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::hash::{Hash, Hasher};
use std::convert::TryFrom;

use rustc::mir;
use rustc::ty::{self, Ty};
use rustc::ty::layout::{self, Size, Align, LayoutOf, TyLayout, HasDataLayout};
use rustc_data_structures::indexed_vec::Idx;

use rustc::mir::interpret::{
    GlobalId, Scalar, EvalResult, Pointer, ScalarMaybeUndef
};
use super::{EvalContext, Machine, Value, ValTy, Operand, OpTy, MemoryKind};

#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub struct MemPlace {
    /// A place may have an integral pointer for ZSTs, and since it might
    /// be turned back into a reference before ever being dereferenced.
    /// However, it may never be undef.
    pub ptr: Scalar,
    pub align: Align,
    pub extra: PlaceExtra,
}

#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub enum Place {
    /// A place referring to a value allocated in the `Memory` system.
    Ptr(MemPlace),

    /// To support alloc-free locals, we are able to write directly to a local.
    /// (Without that optimization, we'd just always be a `MemPlace`.)
    Local {
        frame: usize,
        local: mir::Local,
    },
}

// Extra information for fat pointers / places
#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub enum PlaceExtra {
    None,
    Length(u64),
    Vtable(Pointer),
}

#[derive(Copy, Clone, Debug)]
pub struct PlaceTy<'tcx> {
    place: Place,
    pub layout: TyLayout<'tcx>,
}

impl<'tcx> ::std::ops::Deref for PlaceTy<'tcx> {
    type Target = Place;
    #[inline(always)]
    fn deref(&self) -> &Place {
        &self.place
    }
}

/// A MemPlace with its layout. Constructing it is only possible in this module.
#[derive(Copy, Clone, Debug)]
pub struct MPlaceTy<'tcx> {
    mplace: MemPlace,
    pub layout: TyLayout<'tcx>,
}

impl<'tcx> ::std::ops::Deref for MPlaceTy<'tcx> {
    type Target = MemPlace;
    #[inline(always)]
    fn deref(&self) -> &MemPlace {
        &self.mplace
    }
}

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

impl MemPlace {
    #[inline(always)]
    pub fn from_scalar_ptr(ptr: Scalar, align: Align) -> Self {
        MemPlace {
            ptr,
            align,
            extra: PlaceExtra::None,
        }
    }

    #[inline(always)]
    pub fn from_ptr(ptr: Pointer, align: Align) -> Self {
        Self::from_scalar_ptr(ptr.into(), align)
    }

    #[inline(always)]
    pub fn to_scalar_ptr_align(self) -> (Scalar, Align) {
        assert_eq!(self.extra, PlaceExtra::None);
        (self.ptr, self.align)
    }

    /// Extract the ptr part of the mplace
    #[inline(always)]
    pub fn to_ptr(self) -> EvalResult<'tcx, Pointer> {
        // At this point, we forget about the alignment information -- the place has been turned into a reference,
        // and no matter where it came from, it now must be aligned.
        self.to_scalar_ptr_align().0.to_ptr()
    }

    /// Turn a mplace into a (thin or fat) pointer, as a reference, pointing to the same space.
    /// This is the inverse of `ref_to_mplace`.
    pub fn to_ref(self, cx: impl HasDataLayout) -> Value {
        // We ignore the alignment of the place here -- special handling for packed structs ends
        // at the `&` operator.
        match self.extra {
            PlaceExtra::None => Value::Scalar(self.ptr.into()),
            PlaceExtra::Length(len) => Value::new_slice(self.ptr.into(), len, cx),
            PlaceExtra::Vtable(vtable) => Value::new_dyn_trait(self.ptr.into(), vtable),
        }
    }
}

impl<'tcx> MPlaceTy<'tcx> {
    #[inline]
    fn from_aligned_ptr(ptr: Pointer, layout: TyLayout<'tcx>) -> Self {
        MPlaceTy { mplace: MemPlace::from_ptr(ptr, layout.align), layout }
    }

