rust/compiler/rustc_const_eval/src/interpret/place.rs

//! Computations on places -- field projections, going from mir::Place, and writing
//! into a place.
//! All high-level functions to write to memory work on places as destinations.

use std::assert_matches::assert_matches;

use either::{Either, Left, Right};

use rustc_ast::Mutability;
use rustc_index::IndexSlice;
use rustc_middle::mir;
use rustc_middle::ty;
use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
use rustc_middle::ty::Ty;
use rustc_target::abi::{Abi, Align, FieldIdx, HasDataLayout, Size, FIRST_VARIANT};

use super::{
    alloc_range, mir_assign_valid_types, AllocId, AllocRef, AllocRefMut, CheckInAllocMsg, ImmTy,
    Immediate, InterpCx, InterpResult, Machine, MemoryKind, OpTy, Operand, Pointer,
    PointerArithmetic, Projectable, Provenance, Readable, Scalar,
};

#[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
/// Information required for the sound usage of a `MemPlace`.
pub enum MemPlaceMeta<Prov: Provenance = AllocId> {
    /// The unsized payload (e.g. length for slices or vtable pointer for trait objects).
    Meta(Scalar<Prov>),
    /// `Sized` types or unsized `extern type`
    None,
}

impl<Prov: Provenance> MemPlaceMeta<Prov> {
    #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
    pub fn unwrap_meta(self) -> Scalar<Prov> {
        match self {
            Self::Meta(s) => s,
            Self::None => {
                bug!("expected wide pointer extra data (e.g. slice length or trait object vtable)")
            }
        }
    }

    #[inline(always)]
    pub fn has_meta(self) -> bool {
        match self {
            Self::Meta(_) => true,
            Self::None => false,
        }
    }
}

#[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
pub(super) struct MemPlace<Prov: Provenance = AllocId> {
    /// The pointer can be a pure integer, with the `None` provenance.
    pub ptr: Pointer<Option<Prov>>,
    /// Metadata for unsized places. Interpretation is up to the type.
    /// Must not be present for sized types, but can be missing for unsized types
    /// (e.g., `extern type`).
    pub meta: MemPlaceMeta<Prov>,
}

impl<Prov: Provenance> MemPlace<Prov> {
    #[inline(always)]
    pub fn from_ptr(ptr: Pointer<Option<Prov>>) -> Self {
        MemPlace { ptr, meta: MemPlaceMeta::None }
    }

    #[inline(always)]
    pub fn from_ptr_with_meta(ptr: Pointer<Option<Prov>>, meta: MemPlaceMeta<Prov>) -> Self {
        MemPlace { ptr, meta }
    }

    /// Adjust the provenance of the main pointer (metadata is unaffected).
    pub fn map_provenance(self, f: impl FnOnce(Option<Prov>) -> Option<Prov>) -> Self {
        MemPlace { ptr: self.ptr.map_provenance(f), ..self }
    }

    /// Turn a mplace into a (thin or wide) pointer, as a reference, pointing to the same space.
    #[inline]
    pub fn to_ref(self, cx: &impl HasDataLayout) -> Immediate<Prov> {
        match self.meta {
            MemPlaceMeta::None => Immediate::from(Scalar::from_maybe_pointer(self.ptr, cx)),
            MemPlaceMeta::Meta(meta) => {
                Immediate::ScalarPair(Scalar::from_maybe_pointer(self.ptr, cx), meta)
            }
        }
    }

    #[inline]
    // Not called `offset_with_meta` to avoid confusion with the trait method.
    fn offset_with_meta_<'tcx>(
        self,
        offset: Size,
        meta: MemPlaceMeta<Prov>,
        cx: &impl HasDataLayout,
    ) -> InterpResult<'tcx, Self> {
        debug_assert!(
            !meta.has_meta() || self.meta.has_meta(),
            "cannot use `offset_with_meta` to add metadata to a place"
        );
        Ok(MemPlace { ptr: self.ptr.offset(offset, cx)?, meta })
    }
}

/// A MemPlace with its layout. Constructing it is only possible in this module.
#[derive(Clone, Hash, Eq, PartialEq)]
pub struct MPlaceTy<'tcx, Prov: Provenance = AllocId> {
    mplace: MemPlace<Prov>,
    pub layout: TyAndLayout<'tcx>,
    /// rustc does not have a proper way to represent the type of a field of a `repr(packed)` struct:
    /// it needs to have a different alignment than the field type would usually have.
    /// So we represent this here with a separate field that "overwrites" `layout.align`.
    /// This means `layout.align` should never be used for a `MPlaceTy`!
    pub align: Align,
}

impl<Prov: Provenance> std::fmt::Debug for MPlaceTy<'_, Prov> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        // Printing `layout` results in too much noise; just print a nice version of the type.
        f.debug_struct("MPlaceTy")
            .field("mplace", &self.mplace)
            .field("ty", &format_args!("{}", self.layout.ty))
            .finish()
    }
}

impl<'tcx, Prov: Provenance> MPlaceTy<'tcx, Prov> {
    /// Produces a MemPlace that works for ZST but nothing else.
    /// Conceptually this is a new allocation, but it doesn't actually create an allocation so you
    /// don't need to worry about memory leaks.
    #[inline]
    pub fn fake_alloc_zst(layout: TyAndLayout<'tcx>) -> Self {
        assert!(layout.is_zst());
        let align = layout.align.abi;
        let ptr = Pointer::from_addr_invalid(align.bytes()); // no provenance, absolute address
        MPlaceTy { mplace: MemPlace { ptr, meta: MemPlaceMeta::None }, layout, align }
    }

