args);
         };
-        // Now, check
+        // Check compatibility
         if !Self::check_argument_compat(caller_abi, callee_abi) {
-            let callee_ty = format!("{}", callee_arg.layout.ty);
+            let callee_ty = format!("{}", callee_ty);
             let caller_ty = format!("{}", caller_arg.layout().ty);
             throw_ub_custom!(
                 fluent::const_eval_incompatible_types,
@@ -386,35 +393,37 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // will later protect the source it comes from. This means the callee cannot observe if we
         // did in-place of by-copy argument passing, except for pointer equality tests.
         let caller_arg_copy = self.copy_fn_arg(&caller_arg)?;
-        // Special handling for unsized parameters.
+        if !already_live {
+            // Special handling for unsized parameters: they are harder to make live.
             if caller_arg_copy.layout.is_unsized() {
                 // `check_argument_compat` ensures that both have the same type, so we know they will use the metadata the same way.
-            assert_eq!(caller_arg_copy.layout.ty, callee_arg.layout.ty);
+                assert_eq!(caller_arg_copy.layout.ty, callee_ty);
                 // We have to properly pre-allocate the memory for the callee.
                 // So let's tear down some abstractions.
                 // This all has to be in memory, there are no immediate unsized values.
                 let src = caller_arg_copy.assert_mem_place();
                 // The destination cannot be one of these "spread args".
-            let (dest_frame, dest_local, dest_offset) = callee_arg
+                let dest_local = callee_arg.as_local().expect("unsized arguments cannot be spread");
-                .as_mplace_or_local()
-                .right()
-                .expect("callee fn arguments must be locals");
-            // We are just initializing things, so there can't be anything here yet.
-            assert!(matches!(
-                *self.local_to_op(&self.stack()[dest_frame], dest_local, None)?,
-                Operand::Immediate(Immediate::Uninit)
-            ));
-            assert_eq!(dest_offset, None);
                 // Allocate enough memory to hold `src`.
                 let dest_place = self.allocate_dyn(src.layout, MemoryKind::Stack, src.meta)?;
-            // Update the local to be that new place.
+                // Update the local to be that new place. This is essentially a "dyn-sized StorageLive".
-            *M::access_local_mut(self, dest_frame, dest_local)? = Operand::Indirect(*dest_place);
+                let old = mem::replace(
+                    &mut self.frame_mut().locals[dest_local].value,
+                    LocalValue::Live(Operand::Indirect(*dest_place)),
+                );
+                assert!(matches!(old, LocalValue::Dead));
+            } else {
+                // Just make the local live.
+                self.storage_live(callee_arg.as_local().unwrap())?;
             }
+        }
+        // Now we can finally actually evaluate the callee place.
+        let callee_arg = self.eval_place(*callee_arg)?;
         // We allow some transmutes here.
         // FIXME: Depending on the PassMode, this should reset some padding to uninitialized. (This
         // is true for all `copy_op`, but there are a lot of special cases for argument passing
         // specifically.)
-        self.copy_op(&caller_arg_copy, callee_arg, /*allow_transmute*/ true)?;
+        self.copy_op(&caller_arg_copy, &callee_arg, /*allow_transmute*/ true)?;
         // If this was an in-place pass, protect the place it comes from for the duration of the call.
         if let FnArg::InPlace(place) = caller_arg {
             M::protect_in_place_function_argument(self, place)?;
@@ -600,18 +609,47 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     // not advance `caller_iter` for ZSTs.
                     let mut callee_args_abis = callee_fn_abi.args.iter();
                     for local in body.args_iter() {
-                        let dest = self.eval_place(mir::Place::from(local))?;
+                        // Construct the destination place for this argument. At this point all
+                        // locals are still dead, so we cannot construct a `PlaceTy`.
+                        let dest = mir::Place::from(local);
+                        // `layout_of_local` does more than just the substitution we need to get the
+                        // type, but the result gets cached so this avoids calling the substitution
+                        // query *again* the next time this local is accessed.
+                        let ty = self.layout_of_local(self.frame(), local, None)?.ty;
                         if Some(local) == body.spread_arg {
+                            // Make the local live once, then fill in the value field by field.
+                            self.storage_live(local)?;
                             // Must be a tuple
-                            for i in 0..dest.layout.fields.count() {
+                            let ty::Tuple(fields) = ty.kind() else {
-                                let dest = self.project_field(&dest, i)?;
+                                span_bug!(
+                                    self.cur_span(),
+                                    "non-tuple type for `spread_arg`: {ty:?}"
+                                )
+                            };
+                            for (i, field_ty) in fields.iter().enumerate() {
+                                let dest = dest.project_deeper(
+                                    &[ProjectionElem::Field(FieldIdx::from_usize(i), field_ty)],
+                                    *self.tcx,
+                                );
                                 let callee_abi = callee_args_abis.next().unwrap();
-                                self.pass_argument(&mut caller_args, callee_abi, &dest)?;
+                                self.pass_argument(
+                                    &mut caller_args,
+                                    callee_abi,
+                                    &dest,
+                                    field_ty,
+                                    /* already_live */ true,
+                                )?;
                             }
                         } else {
-                            // Normal argument
+                            // Normal argument. Cannot mark it as live yet, it might be unsized!
                             let callee_abi = callee_args_abis.next().unwrap();
-                            self.pass_argument(&mut caller_args, callee_abi, &dest)?;
+                            self.pass_argument(
+                                &mut caller_args,
+                                callee_abi,
+                                &dest,
+                                ty,
+                                /* already_live */ false,
+                            )?;
                         }
                     }
                     // If the callee needs a caller location, pretend we consume one more argument from the ABI.
@@ -644,6 +682,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         // Nothing to do for locals, they are always properly allocated and aligned.
                     }
                     M::protect_in_place_function_argument(self, destination)?;
+
+                    // Don't forget to mark "initially live" locals as live.
+                    self.storage_live_for_always_live_locals()?;
                 };
                 match res {
                     Err(err) => {