    #[inline]
    pub(super) fn len(self) -> u64 {
        // Sanity check
        let ty_len = match self.layout.fields {
            layout::FieldPlacement::Array { count, .. } => count,
            _ => bug!("Length for non-array layout {:?} requested", self.layout),
        };
        if let PlaceExtra::Length(len) = self.extra {
            len
        } else {
            ty_len
        }
    }
}

// Validation needs to hash MPlaceTy, but we cannot hash Layout -- so we just hash the type
impl<'tcx> Hash for MPlaceTy<'tcx> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.mplace.hash(state);
        self.layout.ty.hash(state);
    }
}
impl<'tcx> PartialEq for MPlaceTy<'tcx> {
    fn eq(&self, other: &Self) -> bool {
        self.mplace == other.mplace && self.layout.ty == other.layout.ty
    }
}
impl<'tcx> Eq for MPlaceTy<'tcx> {}

impl<'tcx> OpTy<'tcx> {
    #[inline(always)]
    pub fn try_as_mplace(self) -> Result<MPlaceTy<'tcx>, Value> {
        match *self {
            Operand::Indirect(mplace) => Ok(MPlaceTy { mplace, layout: self.layout }),
            Operand::Immediate(value) => Err(value),
        }
    }

    #[inline(always)]
    pub fn to_mem_place(self) -> MPlaceTy<'tcx> {
        self.try_as_mplace().unwrap()
    }
}

impl<'tcx> Place {
    /// Produces a Place that will error if attempted to be read from or written to
    #[inline]
    pub fn null(cx: impl HasDataLayout) -> Self {
        Self::from_scalar_ptr(Scalar::ptr_null(cx), Align::from_bytes(1, 1).unwrap())
    }

    #[inline]
    pub fn from_scalar_ptr(ptr: Scalar, align: Align) -> Self {
        Place::Ptr(MemPlace::from_scalar_ptr(ptr, align))
    }

    #[inline]
    pub fn from_ptr(ptr: Pointer, align: Align) -> Self {
        Place::Ptr(MemPlace::from_ptr(ptr, align))
    }

    #[inline]
    pub fn to_mem_place(self) -> MemPlace {
        match self {
            Place::Ptr(mplace) => mplace,
            _ => bug!("to_mem_place: expected Place::Ptr, got {:?}", self),

        }
    }

    #[inline]
    pub fn to_scalar_ptr_align(self) -> (Scalar, Align) {
        self.to_mem_place().to_scalar_ptr_align()
    }

    #[inline]
    pub fn to_ptr(self) -> EvalResult<'tcx, Pointer> {
        self.to_mem_place().to_ptr()
    }
}

impl<'tcx> PlaceTy<'tcx> {
    /// Produces a Place that will error if attempted to be read from or written to
    #[inline]
    pub fn null(cx: impl HasDataLayout, layout: TyLayout<'tcx>) -> Self {
        PlaceTy { place: Place::from_scalar_ptr(Scalar::ptr_null(cx), layout.align), layout }
    }

    #[inline]
    pub fn to_mem_place(self) -> MPlaceTy<'tcx> {
        MPlaceTy { mplace: self.place.to_mem_place(), layout: self.layout }
    }
}

impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
    /// Take a value, which represents a (thin or fat) reference, and make it a place.
    /// Alignment is just based on the type.  This is the inverse of `MemPlace::to_ref`.
    pub fn ref_to_mplace(
        &self, val: ValTy<'tcx>
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        let pointee_type = val.layout.ty.builtin_deref(true).unwrap().ty;
        let layout = self.layout_of(pointee_type)?;
        let mplace = match self.tcx.struct_tail(pointee_type).sty {
            ty::TyDynamic(..) => {
                let (ptr, vtable) = val.to_scalar_dyn_trait()?;
                MemPlace {
                    ptr,
                    align: layout.align,
                    extra: PlaceExtra::Vtable(vtable),
                }
            }
            ty::TyStr | ty::TySlice(_) => {
                let (ptr, len) = val.to_scalar_slice(self)?;
                MemPlace {
                    ptr,
                    align: layout.align,
                    extra: PlaceExtra::Length(len),
                }
            }
            _ => MemPlace {
                ptr: val.to_scalar()?,
                align: layout.align,
                extra: PlaceExtra::None,
            },
        };
        Ok(MPlaceTy { mplace, layout })
    }

    /// Offset a pointer to project to a field. Unlike place_field, this is always
    /// possible without allocating, so it can take &self. Also return the field's layout.
    /// This supports both struct and array fields.
    #[inline(always)]
    pub fn mplace_field(
        &self,
        base: MPlaceTy<'tcx>,
        field: u64,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        // Not using the layout method because we want to compute on u64
        let offset = match base.layout.fields {
            layout::FieldPlacement::Arbitrary { ref offsets, .. } =>
                offsets[usize::try_from(field).unwrap()],
            layout::FieldPlacement::Array { stride, .. } => {
                let len = base.len();
                assert!(field < len, "Tried to access element {} of array/slice with length {}", field, len);
                stride * field
            }
            layout::FieldPlacement::Union(count) => {
                assert!(field < count as u64, "Tried to access field {} of union with {} fields", field, count);
                // Offset is always 0
                Size::from_bytes(0)
            }
        };
        // the only way conversion can fail if is this is an array (otherwise we already panicked
        // above). In that case, all fields are equal.
        let field_layout = base.layout.field(self, usize::try_from(field).unwrap_or(0))?;

        // Adjust offset
        let offset = match base.extra {
            PlaceExtra::Vtable(vtable) => {
                let (_, align) = self.read_size_and_align_from_vtable(vtable)?;
                // FIXME: Is this right? Should we always do this, or only when actually
                // accessing the field to which the vtable applies?
                offset.abi_align(align)
            }
            _ => {
                // No adjustment needed
                offset
            }
        };

        let ptr = base.ptr.ptr_offset(offset, self)?;
        let align = base.align.min(field_layout.align);
        let extra = if !field_layout.is_unsized() {
            PlaceExtra::None
        } else {
            assert!(base.extra != PlaceExtra::None, "Expected fat ptr");
            base.extra
        };

        Ok(MPlaceTy { mplace: MemPlace { ptr, align, extra }, layout: field_layout })
    }

    // Iterates over all fields of an array. Much more efficient than doing the
    // same by repeatedly calling `mplace_array`.
    pub fn mplace_array_fields(
        &self,
        base: MPlaceTy<'tcx>,
    ) -> EvalResult<'tcx, impl Iterator<Item=EvalResult<'tcx, MPlaceTy<'tcx>>> + 'a> {
        let len = base.len();
        let stride = match base.layout.fields {
            layout::FieldPlacement::Array { stride, .. } => stride,
            _ => bug!("mplace_array_fields: expected an array layout"),
        };
        let layout = base.layout.field(self, 0)?;
        let dl = &self.tcx.data_layout;
        Ok((0..len).map(move |i| {
            let ptr = base.ptr.ptr_offset(i * stride, dl)?;
            Ok(MPlaceTy {
                mplace: MemPlace { ptr, align: base.align, extra: PlaceExtra::None },
                layout
            })
        }))
    }

    pub fn mplace_subslice(
        &self,
        base: MPlaceTy<'tcx>,
        from: u64,
        to: u64,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        let len = base.len();
        assert!(from <= len - to);

        // Not using layout method because that works with usize, and does not work with slices
        // (that have count 0 in their layout).
        let from_offset = match base.layout.fields {
            layout::FieldPlacement::Array { stride, .. } =>
                stride * from,
            _ => bug!("Unexpected layout of index access: {:#?}", base.layout),
        };
        let ptr = base.ptr.ptr_offset(from_offset, self)?;

        // Compute extra and new layout
        let inner_len = len - to - from;
        let (extra, ty) = match base.layout.ty.sty {
            ty::TyArray(inner, _) =>
                (PlaceExtra::None, self.tcx.mk_array(inner, inner_len)),
            ty::TySlice(..) =>
                (PlaceExtra::Length(inner_len), base.layout.ty),
            _ =>
                bug!("cannot subslice non-array type: `{:?}`", base.layout.ty),
        };
        let layout = self.layout_of(ty)?;

        Ok(MPlaceTy {
            mplace: MemPlace { ptr, align: base.align, extra },
            layout
        })
    }

    pub fn mplace_downcast(
        &self,
        base: MPlaceTy<'tcx>,
        variant: usize,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        // Downcasts only change the layout
        assert_eq!(base.extra, PlaceExtra::None);
        Ok(MPlaceTy { layout: base.layout.for_variant(self, variant), ..base })
    }

    /// Project into an mplace
    pub fn mplace_projection(
        &self,
        base: MPlaceTy<'tcx>,
        proj_elem: &mir::PlaceElem<'tcx>,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        use rustc::mir::ProjectionElem::*;
        Ok(match *proj_elem {
            Field(field, _) => self.mplace_field(base, field.index() as u64)?,
            Downcast(_, variant) => self.mplace_downcast(base, variant)?,
            Deref => self.deref_operand(base.into())?,

            Index(local) => {
                let n = *self.frame().locals[local].access()?;
                let n_layout = self.layout_of(self.tcx.types.usize)?;
                let n = self.read_scalar(OpTy { op: n, layout: n_layout })?;
                let n = n.to_bits(self.tcx.data_layout.pointer_size)?;
                self.mplace_field(base, u64::try_from(n).unwrap())?
            }

            ConstantIndex {
                offset,
                min_length,
                from_end,
            } => {
                let n = base.len();
                assert!(n >= min_length as u64);

                let index = if from_end {
                    n - u64::from(offset)
                } else {
                    u64::from(offset)
                };

                self.mplace_field(base, index)?
            }

            Subslice { from, to } =>
                self.mplace_subslice(base, u64::from(from), u64::from(to))?,
        })
    }

    /// Get the place of a field inside the place, and also the field's type.
    /// Just a convenience function, but used quite a bit.
    pub fn place_field(
        &mut self,
        base: PlaceTy<'tcx>,
        field: u64,
    ) -> EvalResult<'tcx, PlaceTy<'tcx>> {
        // FIXME: We could try to be smarter and avoid allocation for fields that span the
        // entire place.
        let mplace = self.force_allocation(base)?;
        Ok(self.mplace_field(mplace, field)?.into())
    }

    pub fn place_downcast(
        &mut self,
        base: PlaceTy<'tcx>,
        variant: usize,
    ) -> EvalResult<'tcx, PlaceTy<'tcx>> {
        // Downcast just changes the layout
        Ok(match base.place {
            Place::Ptr(mplace) =>
                self.mplace_downcast(MPlaceTy { mplace, layout: base.layout }, variant)?.into(),
            Place::Local { .. } => {
                let layout = base.layout.for_variant(&self, variant);
                PlaceTy { layout, ..base }
            }
        })
    }

    /// Project into a place
    pub fn place_projection(
        &mut self,
        base: PlaceTy<'tcx>,
        proj_elem: &mir::ProjectionElem<'tcx, mir::Local, Ty<'tcx>>,
    ) -> EvalResult<'tcx, PlaceTy<'tcx>> {
        use rustc::mir::ProjectionElem::*;
        Ok(match *proj_elem {
            Field(field, _) =>  self.place_field(base, field.index() as u64)?,
            Downcast(_, variant) => self.place_downcast(base, variant)?,
            Deref => self.deref_operand(self.place_to_op(base)?)?.into(),
            // For the other variants, we have to force an allocation.
            // This matches `operand_projection`.
            Subslice { .. } | ConstantIndex { .. } | Index(_) => {
                let mplace = self.force_allocation(base)?;
                self.mplace_projection(mplace, proj_elem)?.into()
            }
        })
    }

    /// Compute a place.  You should only use this if you intend to write into this
    /// place; for reading, a more efficient alternative is `eval_place_for_read`.
    pub fn eval_place(&mut self, mir_place: &mir::Place<'tcx>) -> EvalResult<'tcx, PlaceTy<'tcx>> {
        use rustc::mir::Place::*;
        let place = match *mir_place {
            Local(mir::RETURN_PLACE) => PlaceTy {
                place: self.frame().return_place,
                layout: self.layout_of_local(self.cur_frame(), mir::RETURN_PLACE)?,
            },
            Local(local) => PlaceTy {
                place: Place::Local {
                    frame: self.cur_frame(),
                    local,
                },
                layout: self.layout_of_local(self.cur_frame(), local)?,
            },

            Promoted(ref promoted) => {
                let instance = self.frame().instance;
                let op = self.global_to_op(GlobalId {
                    instance,
                    promoted: Some(promoted.0),
                })?;
                let mplace = op.to_mem_place();
                let ty = self.monomorphize(promoted.1, self.substs());
                PlaceTy {
                    place: Place::Ptr(mplace),
                    layout: self.layout_of(ty)?,
                }
            }

            Static(ref static_) => {
                let ty = self.monomorphize(static_.ty, self.substs());
                let layout = self.layout_of(ty)?;
                let instance = ty::Instance::mono(*self.tcx, static_.def_id);
                let cid = GlobalId {
                    instance,
                    promoted: None
                };
                let alloc = Machine::init_static(self, cid)?;
                MPlaceTy::from_aligned_ptr(alloc.into(), layout).into()
            }

            Projection(ref proj) => {
                let place = self.eval_place(&proj.base)?;
                self.place_projection(place, &proj.elem)?
            }
        };

        self.dump_place(place.place);

        Ok(place)
    }

    /// Write a scalar to a place
    pub fn write_scalar(
        &mut self,
        val: impl Into<ScalarMaybeUndef>,
        dest: PlaceTy<'tcx>,
    ) -> EvalResult<'tcx> {
        self.write_value(Value::Scalar(val.into()), dest)
    }

    /// Write a value to a place
    pub fn write_value(
        &mut self,
        src_val: Value,
        dest: PlaceTy<'tcx>,
    ) -> EvalResult<'tcx> {
        trace!("write_value: {:?} <- {:?}", *dest, src_val);
        // See if we can avoid an allocation. This is the counterpart to `try_read_value`,
        // but not factored as a separate function.
        let mplace = match dest.place {
            Place::Local { frame, local } => {
                match *self.stack[frame].locals[local].access_mut()? {
                    Operand::Immediate(ref mut dest_val) => {
                        // Yay, we can just change the local directly.
                        *dest_val = src_val;
                        return Ok(());
                    },
                    Operand::Indirect(mplace) => mplace, // already in memory
                }
            },
            Place::Ptr(mplace) => mplace, // already in memory
        };

        // This is already in memory, write there.
        let dest = MPlaceTy { mplace, layout: dest.layout };
        self.write_value_to_mplace(src_val, dest)
    }

    /// Write a value to memory
    fn write_value_to_mplace(
        &mut self,
        value: Value,
        dest: MPlaceTy<'tcx>,
    ) -> EvalResult<'tcx> {
        let (ptr, ptr_align) = dest.to_scalar_ptr_align();
        // Note that it is really important that the type here is the right one, and matches the type things are read at.
        // In case `src_val` 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.

        // Nothing to do for ZSTs, other than checking alignment
        if dest.layout.size.bytes() == 0 {
            self.memory.check_align(ptr, ptr_align)?;
            return Ok(());
        }

        let ptr = ptr.to_ptr()?;
        match value {
            Value::Scalar(scalar) => {
                self.memory.write_scalar(
                    ptr, ptr_align.min(dest.layout.align), scalar, dest.layout.size
                )
            }
            Value::ScalarPair(a_val, b_val) => {
                let (a, b) = match dest.layout.abi {
                    layout::Abi::ScalarPair(ref a, ref b) => (&a.value, &b.value),
                    _ => bug!("write_value_to_mplace: invalid ScalarPair layout: {:#?}", dest.layout)
                };
                let (a_size, b_size) = (a.size(&self), b.size(&self));
                let (a_align, b_align) = (a.align(&self), b.align(&self));
                let b_offset = a_size.abi_align(b_align);
                let b_ptr = ptr.offset(b_offset, &self)?.into();

                self.memory.write_scalar(ptr, ptr_align.min(a_align), a_val, a_size)?;
                self.memory.write_scalar(b_ptr, ptr_align.min(b_align), b_val, b_size)
            }
        }
    }

    /// Copy the data from an operand to a place
    pub fn copy_op(
        &mut self,
        src: OpTy<'tcx>,
        dest: PlaceTy<'tcx>,
    ) -> EvalResult<'tcx> {
        assert_eq!(src.layout.size, dest.layout.size,
            "Size mismatch when copying!\nsrc: {:#?}\ndest: {:#?}", src, dest);

        // Let us see if the layout is simple so we take a shortcut, avoid force_allocation.
        let (src_ptr, src_align) = match self.try_read_value(src)? {
            Ok(src_val) =>
                // Yay, we got a value that we can write directly.  We write with the
                // *source layout*, because that was used to load, and if they do not match
                // this is a transmute we want to support.
                return self.write_value(src_val, PlaceTy { place: *dest, layout: src.layout }),
            Err(mplace) => mplace.to_scalar_ptr_align(),
        };
        // Slow path, this does not fit into an immediate. Just memcpy.
        trace!("copy_op: {:?} <- {:?}", *dest, *src);
        let (dest_ptr, dest_align) = self.force_allocation(dest)?.to_scalar_ptr_align();
        self.memory.copy(
            src_ptr, src_align,
            dest_ptr, dest_align,
            src.layout.size, false
        )
    }

    /// Make sure that a place is in memory, and return where it is.
    /// This is essentially `force_to_memplace`.
    pub fn force_allocation(
        &mut self,
        place: PlaceTy<'tcx>,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        let mplace = match place.place {
            Place::Local { frame, local } => {
                // FIXME: Consider not doing anything for a ZST, and just returning
                // a fake pointer?

                // We need the layout of the local.  We can NOT use the layout we got,
                // that might e.g. be a downcast variant!
                let local_layout = self.layout_of_local(frame, local)?;
                // Make sure it has a place
                let rval = *self.stack[frame].locals[local].access()?;
                let mplace = self.allocate_op(OpTy { op: rval, layout: local_layout })?.mplace;
                // This might have allocated the flag
                *self.stack[frame].locals[local].access_mut()? =
                    Operand::Indirect(mplace);
                // done
                mplace
            }
            Place::Ptr(mplace) => mplace
        };
        // Return with the original layout, so that the caller can go on
        Ok(MPlaceTy { mplace, layout: place.layout })
    }

    pub fn allocate(
        &mut self,
        layout: TyLayout<'tcx>,
        kind: MemoryKind<M::MemoryKinds>,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        assert!(!layout.is_unsized(), "cannot alloc memory for unsized type");
        let ptr = self.memory.allocate(layout.size, layout.align, kind)?;
        Ok(MPlaceTy::from_aligned_ptr(ptr, layout))
    }

    /// Make a place for an operand, allocating if needed
    pub fn allocate_op(
        &mut self,
        OpTy { op, layout }: OpTy<'tcx>,
    ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        Ok(match op {
            Operand::Indirect(mplace) => MPlaceTy { mplace, layout },
            Operand::Immediate(value) => {
                // FIXME: Is stack always right here?
                let ptr = self.allocate(layout, MemoryKind::Stack)?;
                self.write_value_to_mplace(value, ptr)?;
                ptr
            },
        })
    }

    pub fn write_discriminant_value(
        &mut self,
        variant_index: usize,
        dest: PlaceTy<'tcx>,
    ) -> EvalResult<'tcx> {
        match dest.layout.variants {
            layout::Variants::Single { index } => {
                if index != variant_index {
                    // If the layout of an enum is `Single`, all
                    // other variants are necessarily uninhabited.
                    assert_eq!(dest.layout.for_variant(&self, variant_index).abi,
                               layout::Abi::Uninhabited);
                }
            }
            layout::Variants::Tagged { ref tag, .. } => {
                let discr_val = dest.layout.ty.ty_adt_def().unwrap()
                    .discriminant_for_variant(*self.tcx, variant_index)
                    .val;

                // raw discriminants for enums are isize or bigger during
                // their computation, but the in-memory tag is the smallest possible
                // representation
                let size = tag.value.size(self.tcx.tcx);
                let shift = 128 - size.bits();
                let discr_val = (discr_val << shift) >> shift;

                let discr_dest = self.place_field(dest, 0)?;
                self.write_scalar(Scalar::Bits {
                    bits: discr_val,
                    size: size.bytes() as u8,
                }, discr_dest)?;
            }
            layout::Variants::NicheFilling {
                dataful_variant,
                ref niche_variants,
                niche_start,
                ..
            } => {
                if variant_index != dataful_variant {
                    let niche_dest =
                        self.place_field(dest, 0)?;
                    let niche_value = ((variant_index - niche_variants.start()) as u128)
                        .wrapping_add(niche_start);
                    self.write_scalar(Scalar::Bits {
                        bits: niche_value,
                        size: niche_dest.layout.size.bytes() as u8,
                    }, niche_dest)?;
                }
            }
        }

        Ok(())
    }

    /// Every place can be read from, so we can turm them into an operand
    #[inline(always)]
    pub fn place_to_op(&self, place: PlaceTy<'tcx>) -> EvalResult<'tcx, OpTy<'tcx>> {
        let op = match place.place {
            Place::Ptr(mplace) => {
                Operand::Indirect(mplace)
            }
            Place::Local { frame, local } =>
                *self.stack[frame].locals[local].access()?
        };
        Ok(OpTy { op, layout: place.layout })
    }

    /// Turn a place that is a dyn trait (i.e., PlaceExtra::Vtable and the appropriate layout)
    /// or a slice into the specific fixed-size place and layout that is given by the vtable/len.
    /// This "unpacks" the existential quantifier, so to speak.
    pub fn unpack_unsized_mplace(&self, mplace: MPlaceTy<'tcx>) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
        trace!("Unpacking {:?} ({:?})", *mplace, mplace.layout.ty);
        let layout = match mplace.extra {
            PlaceExtra::Vtable(vtable) => {
                // the drop function signature
                let drop_instance = self.read_drop_type_from_vtable(vtable)?;
                trace!("Found drop fn: {:?}", drop_instance);
                let fn_sig = drop_instance.ty(*self.tcx).fn_sig(*self.tcx);
                let fn_sig = self.tcx.normalize_erasing_late_bound_regions(self.param_env, &fn_sig);
                // the drop function takes *mut T where T is the type being dropped, so get that
                let ty = fn_sig.inputs()[0].builtin_deref(true).unwrap().ty;
                let layout = self.layout_of(ty)?;
                // Sanity checks
                let (size, align) = self.read_size_and_align_from_vtable(vtable)?;
                assert_eq!(size, layout.size);
                assert_eq!(align.abi(), layout.align.abi()); // only ABI alignment is preserved
                // FIXME: More checks for the vtable? We could make sure it is exactly
                // the one one would expect for this type.
                // Done!
                layout
            },
            PlaceExtra::Length(len) => {
                let ty = self.tcx.mk_array(mplace.layout.field(self, 0)?.ty, len);
                self.layout_of(ty)?
            }
            PlaceExtra::None => bug!("Expected a fat pointer"),
        };
        trace!("Unpacked type: {:?}", layout.ty);
        Ok(MPlaceTy {
            mplace: MemPlace { extra: PlaceExtra::None, ..*mplace },
            layout
        })
    }
}