    #[inline]
    pub fn from_aligned_ptr(ptr: Pointer<Option<Prov>>, layout: TyAndLayout<'tcx>) -> Self {
        MPlaceTy { mplace: MemPlace::from_ptr(ptr), layout, align: layout.align.abi }
    }

    #[inline]
    pub fn from_aligned_ptr_with_meta(
        ptr: Pointer<Option<Prov>>,
        layout: TyAndLayout<'tcx>,
        meta: MemPlaceMeta<Prov>,
    ) -> Self {
        MPlaceTy {
            mplace: MemPlace::from_ptr_with_meta(ptr, meta),
            layout,
            align: layout.align.abi,
        }
    }

    /// Adjust the provenance of the main pointer (metadata is unaffected).
    pub fn map_provenance(self, f: impl FnOnce(Option<Prov>) -> Option<Prov>) -> Self {
        MPlaceTy { mplace: self.mplace.map_provenance(f), ..self }
    }

    #[inline(always)]
    pub(super) fn mplace(&self) -> &MemPlace<Prov> {
        &self.mplace
    }

    #[inline(always)]
    pub fn ptr(&self) -> Pointer<Option<Prov>> {
        self.mplace.ptr
    }

    #[inline(always)]
    pub fn to_ref(&self, cx: &impl HasDataLayout) -> Immediate<Prov> {
        self.mplace.to_ref(cx)
    }
}

impl<'tcx, Prov: Provenance> Projectable<'tcx, Prov> for MPlaceTy<'tcx, Prov> {
    #[inline(always)]
    fn layout(&self) -> TyAndLayout<'tcx> {
        self.layout
    }

    #[inline(always)]
    fn meta(&self) -> MemPlaceMeta<Prov> {
        self.mplace.meta
    }

    fn offset_with_meta<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        offset: Size,
        meta: MemPlaceMeta<Prov>,
        layout: TyAndLayout<'tcx>,
        ecx: &InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, Self> {
        Ok(MPlaceTy {
            mplace: self.mplace.offset_with_meta_(offset, meta, ecx)?,
            align: self.align.restrict_for_offset(offset),
            layout,
        })
    }

    fn to_op<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        _ecx: &InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
        Ok(self.clone().into())
    }
}

#[derive(Copy, Clone, Debug)]
pub(super) enum Place<Prov: Provenance = AllocId> {
    /// A place referring to a value allocated in the `Memory` system.
    Ptr(MemPlace<Prov>),

    /// To support alloc-free locals, we are able to write directly to a local. The offset indicates
    /// where in the local this place is located; if it is `None`, no projection has been applied.
    /// Such projections are meaningful even if the offset is 0, since they can change layouts.
    /// (Without that optimization, we'd just always be a `MemPlace`.)
    /// Note that this only stores the frame index, not the thread this frame belongs to -- that is
    /// implicit. This means a `Place` must never be moved across interpreter thread boundaries!
    ///
    /// This variant shall not be used for unsized types -- those must always live in memory.
    Local { frame: usize, local: mir::Local, offset: Option<Size> },
}

#[derive(Clone)]
pub struct PlaceTy<'tcx, Prov: Provenance = AllocId> {
    place: Place<Prov>, // Keep this private; it helps enforce invariants.
    pub layout: TyAndLayout<'tcx>,
    /// rustc does not have a proper way to represent the type of a field of a `repr(packed)` struct:
    /// it needs to have a different alignment than the field type would usually have.
    /// So we represent this here with a separate field that "overwrites" `layout.align`.
    /// This means `layout.align` should never be used for a `PlaceTy`!
    pub align: Align,
}

impl<Prov: Provenance> std::fmt::Debug for PlaceTy<'_, Prov> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        // Printing `layout` results in too much noise; just print a nice version of the type.
        f.debug_struct("PlaceTy")
            .field("place", &self.place)
            .field("ty", &format_args!("{}", self.layout.ty))
            .finish()
    }
}

impl<'tcx, Prov: Provenance> From<MPlaceTy<'tcx, Prov>> for PlaceTy<'tcx, Prov> {
    #[inline(always)]
    fn from(mplace: MPlaceTy<'tcx, Prov>) -> Self {
        PlaceTy { place: Place::Ptr(mplace.mplace), layout: mplace.layout, align: mplace.align }
    }
}

impl<'tcx, Prov: Provenance> PlaceTy<'tcx, Prov> {
    #[inline(always)]
    pub(super) fn place(&self) -> &Place<Prov> {
        &self.place
    }

    /// A place is either an mplace or some local.
    #[inline(always)]
    pub fn as_mplace_or_local(
        &self,
    ) -> Either<MPlaceTy<'tcx, Prov>, (usize, mir::Local, Option<Size>)> {
        match self.place {
            Place::Ptr(mplace) => Left(MPlaceTy { mplace, layout: self.layout, align: self.align }),
            Place::Local { frame, local, offset } => Right((frame, local, offset)),
        }
    }

    #[inline(always)]
    #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
    pub fn assert_mem_place(&self) -> MPlaceTy<'tcx, Prov> {
        self.as_mplace_or_local().left().unwrap_or_else(|| {
            bug!(
                "PlaceTy of type {} was a local when it was expected to be an MPlace",
                self.layout.ty
            )
        })
    }
}

impl<'tcx, Prov: Provenance> Projectable<'tcx, Prov> for PlaceTy<'tcx, Prov> {
    #[inline(always)]
    fn layout(&self) -> TyAndLayout<'tcx> {
        self.layout
    }

    #[inline]
    fn meta(&self) -> MemPlaceMeta<Prov> {
        match self.as_mplace_or_local() {
            Left(mplace) => mplace.meta(),
            Right(_) => {
                debug_assert!(self.layout.is_sized(), "unsized locals should live in memory");
                MemPlaceMeta::None
            }
        }
    }

    fn offset_with_meta<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        offset: Size,
        meta: MemPlaceMeta<Prov>,
        layout: TyAndLayout<'tcx>,
        ecx: &InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, Self> {
        Ok(match self.as_mplace_or_local() {
            Left(mplace) => mplace.offset_with_meta(offset, meta, layout, ecx)?.into(),
            Right((frame, local, old_offset)) => {
                debug_assert!(layout.is_sized(), "unsized locals should live in memory");
                assert_matches!(meta, MemPlaceMeta::None); // we couldn't store it anyway...
                let new_offset = ecx
                    .data_layout()
                    .offset(old_offset.unwrap_or(Size::ZERO).bytes(), offset.bytes())?;
                PlaceTy {
                    place: Place::Local {
                        frame,
                        local,
                        offset: Some(Size::from_bytes(new_offset)),
                    },
                    align: self.align.restrict_for_offset(offset),
                    layout,
                }
            }
        })
    }

    fn to_op<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        ecx: &InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
        ecx.place_to_op(self)
    }
}

// These are defined here because they produce a place.
impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
    #[inline(always)]
    pub fn as_mplace_or_imm(&self) -> Either<MPlaceTy<'tcx, Prov>, ImmTy<'tcx, Prov>> {
        match self.op() {
            Operand::Indirect(mplace) => {
                Left(MPlaceTy { mplace: *mplace, layout: self.layout, align: self.align.unwrap() })
            }
            Operand::Immediate(imm) => Right(ImmTy::from_immediate(*imm, self.layout)),
        }
    }

    #[inline(always)]
    #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
    pub fn assert_mem_place(&self) -> MPlaceTy<'tcx, Prov> {
        self.as_mplace_or_imm().left().unwrap_or_else(|| {
            bug!(
                "OpTy of type {} was immediate when it was expected to be an MPlace",
                self.layout.ty
            )
        })
    }
}

/// The `Weiteable` trait describes interpreter values that can be written to.
pub trait Writeable<'tcx, Prov: Provenance>: Projectable<'tcx, Prov> {
    fn as_mplace_or_local(
        &self,
    ) -> Either<MPlaceTy<'tcx, Prov>, (usize, mir::Local, Option<Size>, Align, TyAndLayout<'tcx>)>;

    fn force_mplace<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        ecx: &mut InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, Prov>>;
}

impl<'tcx, Prov: Provenance> Writeable<'tcx, Prov> for PlaceTy<'tcx, Prov> {
    #[inline(always)]
    fn as_mplace_or_local(
        &self,
    ) -> Either<MPlaceTy<'tcx, Prov>, (usize, mir::Local, Option<Size>, Align, TyAndLayout<'tcx>)>
    {
        self.as_mplace_or_local()
            .map_right(|(frame, local, offset)| (frame, local, offset, self.align, self.layout))
    }

    #[inline(always)]
    fn force_mplace<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        ecx: &mut InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, Prov>> {
        ecx.force_allocation(self)
    }
}

impl<'tcx, Prov: Provenance> Writeable<'tcx, Prov> for MPlaceTy<'tcx, Prov> {
    #[inline(always)]
    fn as_mplace_or_local(
        &self,
    ) -> Either<MPlaceTy<'tcx, Prov>, (usize, mir::Local, Option<Size>, Align, TyAndLayout<'tcx>)>
    {
        Left(self.clone())
    }

    #[inline(always)]
    fn force_mplace<'mir, M: Machine<'mir, 'tcx, Provenance = Prov>>(
        &self,
        _ecx: &mut InterpCx<'mir, 'tcx, M>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, Prov>> {
        Ok(self.clone())
    }
}

// FIXME: Working around https://github.com/rust-lang/rust/issues/54385
impl<'mir, 'tcx: 'mir, Prov, M> InterpCx<'mir, 'tcx, M>
where
    Prov: Provenance,
    M: Machine<'mir, 'tcx, Provenance = Prov>,
{
    /// Take a value, which represents a (thin or wide) reference, and make it a place.
    /// Alignment is just based on the type. This is the inverse of `mplace_to_ref()`.
    ///
    /// Only call this if you are sure the place is "valid" (aligned and inbounds), or do not
    /// want to ever use the place for memory access!
    /// Generally prefer `deref_pointer`.
    pub fn ref_to_mplace(
        &self,
        val: &ImmTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        let pointee_type =
            val.layout.ty.builtin_deref(true).expect("`ref_to_mplace` called on non-ptr type").ty;
        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)),
            Immediate::Uninit => throw_ub!(InvalidUninitBytes(None)),
        };

        // `ref_to_mplace` is called on raw pointers even if they don't actually get dereferenced;
        // we hence can't call `size_and_align_of` since that asserts more validity than we want.
        Ok(MPlaceTy::from_aligned_ptr_with_meta(ptr.to_pointer(self)?, layout, meta))
    }

    /// Turn a mplace into a (thin or wide) mutable raw pointer, pointing to the same space.
    /// `align` information is lost!
    /// This is the inverse of `ref_to_mplace`.
    pub fn mplace_to_ref(
        &self,
        mplace: &MPlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
        let imm = mplace.mplace.to_ref(self);
        let layout = self.layout_of(Ty::new_mut_ptr(self.tcx.tcx, mplace.layout.ty))?;
        Ok(ImmTy::from_immediate(imm, layout))
    }

    /// Take an operand, representing a pointer, and dereference it to a place.
    /// Corresponds to the `*` operator in Rust.
    #[instrument(skip(self), level = "debug")]
    pub fn deref_pointer(
        &self,
        src: &impl Readable<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        let val = self.read_immediate(src)?;
        trace!("deref to {} on {:?}", val.layout.ty, *val);

        if val.layout.ty.is_box() {
            bug!("dereferencing {}", val.layout.ty);
        }

        let mplace = self.ref_to_mplace(&val)?;
        self.check_mplace(&mplace)?;
        Ok(mplace)
    }

    #[inline]
    pub(super) fn get_place_alloc(
        &self,
        mplace: &MPlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, Option<AllocRef<'_, 'tcx, M::Provenance, M::AllocExtra, M::Bytes>>>
    {
        let (size, _align) = self
            .size_and_align_of_mplace(&mplace)?
            .unwrap_or((mplace.layout.size, mplace.layout.align.abi));
        // Due to packed places, only `mplace.align` matters.
        self.get_ptr_alloc(mplace.ptr(), size, mplace.align)
    }

    #[inline]
    pub(super) fn get_place_alloc_mut(
        &mut self,
        mplace: &MPlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, Option<AllocRefMut<'_, 'tcx, M::Provenance, M::AllocExtra, M::Bytes>>>
    {
        let (size, _align) = self
            .size_and_align_of_mplace(&mplace)?
            .unwrap_or((mplace.layout.size, mplace.layout.align.abi));
        // Due to packed places, only `mplace.align` matters.
        self.get_ptr_alloc_mut(mplace.ptr(), size, mplace.align)
    }

    /// Check if this mplace is dereferenceable and sufficiently aligned.
    pub fn check_mplace(&self, mplace: &MPlaceTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
        let (size, _align) = self
            .size_and_align_of_mplace(&mplace)?
            .unwrap_or((mplace.layout.size, mplace.layout.align.abi));
        // Due to packed places, only `mplace.align` matters.
        let align =
            if M::enforce_alignment(self).should_check() { mplace.align } else { Align::ONE };
        self.check_ptr_access_align(mplace.ptr(), size, align, CheckInAllocMsg::DerefTest)?;
        Ok(())
    }

    /// Converts a repr(simd) place into a place where `place_index` accesses the SIMD elements.
    /// Also returns the number of elements.
    pub fn mplace_to_simd(
        &self,
        mplace: &MPlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::Provenance>, u64)> {
        // Basically we just transmute this place into an array following simd_size_and_type.
        // (Transmuting is okay since this is an in-memory place. We also double-check the size
        // stays the same.)
        let (len, e_ty) = mplace.layout.ty.simd_size_and_type(*self.tcx);
        let array = Ty::new_array(self.tcx.tcx, e_ty, len);
        let layout = self.layout_of(array)?;
        assert_eq!(layout.size, mplace.layout.size);
        Ok((MPlaceTy { layout, ..*mplace }, len))
    }

    /// Converts a repr(simd) place into a place where `place_index` accesses the SIMD elements.
    /// Also returns the number of elements.
    pub fn place_to_simd(
        &mut self,
        place: &PlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::Provenance>, u64)> {
        let mplace = self.force_allocation(place)?;
        self.mplace_to_simd(&mplace)
    }

    pub fn local_to_place(
        &self,
        frame: usize,
        local: mir::Local,
    ) -> InterpResult<'tcx, PlaceTy<'tcx, M::Provenance>> {
        // Other parts of the system rely on `Place::Local` never being unsized.
        // 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 = if layout.is_sized() {
            // 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 })
    }