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

@@ -415,6 +415,8 @@ pub enum UnsupportedOpInfo {
     /// Free-form case. Only for errors that are never caught!
     // FIXME still use translatable diagnostics
     Unsupported(String),
+    /// Unsized local variables.
+    UnsizedLocal,
     //
     // The variants below are only reachable from CTFE/const prop, miri will never emit them.
     //

compiler/rustc_mir_transform/src/const_prop.rs

@@ -376,6 +376,16 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         )
         .expect("failed to push initial stack frame");
 
+        for local in body.local_decls.indices() {
+            // Mark everything initially live.
+            // This is somewhat dicey since some of them might be unsized and it is incoherent to
+            // mark those as live... We rely on `local_to_place`/`local_to_op` in the interpreter
+            // stopping us before those unsized immediates can cause issues deeper in the
+            // interpreter.
+            ecx.frame_mut().locals[local].value =
+                LocalValue::Live(interpret::Operand::Immediate(Immediate::Uninit));
+        }
+
         ConstPropagator { ecx, tcx, param_env, local_decls: &dummy_body.local_decls }
     }
 

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -206,6 +206,16 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         )
         .expect("failed to push initial stack frame");
 
+        for local in body.local_decls.indices() {
+            // Mark everything initially live.
+            // This is somewhat dicey since some of them might be unsized and it is incoherent to
+            // mark those as live... We rely on `local_to_place`/`local_to_op` in the interpreter
+            // stopping us before those unsized immediates can cause issues deeper in the
+            // interpreter.
+            ecx.frame_mut().locals[local].value =
+                LocalValue::Live(interpret::Operand::Immediate(Immediate::Uninit));
+        }
+
         ConstPropagator {
             ecx,
             tcx,