    /// Computes a place. You should only use this if you intend to write into this
    /// place; for reading, a more efficient alternative is `eval_place_to_op`.
    #[instrument(skip(self), level = "debug")]
    pub fn eval_place(
        &self,
        mir_place: mir::Place<'tcx>,
    ) -> InterpResult<'tcx, PlaceTy<'tcx, M::Provenance>> {
        let mut place = self.local_to_place(self.frame_idx(), mir_place.local)?;
        // Using `try_fold` turned out to be bad for performance, hence the loop.
        for elem in mir_place.projection.iter() {
            place = self.project(&place, elem)?
        }

        trace!("{:?}", self.dump_place(&place));
        // Sanity-check the type we ended up with.
        if cfg!(debug_assertions) {
            let normalized_place_ty = self.subst_from_current_frame_and_normalize_erasing_regions(
                mir_place.ty(&self.frame().body.local_decls, *self.tcx).ty,
            )?;
            if !mir_assign_valid_types(
                *self.tcx,
                self.param_env,
                self.layout_of(normalized_place_ty)?,
                place.layout,
            ) {
                span_bug!(
                    self.cur_span(),
                    "eval_place of a MIR place with type {} produced an interpreter place with type {}",
                    normalized_place_ty,
                    place.layout.ty,
                )
            }
        }
        Ok(place)
    }

    /// Write an immediate to a place
    #[inline(always)]
    #[instrument(skip(self), level = "debug")]
    pub fn write_immediate(
        &mut self,
        src: Immediate<M::Provenance>,
        dest: &impl Writeable<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx> {
        self.write_immediate_no_validate(src, dest)?;

        if M::enforce_validity(self, dest.layout()) {
            // Data got changed, better make sure it matches the type!
            self.validate_operand(&dest.to_op(self)?)?;
        }

        Ok(())
    }

    /// Write a scalar to a place
    #[inline(always)]
    pub fn write_scalar(
        &mut self,
        val: impl Into<Scalar<M::Provenance>>,
        dest: &impl Writeable<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx> {
        self.write_immediate(Immediate::Scalar(val.into()), dest)
    }

    /// Write a pointer to a place
    #[inline(always)]
    pub fn write_pointer(
        &mut self,
        ptr: impl Into<Pointer<Option<M::Provenance>>>,
        dest: &impl Writeable<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx> {
        self.write_scalar(Scalar::from_maybe_pointer(ptr.into(), self), dest)
    }

    /// Write an immediate to a place.
    /// If you use this you are responsible for validating that things got copied at the
    /// right type.
    fn write_immediate_no_validate(
        &mut self,
        src: Immediate<M::Provenance>,
        dest: &impl Writeable<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx> {
        assert!(dest.layout().is_sized(), "Cannot write unsized immediate data");

        // See if we can avoid an allocation. This is the counterpart to `read_immediate_raw`,
        // but not factored as a separate function.
        let mplace = match dest.as_mplace_or_local() {
            Right((frame, local, offset, align, layout)) => {
                if offset.is_some() {
                    // This has been projected to a part of this local. We could have complicated
                    // logic to still keep this local as an `Operand`... but it's much easier to
                    // just fall back to the indirect path.
                    dest.force_mplace(self)?
                } else {
                    M::before_access_local_mut(self, frame, local)?;
                    match self.stack_mut()[frame].locals[local].access_mut()? {
                        Operand::Immediate(local_val) => {
                            // Local can be updated in-place.
                            *local_val = src;
                            // Double-check that the value we are storing and the local fit to each other.
                            // (*After* doing the update for borrow checker reasons.)
                            if cfg!(debug_assertions) {
                                let local_layout =
                                    self.layout_of_local(&self.stack()[frame], local, None)?;
                                match (src, local_layout.abi) {
                                    (Immediate::Scalar(scalar), Abi::Scalar(s)) => {
                                        assert_eq!(scalar.size(), s.size(self))
                                    }
                                    (
                                        Immediate::ScalarPair(a_val, b_val),
                                        Abi::ScalarPair(a, b),
                                    ) => {
                                        assert_eq!(a_val.size(), a.size(self));
                                        assert_eq!(b_val.size(), b.size(self));
                                    }
                                    (Immediate::Uninit, _) => {}
                                    (src, abi) => {
                                        bug!(
                                            "value {src:?} cannot be written into local with type {} (ABI {abi:?})",
                                            local_layout.ty
                                        )
                                    }
                                };
                            }
                            return Ok(());
                        }
                        Operand::Indirect(mplace) => {
                            // The local is in memory, go on below.
                            MPlaceTy { mplace: *mplace, align, layout }
                        }
                    }
                }
            }
            Left(mplace) => mplace, // already referring to memory
        };

        // This is already in memory, write there.
        self.write_immediate_to_mplace_no_validate(src, mplace.layout, mplace.align, mplace.mplace)
    }

    /// Write an immediate to memory.
    /// If you use this you are responsible for validating that things got copied at the
    /// right layout.
    fn write_immediate_to_mplace_no_validate(
        &mut self,
        value: Immediate<M::Provenance>,
        layout: TyAndLayout<'tcx>,
        align: Align,
        dest: MemPlace<M::Provenance>,
    ) -> InterpResult<'tcx> {
        // Note that it is really important that the type here is the right one, and matches the
        // type things are read at. In case `value` is a `ScalarPair`, we don't do any magic here
        // to handle padding properly, which is only correct if we never look at this data with the
        // wrong type.

        let tcx = *self.tcx;
        let Some(mut alloc) =
            self.get_place_alloc_mut(&MPlaceTy { mplace: dest, layout, align })?
        else {
            // zero-sized access
            return Ok(());
        };

        match value {
            Immediate::Scalar(scalar) => {
                let Abi::Scalar(s) = layout.abi else {
                    span_bug!(
                        self.cur_span(),
                        "write_immediate_to_mplace: invalid Scalar layout: {layout:#?}",
                    )
                };
                let size = s.size(&tcx);
                assert_eq!(size, layout.size, "abi::Scalar size does not match layout size");
                alloc.write_scalar(alloc_range(Size::ZERO, size), scalar)
            }
            Immediate::ScalarPair(a_val, b_val) => {
                // 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 Abi::ScalarPair(a, b) = layout.abi else {
                    span_bug!(
                        self.cur_span(),
                        "write_immediate_to_mplace: invalid ScalarPair layout: {:#?}",
                        layout
                    )
                };
                let (a_size, b_size) = (a.size(&tcx), b.size(&tcx));
                let b_offset = a_size.align_to(b.align(&tcx).abi);
                assert!(b_offset.bytes() > 0); // in `operand_field` we use the offset to tell apart the fields

                // It is tempting to verify `b_offset` against `layout.fields.offset(1)`,
                // but that does not work: We could be a newtype around a pair, then the
                // fields do not match the `ScalarPair` components.

                alloc.write_scalar(alloc_range(Size::ZERO, a_size), a_val)?;
                alloc.write_scalar(alloc_range(b_offset, b_size), b_val)
            }
            Immediate::Uninit => alloc.write_uninit(),
        }
    }

    pub fn write_uninit(
        &mut self,
        dest: &impl Writeable<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx> {
        let mplace = match dest.as_mplace_or_local() {
            Left(mplace) => mplace,
            Right((frame, local, offset, align, layout)) => {
                if offset.is_some() {
                    // This has been projected to a part of this local. We could have complicated
                    // logic to still keep this local as an `Operand`... but it's much easier to
                    // just fall back to the indirect path.
                    // FIXME: share the logic with `write_immediate_no_validate`.
                    dest.force_mplace(self)?
                } else {
                    M::before_access_local_mut(self, frame, local)?;
                    match self.stack_mut()[frame].locals[local].access_mut()? {
                        Operand::Immediate(local) => {
                            *local = Immediate::Uninit;
                            return Ok(());
                        }
                        Operand::Indirect(mplace) => {
                            // The local is in memory, go on below.
                            MPlaceTy { mplace: *mplace, layout, align }
                        }
                    }
                }
            }
        };
        let Some(mut alloc) = self.get_place_alloc_mut(&mplace)? else {
            // Zero-sized access
            return Ok(());
        };
        alloc.write_uninit()?;
        Ok(())
    }

    /// Copies the data from an operand to a place.
    /// `allow_transmute` indicates whether the layouts may disagree.
    #[inline(always)]
    #[instrument(skip(self), level = "debug")]
    pub fn copy_op(
        &mut self,
        src: &impl Readable<'tcx, M::Provenance>,
        dest: &impl Writeable<'tcx, M::Provenance>,
        allow_transmute: bool,
    ) -> InterpResult<'tcx> {
        // Generally for transmutation, data must be valid both at the old and new type.
        // But if the types are the same, the 2nd validation below suffices.
        if src.layout().ty != dest.layout().ty && M::enforce_validity(self, src.layout()) {
            self.validate_operand(&src.to_op(self)?)?;
        }

        // Do the actual copy.
        self.copy_op_no_validate(src, dest, allow_transmute)?;

        if M::enforce_validity(self, dest.layout()) {
            // Data got changed, better make sure it matches the type!
            self.validate_operand(&dest.to_op(self)?)?;
        }

        Ok(())
    }

    /// Copies the data from an operand to a place.
    /// `allow_transmute` indicates whether the layouts may disagree.
    /// Also, if you use this you are responsible for validating that things get copied at the
    /// right type.
    #[instrument(skip(self), level = "debug")]
    fn copy_op_no_validate(
        &mut self,
        src: &impl Readable<'tcx, M::Provenance>,
        dest: &impl Writeable<'tcx, M::Provenance>,
        allow_transmute: bool,
    ) -> InterpResult<'tcx> {
        // We do NOT compare the types for equality, because well-typed code can
        // actually "transmute" `&mut T` to `&T` in an assignment without a cast.
        let layout_compat =
            mir_assign_valid_types(*self.tcx, self.param_env, src.layout(), dest.layout());
        if !allow_transmute && !layout_compat {
            span_bug!(
                self.cur_span(),
                "type mismatch when copying!\nsrc: {},\ndest: {}",
                src.layout().ty,
                dest.layout().ty,
            );
        }

        // Let us see if the layout is simple so we take a shortcut,
        // avoid force_allocation.
        let src = match self.read_immediate_raw(src)? {
            Right(src_val) => {
                // FIXME(const_prop): Const-prop can possibly evaluate an
                // unsized copy operation when it thinks that the type is
                // actually sized, due to a trivially false where-clause
                // predicate like `where Self: Sized` with `Self = dyn Trait`.
                // See #102553 for an example of such a predicate.
                if src.layout().is_unsized() {
                    throw_inval!(ConstPropNonsense);
                }
                if dest.layout().is_unsized() {
                    throw_inval!(ConstPropNonsense);
                }
                assert_eq!(src.layout().size, dest.layout().size);
                // Yay, we got a value that we can write directly.
                return if layout_compat {
                    self.write_immediate_no_validate(*src_val, dest)
                } else {
                    // This is tricky. The problematic case is `ScalarPair`: the `src_val` was
                    // loaded using the offsets defined by `src.layout`. When we put this back into
                    // the destination, we have to use the same offsets! So (a) we make sure we
                    // write back to memory, and (b) we use `dest` *with the source layout*.
                    let dest_mem = dest.force_mplace(self)?;
                    self.write_immediate_to_mplace_no_validate(
                        *src_val,
                        src.layout(),
                        dest_mem.align,
                        dest_mem.mplace,
                    )
                };
            }
            Left(mplace) => mplace,
        };
        // Slow path, this does not fit into an immediate. Just memcpy.
        trace!("copy_op: {:?} <- {:?}: {}", *dest, src, dest.layout().ty);

        let dest = dest.force_mplace(self)?;
        let Some((dest_size, _)) = self.size_and_align_of_mplace(&dest)? else {
            span_bug!(self.cur_span(), "copy_op needs (dynamically) sized values")
        };
        if cfg!(debug_assertions) {
            let src_size = self.size_and_align_of_mplace(&src)?.unwrap().0;
            assert_eq!(src_size, dest_size, "Cannot copy differently-sized data");
        } else {
            // As a cheap approximation, we compare the fixed parts of the size.
            assert_eq!(src.layout.size, dest.layout.size);
        }

        // Setting `nonoverlapping` here only has an effect when we don't hit the fast-path above,
        // but that should at least match what LLVM does where `memcpy` is also only used when the
        // type does not have Scalar/ScalarPair layout.
        // (Or as the `Assign` docs put it, assignments "not producing primitives" must be
        // non-overlapping.)
        self.mem_copy(
            src.ptr(),
            src.align,
            dest.ptr(),
            dest.align,
            dest_size,
            /*nonoverlapping*/ true,
        )
    }

    /// Ensures that a place is in memory, and returns where it is.
    /// If the place currently refers to a local that doesn't yet have a matching allocation,
    /// create such an allocation.
    /// This is essentially `force_to_memplace`.
    #[instrument(skip(self), level = "debug")]
    pub fn force_allocation(
        &mut self,
        place: &PlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        let mplace = match place.place {
            Place::Local { frame, local, offset } => {
                M::before_access_local_mut(self, frame, local)?;
                let whole_local = match self.stack_mut()[frame].locals[local].access_mut()? {
                    &mut Operand::Immediate(local_val) => {
                        // We need to make an allocation.

                        // We need the layout of the local. We can NOT use the layout we got,
                        // that might e.g., be an inner field of a struct with `Scalar` layout,
                        // that has different alignment than the outer field.
                        let local_layout =
                            self.layout_of_local(&self.stack()[frame], local, None)?;
                        assert!(local_layout.is_sized(), "unsized locals cannot be immediate");
                        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) {
                            // We don't have to validate as we can assume the local was already
                            // valid for its type. We must not use any part of `place` here, that
                            // could be a projection to a part of the local!
                            self.write_immediate_to_mplace_no_validate(
                                local_val,
                                local_layout,
                                local_layout.align.abi,
                                mplace.mplace,
                            )?;
                        }
                        M::after_local_allocated(self, frame, local, &mplace)?;
                        // Now we can call `access_mut` again, asserting it goes well, and actually
                        // overwrite things. This points to the entire allocation, not just the part
                        // the place refers to, i.e. we do this before we apply `offset`.
                        *self.stack_mut()[frame].locals[local].access_mut().unwrap() =
                            Operand::Indirect(mplace.mplace);
                        mplace.mplace
                    }
                    &mut Operand::Indirect(mplace) => mplace, // this already was an indirect local
                };
                if let Some(offset) = offset {
                    whole_local.offset_with_meta_(offset, MemPlaceMeta::None, self)?
                } else {
                    // Preserve wide place metadata, do not call `offset`.
                    whole_local
                }
            }
            Place::Ptr(mplace) => mplace,
        };
        // Return with the original layout and align, so that the caller can go on
        Ok(MPlaceTy { mplace, layout: place.layout, align: place.align })
    }

    pub fn allocate_dyn(
        &mut self,
        layout: TyAndLayout<'tcx>,
        kind: MemoryKind<M::MemoryKind>,
        meta: MemPlaceMeta<M::Provenance>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        let Some((size, align)) = self.size_and_align_of(&meta, &layout)? else {
            span_bug!(self.cur_span(), "cannot allocate space for `extern` type, size is not known")
        };
        let ptr = self.allocate_ptr(size, align, kind)?;
        Ok(MPlaceTy::from_aligned_ptr_with_meta(ptr.into(), layout, meta))
    }

    pub fn allocate(
        &mut self,
        layout: TyAndLayout<'tcx>,
        kind: MemoryKind<M::MemoryKind>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        assert!(layout.is_sized());
        self.allocate_dyn(layout, kind, MemPlaceMeta::None)
    }

    /// Returns a wide MPlace of type `&'static [mut] str` to a new 1-aligned allocation.
    pub fn allocate_str(
        &mut self,
        str: &str,
        kind: MemoryKind<M::MemoryKind>,
        mutbl: Mutability,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        let ptr = self.allocate_bytes_ptr(str.as_bytes(), Align::ONE, kind, mutbl)?;
        let meta = Scalar::from_target_usize(u64::try_from(str.len()).unwrap(), self);
        let mplace = MemPlace { ptr: ptr.into(), meta: MemPlaceMeta::Meta(meta) };

        let ty = Ty::new_ref(
            self.tcx.tcx,
            self.tcx.lifetimes.re_static,
            ty::TypeAndMut { ty: self.tcx.types.str_, mutbl },
        );
        let layout = self.layout_of(ty).unwrap();
        Ok(MPlaceTy { mplace, layout, align: layout.align.abi })
    }

    /// Writes the aggregate to the destination.
    #[instrument(skip(self), level = "trace")]
    pub fn write_aggregate(
        &mut self,
        kind: &mir::AggregateKind<'tcx>,
        operands: &IndexSlice<FieldIdx, mir::Operand<'tcx>>,
        dest: &PlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx> {
        self.write_uninit(dest)?;
        let (variant_index, variant_dest, active_field_index) = match *kind {
            mir::AggregateKind::Adt(_, variant_index, _, _, active_field_index) => {
                let variant_dest = self.project_downcast(dest, variant_index)?;
                (variant_index, variant_dest, active_field_index)
            }
            _ => (FIRST_VARIANT, dest.clone(), None),
        };
        if active_field_index.is_some() {
            assert_eq!(operands.len(), 1);
        }
        for (field_index, operand) in operands.iter_enumerated() {
            let field_index = active_field_index.unwrap_or(field_index);
            let field_dest = self.project_field(&variant_dest, field_index.as_usize())?;
            let op = self.eval_operand(operand, Some(field_dest.layout))?;
            self.copy_op(&op, &field_dest, /*allow_transmute*/ false)?;
        }
        self.write_discriminant(variant_index, dest)
    }

    pub fn raw_const_to_mplace(
        &self,
        raw: mir::ConstAlloc<'tcx>,
    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
        // This must be an allocation in `tcx`
        let _ = self.tcx.global_alloc(raw.alloc_id);
        let ptr = self.global_base_pointer(Pointer::from(raw.alloc_id))?;
        let layout = self.layout_of(raw.ty)?;
        Ok(MPlaceTy::from_aligned_ptr(ptr.into(), layout))
    }

    /// Turn a place with a `dyn Trait` type into a place with the actual dynamic type.
    /// Aso returns the vtable.
    pub(super) fn unpack_dyn_trait(
        &self,
        mplace: &MPlaceTy<'tcx, M::Provenance>,
    ) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::Provenance>, Pointer<Option<M::Provenance>>)> {
        assert!(
            matches!(mplace.layout.ty.kind(), ty::Dynamic(_, _, ty::Dyn)),
            "`unpack_dyn_trait` only makes sense on `dyn*` types"
        );
        let vtable = mplace.meta().unwrap_meta().to_pointer(self)?;
        let (ty, _) = self.get_ptr_vtable(vtable)?;
        let layout = self.layout_of(ty)?;

        let mplace = MPlaceTy {
            mplace: MemPlace { meta: MemPlaceMeta::None, ..mplace.mplace },
            layout,
            align: layout.align.abi,
        };
        Ok((mplace, vtable))
    }

    /// Turn a `dyn* Trait` type into an value with the actual dynamic type.
    /// Also returns the vtable.
    pub(super) fn unpack_dyn_star<P: Projectable<'tcx, M::Provenance>>(
        &self,
        val: &P,
    ) -> InterpResult<'tcx, (P, Pointer<Option<M::Provenance>>)> {
        assert!(
            matches!(val.layout().ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
            "`unpack_dyn_star` only makes sense on `dyn*` types"
        );
        let data = self.project_field(val, 0)?;
        let vtable = self.project_field(val, 1)?;
        let vtable = self.read_pointer(&vtable.to_op(self)?)?;
        let (ty, _) = self.get_ptr_vtable(vtable)?;
        let layout = self.layout_of(ty)?;
        // `data` is already the right thing but has the wrong type. So we transmute it, by
        // projecting with offset 0.
        let data = data.transmute(layout, self)?;
        Ok((data, vtable))
    }
}

// Some nodes are used a lot. Make sure they don't unintentionally get bigger.
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
mod size_asserts {
    use super::*;
    use rustc_data_structures::static_assert_size;
    // tidy-alphabetical-start
    static_assert_size!(MemPlace, 40);
    static_assert_size!(MemPlaceMeta, 24);
    static_assert_size!(MPlaceTy<'_>, 64);
    static_assert_size!(Place, 40);
    static_assert_size!(PlaceTy<'_>, 64);
    // tidy-alphabetical-end
}