src/tools/miri/src/diagnostics.rs

@@ -283,7 +283,7 @@ pub fn report_error<'tcx, 'mir>(
                 "resource exhaustion",
             Unsupported(
                 // We list only the ones that can actually happen.
-                UnsupportedOpInfo::Unsupported(_)
+                UnsupportedOpInfo::Unsupported(_) | UnsupportedOpInfo::UnsizedLocal
             ) =>
                 "unsupported operation",
             InvalidProgram(

src/tools/miri/src/helpers.rs

@@ -14,7 +14,7 @@ use rustc_middle::mir;
 use rustc_middle::ty::{
     self,
     layout::{IntegerExt as _, LayoutOf, TyAndLayout},
-    List, Ty, TyCtxt,
+    Ty, TyCtxt,
 };
 use rustc_span::{def_id::CrateNum, sym, Span, Symbol};
 use rustc_target::abi::{Align, FieldIdx, FieldsShape, Integer, Size, Variants};
@@ -282,13 +282,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         Ok(ptr.addr().bytes() == 0)
     }
 
-    /// Get the `Place` for a local
-    fn local_place(&self, local: mir::Local) -> InterpResult<'tcx, PlaceTy<'tcx, Provenance>> {
-        let this = self.eval_context_ref();
-        let place = mir::Place { local, projection: List::empty() };
-        this.eval_place(place)
-    }
-
     /// Generate some random bytes, and write them to `dest`.
     fn gen_random(&mut self, ptr: Pointer<Option<Provenance>>, len: u64) -> InterpResult<'tcx> {
         // Some programs pass in a null pointer and a length of 0
@@ -350,17 +343,21 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         // Initialize arguments.
         let mut callee_args = this.frame().body.args_iter();
         for arg in args {
-            let callee_arg = this.local_place(
+            let local = callee_args
-                callee_args
                 .next()
-                    .ok_or_else(|| err_ub_format!("callee has fewer arguments than expected"))?,
+                .ok_or_else(|| err_ub_format!("callee has fewer arguments than expected"))?;
-            )?;
+            // Make the local live, and insert the initial value.
+            this.storage_live(local)?;
+            let callee_arg = this.local_to_place(this.frame_idx(), local)?;
             this.write_immediate(*arg, &callee_arg)?;
         }
         if callee_args.next().is_some() {
             throw_ub_format!("callee has more arguments than expected");
         }
 
+        // Initialize remaining locals.
+        this.storage_live_for_always_live_locals()?;
+
         Ok(())
     }
 

src/tools/miri/tests/fail/type-too-large.rs

@@ -1,6 +1,6 @@
 //@ignore-32bit
 
 fn main() {
-    let _fat: [u8; (1 << 61) + (1 << 31)];
+    let _fat: [u8; (1 << 61) + (1 << 31)]; //~ ERROR: post-monomorphization error
-    _fat = [0; (1u64 << 61) as usize + (1u64 << 31) as usize]; //~ ERROR: post-monomorphization error
+    _fat = [0; (1u64 << 61) as usize + (1u64 << 31) as usize];
 }

src/tools/miri/tests/fail/type-too-large.stderr

@@ -1,8 +1,8 @@
 error: post-monomorphization error: values of the type `[u8; 2305843011361177600]` are too big for the current architecture
   --> $DIR/type-too-large.rs:LL:CC
    |
-LL |     _fat = [0; (1u64 << 61) as usize + (1u64 << 31) as usize];
+LL |     let _fat: [u8; (1 << 61) + (1 << 31)];
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ values of the type `[u8; 2305843011361177600]` are too big for the current architecture
+   |         ^^^^ values of the type `[u8; 2305843011361177600]` are too big for the current architecture
    |
    = note: inside `main` at $DIR/type-too-large.rs:LL:CC
 

src/tools/miri/tests/fail/unsized-local.stderr

@@ -2,7 +2,7 @@ error: unsupported operation: unsized locals are not supported
   --> $DIR/unsized-local.rs:LL:CC
    |
 LL |     let x = *(Box::new(A) as Box<dyn Foo>);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsized locals are not supported
+   |         ^ unsized locals are not supported
    |
    = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
    = note: BACKTRACE: