rust/src/interpreter/mod.rs

use rustc::middle::const_val::ConstVal;
use rustc::hir::def_id::DefId;
use rustc::mir::mir_map::MirMap;
use rustc::mir::repr as mir;
use rustc::traits::Reveal;
use rustc::ty::layout::{self, Layout, Size};
use rustc::ty::subst::{self, Subst, Substs};
use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
use rustc::session::Session;
use rustc::util::nodemap::DefIdMap;
use rustc_data_structures::indexed_vec::Idx;
use std::cell::RefCell;
use std::ops::Deref;
use std::rc::Rc;
use std::iter;
use syntax::codemap::{self, DUMMY_SP};

use error::{EvalError, EvalResult};
use memory::{Memory, Pointer, AllocId};
use primval::{self, PrimVal};
use self::value::Value;

use std::collections::HashMap;

mod step;
mod terminator;
mod cast;
mod vtable;
mod value;

pub struct EvalContext<'a, 'tcx: 'a> {
    /// The results of the type checker, from rustc.
    tcx: TyCtxt<'a, 'tcx, 'tcx>,

    /// The Session, from rustc.
    /// Used to extract info from other crates
    session: &'a Session,

    /// A mapping from NodeIds to Mir, from rustc. Only contains MIR for crate-local items.
    mir_map: &'a MirMap<'tcx>,

    /// A local cache from DefIds to Mir for non-crate-local items.
    mir_cache: RefCell<DefIdMap<Rc<mir::Mir<'tcx>>>>,

    /// The virtual memory system.
    memory: Memory<'a, 'tcx>,

    /// Precomputed statics, constants and promoteds.
    statics: HashMap<ConstantId<'tcx>, Pointer>,

    /// The virtual call stack.
    stack: Vec<Frame<'a, 'tcx>>,

    /// The maximum number of stack frames allowed
    stack_limit: usize,
}

/// A stack frame.
pub struct Frame<'a, 'tcx: 'a> {
    ////////////////////////////////////////////////////////////////////////////////
    // Function and callsite information
    ////////////////////////////////////////////////////////////////////////////////

    /// The MIR for the function called on this frame.
    pub mir: CachedMir<'a, 'tcx>,

    /// The def_id of the current function.
    pub def_id: DefId,

    /// type substitutions for the current function invocation.
    pub substs: &'tcx Substs<'tcx>,

    /// The span of the call site.
    pub span: codemap::Span,

    ////////////////////////////////////////////////////////////////////////////////
    // Return pointer and local allocations
    ////////////////////////////////////////////////////////////////////////////////

    /// A pointer for writing the return value of the current call if it's not a diverging call.
    pub return_ptr: Option<Pointer>,

    /// The block to return to when returning from the current stack frame
    pub return_to_block: StackPopCleanup,

    /// The list of locals for the current function, stored in order as
    /// `[arguments..., variables..., temporaries...]`. The variables begin at `self.var_offset`
    /// and the temporaries at `self.temp_offset`.
    pub locals: Vec<Pointer>,

    /// The offset of the first variable in `self.locals`.
    pub var_offset: usize,

    /// The offset of the first temporary in `self.locals`.
    pub temp_offset: usize,

    ////////////////////////////////////////////////////////////////////////////////
    // Current position within the function
    ////////////////////////////////////////////////////////////////////////////////

    /// The block that is currently executed (or will be executed after the above call stacks
    /// return).
    pub block: mir::BasicBlock,

    /// The index of the currently evaluated statment.
    pub stmt: usize,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
struct Lvalue {
    ptr: Pointer,
    extra: LvalueExtra,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
enum LvalueExtra {
    None,
    Length(u64),
    Vtable(Pointer),
    DowncastVariant(usize),
}

#[derive(Clone)]
pub enum CachedMir<'mir, 'tcx: 'mir> {
    Ref(&'mir mir::Mir<'tcx>),
    Owned(Rc<mir::Mir<'tcx>>)
}

#[derive(Clone, Debug, Eq, PartialEq, Hash)]
/// Uniquely identifies a specific constant or static
struct ConstantId<'tcx> {
    /// the def id of the constant/static or in case of promoteds, the def id of the function they belong to
    def_id: DefId,
    /// In case of statics and constants this is `Substs::empty()`, so only promoteds and associated
    /// constants actually have something useful here. We could special case statics and constants,
    /// but that would only require more branching when working with constants, and not bring any
    /// real benefits.
    substs: &'tcx Substs<'tcx>,
    kind: ConstantKind,
}

#[derive(Clone, Debug, Eq, PartialEq, Hash)]
enum ConstantKind {
    Promoted(mir::Promoted),
    /// Statics, constants and associated constants
    Global,
}

#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub enum StackPopCleanup {
    /// The stackframe existed to compute the initial value of a static/constant, make sure the
    /// static isn't modifyable afterwards
    Freeze(AllocId),
    /// A regular stackframe added due to a function call will need to get forwarded to the next
    /// block
    Goto(mir::BasicBlock),
    /// The main function and diverging functions have nowhere to return to
    None,
}

impl<'a, 'tcx> EvalContext<'a, 'tcx> {
    pub fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>, mir_map: &'a MirMap<'tcx>, memory_size: usize, stack_limit: usize, session: &'a Session) -> Self {
        EvalContext {
            tcx: tcx,
            mir_map: mir_map,
            mir_cache: RefCell::new(DefIdMap()),
            memory: Memory::new(&tcx.data_layout, memory_size),
            statics: HashMap::new(),
            stack: Vec::new(),
            stack_limit: stack_limit,
            session: session,
        }
    }

    pub fn alloc_ret_ptr(&mut self, ty: Ty<'tcx>, substs: &'tcx Substs<'tcx>) -> EvalResult<'tcx, Pointer> {
        let size = self.type_size_with_substs(ty, substs);
        let align = self.type_align_with_substs(ty, substs);
        self.memory.allocate(size, align)
    }

    pub fn memory(&self) -> &Memory<'a, 'tcx> {
        &self.memory
    }

    pub fn memory_mut(&mut self) -> &mut Memory<'a, 'tcx> {
        &mut self.memory
    }

    pub fn stack(&self) -> &[Frame<'a, 'tcx>] {
        &self.stack
    }

    fn target_isize_primval(&self, n: i64) -> PrimVal {
        match self.memory.pointer_size() {
            1 => PrimVal::I8(n as i8),
            2 => PrimVal::I16(n as i16),
            4 => PrimVal::I32(n as i32),
            8 => PrimVal::I64(n as i64),
            p => bug!("unsupported target pointer size: {}", p),
        }
    }

    fn target_usize_primval(&self, n: u64) -> PrimVal {
        match self.memory.pointer_size() {
            1 => PrimVal::U8(n as u8),
            2 => PrimVal::U16(n as u16),
            4 => PrimVal::U32(n as u32),
            8 => PrimVal::U64(n as u64),
            p => bug!("unsupported target pointer size: {}", p),
        }
    }

    fn str_to_value(&mut self, s: &str) -> EvalResult<'tcx, Value> {
        // FIXME: cache these allocs
        let ptr = self.memory.allocate(s.len(), 1)?;
        self.memory.write_bytes(ptr, s.as_bytes())?;
        self.memory.freeze(ptr.alloc_id)?;
        Ok(Value::ByValPair(PrimVal::Ptr(ptr), self.target_usize_primval(s.len() as u64)))
    }

    fn const_to_value(&mut self, const_val: &ConstVal) -> EvalResult<'tcx, Value> {
        use rustc::middle::const_val::ConstVal::*;
        use rustc_const_math::{ConstInt, ConstIsize, ConstUsize, ConstFloat};

        let primval = match *const_val {
            Integral(ConstInt::I8(i)) => PrimVal::I8(i),
            Integral(ConstInt::U8(i)) => PrimVal::U8(i),
            Integral(ConstInt::Isize(ConstIsize::Is16(i))) |
            Integral(ConstInt::I16(i)) => PrimVal::I16(i),
            Integral(ConstInt::Usize(ConstUsize::Us16(i))) |
            Integral(ConstInt::U16(i)) => PrimVal::U16(i),
            Integral(ConstInt::Isize(ConstIsize::Is32(i))) |
            Integral(ConstInt::I32(i)) => PrimVal::I32(i),
            Integral(ConstInt::Usize(ConstUsize::Us32(i))) |
            Integral(ConstInt::U32(i)) => PrimVal::U32(i),
            Integral(ConstInt::Isize(ConstIsize::Is64(i))) |
            Integral(ConstInt::I64(i)) => PrimVal::I64(i),
            Integral(ConstInt::Usize(ConstUsize::Us64(i))) |
            Integral(ConstInt::U64(i)) => PrimVal::U64(i),
            Float(ConstFloat::F32(f)) => PrimVal::F32(f),
            Float(ConstFloat::F64(f)) => PrimVal::F64(f),
            Bool(b) => PrimVal::Bool(b),
            Char(c) => PrimVal::Char(c),

            Str(ref s) => return self.str_to_value(s),

            ByteStr(ref bs) => {
                let ptr = self.memory.allocate(bs.len(), 1)?;
                self.memory.write_bytes(ptr, bs)?;
                self.memory.freeze(ptr.alloc_id)?;
                PrimVal::Ptr(ptr)
            }

            Struct(_)    => unimplemented!(),
            Tuple(_)     => unimplemented!(),
            Function(_)  => unimplemented!(),
            Array(_, _)  => unimplemented!(),
            Repeat(_, _) => unimplemented!(),
            Dummy        => unimplemented!(),

            Float(ConstFloat::FInfer{..}) |
            Integral(ConstInt::Infer(_)) |
            Integral(ConstInt::InferSigned(_)) =>
                bug!("uninferred constants only exist before typeck"),
        };

        Ok(Value::ByVal(primval))
    }

    fn type_is_sized(&self, ty: Ty<'tcx>) -> bool {
        // generics are weird, don't run this function on a generic
        assert!(!ty.needs_subst());
        ty.is_sized(self.tcx, &self.tcx.empty_parameter_environment(), DUMMY_SP)
    }

    pub fn load_mir(&self, def_id: DefId) -> EvalResult<'tcx, CachedMir<'a, 'tcx>> {
        trace!("load mir {:?}", def_id);
        if def_id.is_local() {
            Ok(CachedMir::Ref(self.mir_map.map.get(&def_id).unwrap()))
        } else {
            let mut mir_cache = self.mir_cache.borrow_mut();
            if let Some(mir) = mir_cache.get(&def_id) {
                return Ok(CachedMir::Owned(mir.clone()));
            }

            let cs = &self.tcx.sess.cstore;
            match cs.maybe_get_item_mir(self.tcx, def_id) {
                Some(mir) => {
                    let cached = Rc::new(mir);
                    mir_cache.insert(def_id, cached.clone());
                    Ok(CachedMir::Owned(cached))
                },
                None => Err(EvalError::NoMirFor(self.tcx.item_path_str(def_id))),
            }
        }
    }

    pub fn monomorphize_field_ty(&self, f: ty::FieldDef<'tcx>, substs: &'tcx Substs<'tcx>) -> Ty<'tcx> {
        let substituted = &f.ty(self.tcx, substs);
        self.tcx.normalize_associated_type(&substituted)
    }

    pub fn monomorphize(&self, ty: Ty<'tcx>, substs: &'tcx Substs<'tcx>) -> Ty<'tcx> {
        let substituted = ty.subst(self.tcx, substs);
        self.tcx.normalize_associated_type(&substituted)
    }

    fn type_size(&self, ty: Ty<'tcx>) -> usize {
        self.type_size_with_substs(ty, self.substs())
    }

    fn type_align(&self, ty: Ty<'tcx>) -> usize {
        self.type_align_with_substs(ty, self.substs())
    }

    fn type_size_with_substs(&self, ty: Ty<'tcx>, substs: &'tcx Substs<'tcx>) -> usize {
        self.type_layout_with_substs(ty, substs).size(&self.tcx.data_layout).bytes() as usize
    }

    fn type_align_with_substs(&self, ty: Ty<'tcx>, substs: &'tcx Substs<'tcx>) -> usize {
        self.type_layout_with_substs(ty, substs).align(&self.tcx.data_layout).abi() as usize
    }

    fn type_layout(&self, ty: Ty<'tcx>) -> &'tcx Layout {
        self.type_layout_with_substs(ty, self.substs())
    }

    fn type_layout_with_substs(&self, ty: Ty<'tcx>, substs: &'tcx Substs<'tcx>) -> &'tcx Layout {
        // TODO(solson): Is this inefficient? Needs investigation.
        let ty = self.monomorphize(ty, substs);

        self.tcx.infer_ctxt(None, None, Reveal::All).enter(|infcx| {
            // TODO(solson): Report this error properly.
            ty.layout(&infcx).unwrap()
        })
    }

    pub fn push_stack_frame(
        &mut self,
        def_id: DefId,
        span: codemap::Span,
        mir: CachedMir<'a, 'tcx>,
        substs: &'tcx Substs<'tcx>,
        return_ptr: Option<Pointer>,
        return_to_block: StackPopCleanup,
    ) -> EvalResult<'tcx, ()> {
        let arg_tys = mir.arg_decls.iter().map(|a| a.ty);
        let var_tys = mir.var_decls.iter().map(|v| v.ty);
        let temp_tys = mir.temp_decls.iter().map(|t| t.ty);

        let num_args = mir.arg_decls.len();
        let num_vars = mir.var_decls.len();

        ::log_settings::settings().indentation += 1;

        let locals: EvalResult<'tcx, Vec<Pointer>> = arg_tys.chain(var_tys).chain(temp_tys).map(|ty| {
            let size = self.type_size_with_substs(ty, substs);
            let align = self.type_align_with_substs(ty, substs);
            self.memory.allocate(size, align)
        }).collect();

        self.stack.push(Frame {
            mir: mir.clone(),
            block: mir::START_BLOCK,
            return_ptr: return_ptr,
            return_to_block: return_to_block,
            locals: locals?,
            var_offset: num_args,
            temp_offset: num_args + num_vars,
            span: span,
            def_id: def_id,
            substs: substs,
            stmt: 0,
        });
        if self.stack.len() > self.stack_limit {
            Err(EvalError::StackFrameLimitReached)
        } else {
            Ok(())
        }
    }

    fn pop_stack_frame(&mut self) -> EvalResult<'tcx, ()> {
        ::log_settings::settings().indentation -= 1;
        let frame = self.stack.pop().expect("tried to pop a stack frame, but there were none");
        match frame.return_to_block {
            StackPopCleanup::Freeze(alloc_id) => self.memory.freeze(alloc_id)?,
            StackPopCleanup::Goto(target) => self.goto_block(target),
            StackPopCleanup::None => {},
        }
        // TODO(solson): Deallocate local variables.
        Ok(())
    }

    /// Applies the binary operation `op` to the two operands and writes a tuple of the result
    /// and a boolean signifying the potential overflow to the destination.
    fn intrinsic_with_overflow(
        &mut self,
        op: mir::BinOp,
        left: &mir::Operand<'tcx>,
        right: &mir::Operand<'tcx>,
        dest: Pointer,
        dest_layout: &'tcx Layout,
    ) -> EvalResult<'tcx, ()> {
        use rustc::ty::layout::Layout::*;
        let tup_layout = match *dest_layout {
            Univariant { ref variant, .. } => variant,
            _ => bug!("checked bin op returns something other than a tuple"),
        };

        let overflowed = self.intrinsic_overflowing(op, left, right, dest)?;
        let offset = tup_layout.field_offset(1).bytes() as isize;
        self.memory.write_bool(dest.offset(offset), overflowed)
    }

    /// Applies the binary operation `op` to the arguments and writes the result to the destination.
    /// Returns `true` if the operation overflowed.
    fn intrinsic_overflowing(
        &mut self,
        op: mir::BinOp,
        left: &mir::Operand<'tcx>,
        right: &mir::Operand<'tcx>,
        dest: Pointer,
    ) -> EvalResult<'tcx, bool> {
        let left_primval = self.eval_operand_to_primval(left)?;
        let right_primval = self.eval_operand_to_primval(right)?;
        let (val, overflow) = primval::binary_op(op, left_primval, right_primval)?;
        self.memory.write_primval(dest, val)?;
        Ok(overflow)
    }

    fn assign_fields<I: IntoIterator<Item = u64>>(
        &mut self,
        dest: Pointer,
        offsets: I,
        operands: &[mir::Operand<'tcx>],
    ) -> EvalResult<'tcx, ()> {
        for (offset, operand) in offsets.into_iter().zip(operands) {
            let value = self.eval_operand(operand)?;
            let value_ty = self.operand_ty(operand);
            let field_dest = dest.offset(offset as isize);
            self.write_value(value, field_dest, value_ty)?;
        }
        Ok(())
    }

    /// Evaluate an assignment statement.
    ///
    /// There is no separate `eval_rvalue` function. Instead, the code for handling each rvalue
    /// type writes its results directly into the memory specified by the lvalue.
    fn eval_rvalue_into_lvalue(
        &mut self,
        rvalue: &mir::Rvalue<'tcx>,
        lvalue: &mir::Lvalue<'tcx>,
    ) -> EvalResult<'tcx, ()> {
        let dest = self.eval_lvalue(lvalue)?.to_ptr();
        let dest_ty = self.lvalue_ty(lvalue);
        let dest_layout = self.type_layout(dest_ty);

        use rustc::mir::repr::Rvalue::*;
        match *rvalue {
            Use(ref operand) => {
                let value = self.eval_operand(operand)?;
                self.write_value(value, dest, dest_ty)?;
            }

            BinaryOp(bin_op, ref left, ref right) => {
                // ignore overflow bit, rustc inserts check branches for us
                self.intrinsic_overflowing(bin_op, left, right, dest)?;
            }

            CheckedBinaryOp(bin_op, ref left, ref right) => {
                self.intrinsic_with_overflow(bin_op, left, right, dest, dest_layout)?;
            }

            UnaryOp(un_op, ref operand) => {
                let val = self.eval_operand_to_primval(operand)?;
                self.memory.write_primval(dest, primval::unary_op(un_op, val)?)?;
            }

            Aggregate(ref kind, ref operands) => {
                use rustc::ty::layout::Layout::*;
                match *dest_layout {
                    Univariant { ref variant, .. } => {
                        let offsets = iter::once(0)
                            .chain(variant.offset_after_field.iter().map(|s| s.bytes()));
                        self.assign_fields(dest, offsets, operands)?;
                    }

                    Array { .. } => {
                        let elem_size = match dest_ty.sty {
                            ty::TyArray(elem_ty, _) => self.type_size(elem_ty) as u64,
                            _ => bug!("tried to assign {:?} to non-array type {:?}", kind, dest_ty),
                        };
                        let offsets = (0..).map(|i| i * elem_size);
                        self.assign_fields(dest, offsets, operands)?;
                    }

                    General { discr, ref variants, .. } => {
                        if let mir::AggregateKind::Adt(adt_def, variant, _, _) = *kind {
                            let discr_val = adt_def.variants[variant].disr_val.to_u64_unchecked();
                            let discr_size = discr.size().bytes() as usize;
                            self.memory.write_uint(dest, discr_val, discr_size)?;

                            let offsets = variants[variant].offset_after_field.iter()
                                .map(|s| s.bytes());
                            self.assign_fields(dest, offsets, operands)?;
                        } else {
                            bug!("tried to assign {:?} to Layout::General", kind);
                        }
                    }

                    RawNullablePointer { nndiscr, .. } => {
                        if let mir::AggregateKind::Adt(_, variant, _, _) = *kind {
                            if nndiscr == variant as u64 {
                                assert_eq!(operands.len(), 1);
                                let operand = &operands[0];
                                let value = self.eval_operand(operand)?;
                                let value_ty = self.operand_ty(operand);
                                self.write_value(value, dest, value_ty)?;
                            } else {
                                assert_eq!(operands.len(), 0);
                                self.memory.write_isize(dest, 0)?;
                            }
                        } else {
                            bug!("tried to assign {:?} to Layout::RawNullablePointer", kind);
                        }
                    }

                    StructWrappedNullablePointer { nndiscr, ref nonnull, ref discrfield } => {
                        if let mir::AggregateKind::Adt(_, variant, _, _) = *kind {
                            if nndiscr == variant as u64 {
                                let offsets = iter::once(0)
                                    .chain(nonnull.offset_after_field.iter().map(|s| s.bytes()));
                                try!(self.assign_fields(dest, offsets, operands));
                            } else {
                                for operand in operands {
                                    let operand_ty = self.operand_ty(operand);
                                    assert_eq!(self.type_size(operand_ty), 0);
                                }
                                let offset = self.nonnull_offset(dest_ty, nndiscr, discrfield)?;
                                let dest = dest.offset(offset.bytes() as isize);
                                try!(self.memory.write_isize(dest, 0));
                            }
                        } else {
                            bug!("tried to assign {:?} to Layout::RawNullablePointer", kind);
                        }
                    }

                    CEnum { discr, signed, .. } => {
                        assert_eq!(operands.len(), 0);
                        if let mir::AggregateKind::Adt(adt_def, variant, _, _) = *kind {
                            let val = adt_def.variants[variant].disr_val.to_u64_unchecked();
                            let size = discr.size().bytes() as usize;

                            if signed {
                                self.memory.write_int(dest, val as i64, size)?;
                            } else {
                                self.memory.write_uint(dest, val, size)?;
                            }
                        } else {
                            bug!("tried to assign {:?} to Layout::CEnum", kind);
                        }
                    }

                    _ => return Err(EvalError::Unimplemented(format!("can't handle destination layout {:?} when assigning {:?}", dest_layout, kind))),
                }
            }

            Repeat(ref operand, _) => {
                let (elem_ty, length) = match dest_ty.sty {
                    ty::TyArray(elem_ty, n) => (elem_ty, n),
                    _ => bug!("tried to assign array-repeat to non-array type {:?}", dest_ty),
                };
                let elem_size = self.type_size(elem_ty);
                let value = self.eval_operand(operand)?;
                for i in 0..length {
                    let elem_dest = dest.offset((i * elem_size) as isize);
                    self.write_value(value, elem_dest, elem_ty)?;
                }
            }

            Len(ref lvalue) => {
                let src = self.eval_lvalue(lvalue)?;
                let ty = self.lvalue_ty(lvalue);
                match ty.sty {
                    ty::TyArray(_, n) => self.memory.write_usize(dest, n as u64)?,
                    ty::TySlice(_) => if let LvalueExtra::Length(len) = src.extra {
                        self.memory.write_usize(dest, len)?;
                    } else {
                        bug!("Rvalue::Len of a slice given non-slice pointer: {:?}", src);
                    },
                    _ => bug!("Rvalue::Len expected array or slice, got {:?}", ty),
                }
            }

            Ref(_, _, ref lvalue) => {
                let lvalue = self.eval_lvalue(lvalue)?;
                self.memory.write_ptr(dest, lvalue.ptr)?;
                let extra_ptr = dest.offset(self.memory.pointer_size() as isize);
                match lvalue.extra {
                    LvalueExtra::None => {},
                    LvalueExtra::Length(len) => self.memory.write_usize(extra_ptr, len)?,
                    LvalueExtra::Vtable(ptr) => self.memory.write_ptr(extra_ptr, ptr)?,
                    LvalueExtra::DowncastVariant(..) =>
                        bug!("attempted to take a reference to an enum downcast lvalue"),
                }
            }

            Box(ty) => {
                let size = self.type_size(ty);
                let align = self.type_align(ty);
                let ptr = self.memory.allocate(size, align)?;
                self.memory.write_ptr(dest, ptr)?;
            }

            Cast(kind, ref operand, cast_ty) => {
                debug_assert_eq!(self.monomorphize(cast_ty, self.substs()), dest_ty);
                use rustc::mir::repr::CastKind::*;
                match kind {
                    Unsize => {
                        let src = self.eval_operand(operand)?;
                        let src_ty = self.operand_ty(operand);
                        self.unsize_into(src, src_ty, dest, dest_ty)?;
                    }

                    Misc => {
                        let src = self.eval_operand(operand)?;
                        let src_ty = self.operand_ty(operand);
                        if self.type_is_fat_ptr(src_ty) {
                            trace!("misc cast: {:?}", src);
                            let ptr_size = self.memory.pointer_size();
                            match (src, self.type_is_fat_ptr(dest_ty)) {
                                (Value::ByValPair(data, meta), true) => {
                                    self.memory.write_primval(dest, data)?;
                                    self.memory.write_primval(dest.offset(ptr_size as isize), meta)?;
                                },
                                (Value::ByValPair(data, _), false) => {
                                    self.memory.write_primval(dest, data)?;
                                },
                                (Value::ByRef(ptr), true) => {
                                    self.memory.copy(ptr, dest, ptr_size * 2, ptr_size)?;
                                },
                                (Value::ByRef(ptr), false) => {
                                    self.memory.copy(ptr, dest, ptr_size, ptr_size)?;
                                },
                                (Value::ByVal(_), _) => bug!("expected fat ptr"),
                            }
                        } else {
                            let src_val = self.value_to_primval(src, src_ty)?;
                            let dest_val = self.cast_primval(src_val, dest_ty)?;
                            self.memory.write_primval(dest, dest_val)?;
                        }
                    }

                    ReifyFnPointer => match self.operand_ty(operand).sty {
                        ty::TyFnDef(def_id, substs, fn_ty) => {
                            let fn_ptr = self.memory.create_fn_ptr(def_id, substs, fn_ty);
                            self.memory.write_ptr(dest, fn_ptr)?;
                        },
                        ref other => bug!("reify fn pointer on {:?}", other),
                    },

                    UnsafeFnPointer => match dest_ty.sty {
                        ty::TyFnPtr(unsafe_fn_ty) => {
                            let src = self.eval_operand(operand)?;
                            let ptr = src.read_ptr(&self.memory)?;
                            let (def_id, substs, _) = self.memory.get_fn(ptr.alloc_id)?;
                            let fn_ptr = self.memory.create_fn_ptr(def_id, substs, unsafe_fn_ty);
                            self.memory.write_ptr(dest, fn_ptr)?;
                        },
                        ref other => bug!("fn to unsafe fn cast on {:?}", other),
                    },
                }
            }

            InlineAsm { .. } => unimplemented!(),
        }

        Ok(())
    }

    fn type_is_fat_ptr(&self, ty: Ty<'tcx>) -> bool {
        match ty.sty {
            ty::TyRawPtr(ty::TypeAndMut{ty, ..}) |
            ty::TyRef(_, ty::TypeAndMut{ty, ..}) |
            ty::TyBox(ty) => !self.type_is_sized(ty),
            _ => false,
        }
    }

    fn nonnull_offset(&self, ty: Ty<'tcx>, nndiscr: u64, discrfield: &[u32]) -> EvalResult<'tcx, Size> {
        // Skip the constant 0 at the start meant for LLVM GEP.
        let mut path = discrfield.iter().skip(1).map(|&i| i as usize);

        // Handle the field index for the outer non-null variant.
        let inner_ty = match ty.sty {
            ty::TyAdt(adt_def, substs) => {
                let variant = &adt_def.variants[nndiscr as usize];
                let index = path.next().unwrap();
                let field = &variant.fields[index];
                field.ty(self.tcx, substs)
            }
            _ => bug!("non-enum for StructWrappedNullablePointer: {}", ty),
        };

        self.field_path_offset(inner_ty, path)
    }

    fn field_path_offset<I: Iterator<Item = usize>>(&self, mut ty: Ty<'tcx>, path: I) -> EvalResult<'tcx, Size> {
        let mut offset = Size::from_bytes(0);

        // Skip the initial 0 intended for LLVM GEP.
        for field_index in path {
            let field_offset = self.get_field_offset(ty, field_index)?;
            ty = self.get_field_ty(ty, field_index)?;
            offset = offset.checked_add(field_offset, &self.tcx.data_layout).unwrap();
        }

        Ok(offset)
    }

    fn get_field_ty(&self, ty: Ty<'tcx>, field_index: usize) -> EvalResult<'tcx, Ty<'tcx>> {
        match ty.sty {
            ty::TyAdt(adt_def, substs) => {
                Ok(adt_def.struct_variant().fields[field_index].ty(self.tcx, substs))
            }

            ty::TyRef(_, ty::TypeAndMut { ty, .. }) |
            ty::TyRawPtr(ty::TypeAndMut { ty, .. }) |
            ty::TyBox(ty) => {
                assert_eq!(field_index, 0);
                Ok(ty)
            }
            _ => Err(EvalError::Unimplemented(format!("can't handle type: {:?}", ty))),
        }
    }

    fn get_field_offset(&self, ty: Ty<'tcx>, field_index: usize) -> EvalResult<'tcx, Size> {
        let layout = self.type_layout(ty);

        use rustc::ty::layout::Layout::*;
        match *layout {
            Univariant { ref variant, .. } => {
                Ok(variant.field_offset(field_index))
            }
            FatPointer { .. } => {
                let bytes = layout::FAT_PTR_ADDR * self.memory.pointer_size();
                Ok(Size::from_bytes(bytes as u64))
            }
            _ => Err(EvalError::Unimplemented(format!("can't handle type: {:?}, with layout: {:?}", ty, layout))),
        }
    }

    fn eval_operand_to_primval(&mut self, op: &mir::Operand<'tcx>) -> EvalResult<'tcx, PrimVal> {
        let value = self.eval_operand(op)?;
        let ty = self.operand_ty(op);
        self.value_to_primval(value, ty)
    }

    fn eval_operand(&mut self, op: &mir::Operand<'tcx>) -> EvalResult<'tcx, Value> {
        use rustc::mir::repr::Operand::*;
        match *op {
            Consume(ref lvalue) => Ok(Value::ByRef(self.eval_lvalue(lvalue)?.to_ptr())),

            Constant(mir::Constant { ref literal, ty, .. }) => {
                use rustc::mir::repr::Literal;
                let value = match *literal {
                    Literal::Value { ref value } => self.const_to_value(value)?,

                    Literal::Item { def_id, substs } => {
                        if let ty::TyFnDef(..) = ty.sty {
                            // function items are zero sized
                            Value::ByRef(self.memory.allocate(0, 0)?)
                        } else {
                            let cid = ConstantId {
                                def_id: def_id,
                                substs: substs,
                                kind: ConstantKind::Global,
                            };
                            let static_ptr = *self.statics.get(&cid)
                                .expect("static should have been cached (rvalue)");
                            Value::ByRef(static_ptr)
                        }
                    }

                    Literal::Promoted { index } => {
                        let cid = ConstantId {
                            def_id: self.frame().def_id,
                            substs: self.substs(),
                            kind: ConstantKind::Promoted(index),
                        };
                        let static_ptr = *self.statics.get(&cid)
                            .expect("a promoted constant hasn't been precomputed");
                        Value::ByRef(static_ptr)
                    }
                };

                Ok(value)
            }
        }
    }

    fn eval_lvalue(&mut self, lvalue: &mir::Lvalue<'tcx>) -> EvalResult<'tcx, Lvalue> {
        use rustc::mir::repr::Lvalue::*;
        let ptr = match *lvalue {
            ReturnPointer => self.frame().return_ptr
                .expect("ReturnPointer used in a function with no return value"),
            Arg(i) => self.frame().locals[i.index()],
            Var(i) => self.frame().locals[self.frame().var_offset + i.index()],
            Temp(i) => self.frame().locals[self.frame().temp_offset + i.index()],

            Static(def_id) => {
                let substs = subst::Substs::empty(self.tcx);
                let cid = ConstantId {
                    def_id: def_id,
                    substs: substs,
                    kind: ConstantKind::Global,
                };
                *self.statics.get(&cid).expect("static should have been cached (lvalue)")
            },

            Projection(ref proj) => {
                let base = self.eval_lvalue(&proj.base)?;
                let base_ty = self.lvalue_ty(&proj.base);
                let base_layout = self.type_layout(base_ty);

                use rustc::mir::repr::ProjectionElem::*;
                match proj.elem {
                    Field(field, field_ty) => {
                        let field_ty = self.monomorphize(field_ty, self.substs());
                        use rustc::ty::layout::Layout::*;
                        let variant = match *base_layout {
                            Univariant { ref variant, .. } => variant,
                            General { ref variants, .. } => {
                                if let LvalueExtra::DowncastVariant(variant_idx) = base.extra {
                                    &variants[variant_idx]
                                } else {
                                    bug!("field access on enum had no variant index");
                                }
                            }
                            RawNullablePointer { .. } => {
                                assert_eq!(field.index(), 0);
                                return Ok(base);
                            }
                            StructWrappedNullablePointer { ref nonnull, .. } => nonnull,
                            _ => bug!("field access on non-product type: {:?}", base_layout),
                        };

                        let offset = variant.field_offset(field.index()).bytes();
                        let ptr = base.ptr.offset(offset as isize);
                        trace!("{:?}", base);
                        trace!("{:?}", field_ty);
                        if self.type_is_sized(field_ty) {
                            ptr
                        } else {
                            match base.extra {
                                LvalueExtra::None => bug!("expected fat pointer"),
                                LvalueExtra::DowncastVariant(..) => bug!("Rust doesn't support unsized fields in enum variants"),
                                LvalueExtra::Vtable(_) |
                                LvalueExtra::Length(_) => {},
                            }
                            return Ok(Lvalue {
                                ptr: ptr,
                                extra: base.extra,
                            });
                        }
                    },

                    Downcast(_, variant) => {
                        use rustc::ty::layout::Layout::*;
                        match *base_layout {
                            General { ref variants, .. } => {
                                return Ok(Lvalue {
                                    ptr: base.ptr.offset(variants[variant].field_offset(1).bytes() as isize),
                                    extra: LvalueExtra::DowncastVariant(variant),
                                });
                            }
                            RawNullablePointer { .. } | StructWrappedNullablePointer { .. } => {
                                return Ok(base);
                            }
                            _ => bug!("variant downcast on non-aggregate: {:?}", base_layout),
                        }
                    },

                    Deref => {
                        use primval::PrimVal::*;
                        use interpreter::value::Value::*;
                        let (ptr, extra) = match self.read_value(base.ptr, base_ty)? {
                            ByValPair(Ptr(ptr), Ptr(vptr)) => (ptr, LvalueExtra::Vtable(vptr)),
                            ByValPair(Ptr(ptr), n) => (ptr, LvalueExtra::Length(n.expect_uint("slice length"))),
                            ByVal(Ptr(ptr)) => (ptr, LvalueExtra::None),
                            _ => bug!("can't deref non pointer types"),
                        };
                        return Ok(Lvalue { ptr: ptr, extra: extra });
                    }

                    Index(ref operand) => {
                        let elem_size = match base_ty.sty {
                            ty::TyArray(elem_ty, _) |
                            ty::TySlice(elem_ty) => self.type_size(elem_ty),
                            _ => bug!("indexing expected an array or slice, got {:?}", base_ty),
                        };
                        let n_ptr = self.eval_operand(operand)?;
                        let usize = self.tcx.types.usize;
                        let n = self.value_to_primval(n_ptr, usize)?.expect_uint("Projection::Index expected usize");
                        base.ptr.offset(n as isize * elem_size as isize)
                    }

                    ConstantIndex { .. } => unimplemented!(),
                    Subslice { .. } => unimplemented!(),
                }
            }
        };

        Ok(Lvalue { ptr: ptr, extra: LvalueExtra::None })
    }

    fn lvalue_ty(&self, lvalue: &mir::Lvalue<'tcx>) -> Ty<'tcx> {
        self.monomorphize(lvalue.ty(&self.mir(), self.tcx).to_ty(self.tcx), self.substs())
    }

    fn operand_ty(&self, operand: &mir::Operand<'tcx>) -> Ty<'tcx> {
        self.monomorphize(operand.ty(&self.mir(), self.tcx), self.substs())
    }

    fn move_(&mut self, src: Pointer, dest: Pointer, ty: Ty<'tcx>) -> EvalResult<'tcx, ()> {
        let size = self.type_size(ty);
        let align = self.type_align(ty);
        self.memory.copy(src, dest, size, align)?;
        Ok(())
    }

    // FIXME(solson): This method unnecessarily allocates and should not be necessary. We can
    // remove it as soon as PrimVal can represent fat pointers.
    fn value_to_ptr_dont_use(&mut self, value: Value, ty: Ty<'tcx>) -> EvalResult<'tcx, Pointer> {
        match value {
            Value::ByRef(ptr) => Ok(ptr),

            Value::ByVal(primval) => {
                let size = self.type_size(ty);
                let align = self.type_align(ty);
                let ptr = self.memory.allocate(size, align)?;
                self.memory.write_primval(ptr, primval)?;
                Ok(ptr)
            }

            Value::ByValPair(a, b) => {
                let size = self.type_size(ty);
                let align = self.type_align(ty);
                let ptr = self.memory.allocate(size, align)?;
                let ptr_size = self.memory.pointer_size() as isize;
                self.memory.write_primval(ptr, a)?;
                self.memory.write_primval(ptr.offset(ptr_size), b)?;
                Ok(ptr)
            }
        }
    }

    fn value_to_primval(&mut self, value: Value, ty: Ty<'tcx>) -> EvalResult<'tcx, PrimVal> {
        match value {
            Value::ByRef(ptr) => match self.read_value(ptr, ty)? {
                Value::ByRef(_) => bug!("read_value can't result in `ByRef`"),
                Value::ByVal(primval) => Ok(primval),
                Value::ByValPair(..) => bug!("value_to_primval can't work with fat pointers"),
            },

            // TODO(solson): Sanity-check the primval type against the input type.
            Value::ByVal(primval) => Ok(primval),
            Value::ByValPair(..) => bug!("value_to_primval can't work with fat pointers"),
        }
    }

    fn write_value(
        &mut self,
        value: Value,
        dest: Pointer,
        dest_ty: Ty<'tcx>
    ) -> EvalResult<'tcx, ()> {
        match value {
            Value::ByRef(ptr) => self.move_(ptr, dest, dest_ty),
            Value::ByVal(primval) => self.memory.write_primval(dest, primval),
            Value::ByValPair(a, b) => {
                self.memory.write_primval(dest, a)?;
                let layout = self.type_layout(dest_ty);
                let offset = match *layout {
                    Layout::Univariant { .. } => {
                        bug!("I don't think this can ever happen until we have custom fat pointers");
                        //variant.field_offset(1).bytes() as isize
                    },
                    Layout::FatPointer { .. } => self.memory.pointer_size() as isize,
                    _ => bug!("tried to write value pair of non-fat pointer type: {:?}", layout),
                };
                let extra_dest = dest.offset(offset);
                self.memory.write_primval(extra_dest, b)
            }
        }
    }

    fn read_value(&mut self, ptr: Pointer, ty: Ty<'tcx>) -> EvalResult<'tcx, Value> {
        use syntax::ast::{IntTy, UintTy, FloatTy};
        let val = match &ty.sty {
            &ty::TyBool => PrimVal::Bool(self.memory.read_bool(ptr)?),
            &ty::TyChar => {
                let c = self.memory.read_uint(ptr, 4)? as u32;
                match ::std::char::from_u32(c) {
                    Some(ch) => PrimVal::Char(ch),
                    None => return Err(EvalError::InvalidChar(c as u64)),
                }
            }

            &ty::TyInt(IntTy::I8)    => PrimVal::I8(self.memory.read_int(ptr, 1)? as i8),
            &ty::TyInt(IntTy::I16)   => PrimVal::I16(self.memory.read_int(ptr, 2)? as i16),
            &ty::TyInt(IntTy::I32)   => PrimVal::I32(self.memory.read_int(ptr, 4)? as i32),
            &ty::TyInt(IntTy::I64)   => PrimVal::I64(self.memory.read_int(ptr, 8)? as i64),
            &ty::TyUint(UintTy::U8)  => PrimVal::U8(self.memory.read_uint(ptr, 1)? as u8),
            &ty::TyUint(UintTy::U16) => PrimVal::U16(self.memory.read_uint(ptr, 2)? as u16),
            &ty::TyUint(UintTy::U32) => PrimVal::U32(self.memory.read_uint(ptr, 4)? as u32),
            &ty::TyUint(UintTy::U64) => PrimVal::U64(self.memory.read_uint(ptr, 8)? as u64),

            &ty::TyInt(IntTy::Is)    => self.target_isize_primval(self.memory.read_isize(ptr)?),
            &ty::TyUint(UintTy::Us)  => self.target_usize_primval(self.memory.read_usize(ptr)?),

            &ty::TyFloat(FloatTy::F32) => PrimVal::F32(self.memory.read_f32(ptr)?),
            &ty::TyFloat(FloatTy::F64) => PrimVal::F64(self.memory.read_f64(ptr)?),

            &ty::TyFnDef(def_id, substs, fn_ty) => {
                PrimVal::FnPtr(self.memory.create_fn_ptr(def_id, substs, fn_ty))
            },
            &ty::TyFnPtr(_) => self.memory.read_ptr(ptr).map(PrimVal::FnPtr)?,
            &ty::TyBox(ty) |
            &ty::TyRef(_, ty::TypeAndMut { ty, .. }) |
            &ty::TyRawPtr(ty::TypeAndMut { ty, .. }) => {
                let p = self.memory.read_ptr(ptr)?;
                if self.type_is_sized(ty) {
                    PrimVal::Ptr(p)
                } else {
                    // FIXME: extract the offset to the tail field for `Box<(i64, i32, [u8])>`
                    let extra = ptr.offset(self.memory.pointer_size() as isize);
                    let extra = match self.tcx.struct_tail(ty).sty {
                        ty::TyTrait(..) => PrimVal::Ptr(self.memory.read_ptr(extra)?),
                        ty::TySlice(..) |
                        ty::TyStr => self.target_usize_primval(self.memory.read_usize(extra)?),
                        _ => bug!("unsized primval ptr read from {:?}", ty),
                    };
                    return Ok(Value::ByValPair(PrimVal::Ptr(p), extra));
                }
            }

            &ty::TyAdt(..) => {
                use rustc::ty::layout::Layout::*;
                if let CEnum { discr, signed, .. } = *self.type_layout(ty) {
                    match (discr.size().bytes(), signed) {
                        (1, true)  => PrimVal::I8(self.memory.read_int(ptr, 1)? as i8),
                        (2, true)  => PrimVal::I16(self.memory.read_int(ptr, 2)? as i16),
                        (4, true)  => PrimVal::I32(self.memory.read_int(ptr, 4)? as i32),
                        (8, true)  => PrimVal::I64(self.memory.read_int(ptr, 8)? as i64),
                        (1, false) => PrimVal::U8(self.memory.read_uint(ptr, 1)? as u8),
                        (2, false) => PrimVal::U16(self.memory.read_uint(ptr, 2)? as u16),
                        (4, false) => PrimVal::U32(self.memory.read_uint(ptr, 4)? as u32),
                        (8, false) => PrimVal::U64(self.memory.read_uint(ptr, 8)? as u64),
                        (size, _) => bug!("CEnum discr size {}", size),
                    }
                } else {
                    bug!("primitive read of non-clike enum: {:?}", ty);
                }
            },

            _ => bug!("primitive read of non-primitive type: {:?}", ty),
        };
        Ok(Value::ByVal(val))
    }

    fn frame(&self) -> &Frame<'a, 'tcx> {
        self.stack.last().expect("no call frames exist")
    }

    pub fn frame_mut(&mut self) -> &mut Frame<'a, 'tcx> {
        self.stack.last_mut().expect("no call frames exist")
    }

    fn mir(&self) -> CachedMir<'a, 'tcx> {
        self.frame().mir.clone()
    }

    fn substs(&self) -> &'tcx Substs<'tcx> {
        self.frame().substs
    }

    fn unsize_into(
        &mut self,
        src: Value,
        src_ty: Ty<'tcx>,
        dest: Pointer,
        dest_ty: Ty<'tcx>,
    ) -> EvalResult<'tcx, ()> {
        match (&src_ty.sty, &dest_ty.sty) {
            (&ty::TyBox(sty), &ty::TyBox(dty)) |
            (&ty::TyRef(_, ty::TypeAndMut { ty: sty, .. }), &ty::TyRef(_, ty::TypeAndMut { ty: dty, .. })) |
            (&ty::TyRef(_, ty::TypeAndMut { ty: sty, .. }), &ty::TyRawPtr(ty::TypeAndMut { ty: dty, .. })) |
            (&ty::TyRawPtr(ty::TypeAndMut { ty: sty, .. }), &ty::TyRawPtr(ty::TypeAndMut { ty: dty, .. })) => {
                // A<Struct> -> A<Trait> conversion
                let (src_pointee_ty, dest_pointee_ty) = self.tcx.struct_lockstep_tails(sty, dty);

                match (&src_pointee_ty.sty, &dest_pointee_ty.sty) {
                    (&ty::TyArray(_, length), &ty::TySlice(_)) => {
                        let ptr = src.read_ptr(&self.memory)?;
                        self.memory.write_ptr(dest, ptr)?;
                        let ptr_size = self.memory.pointer_size() as isize;
                        let dest_extra = dest.offset(ptr_size);
                        self.memory.write_usize(dest_extra, length as u64)?;
                    }
                    (&ty::TyTrait(_), &ty::TyTrait(_)) => {
                        // For now, upcasts are limited to changes in marker
                        // traits, and hence never actually require an actual
                        // change to the vtable.
                        self.write_value(src, dest, dest_ty)?;
                    },
                    (_, &ty::TyTrait(ref data)) => {
                        let trait_ref = data.principal.with_self_ty(self.tcx, src_pointee_ty);
                        let trait_ref = self.tcx.erase_regions(&trait_ref);
                        let vtable = self.get_vtable(trait_ref)?;
                        let ptr = src.read_ptr(&self.memory)?;

                        self.memory.write_ptr(dest, ptr)?;
                        let ptr_size = self.memory.pointer_size() as isize;
                        let dest_extra = dest.offset(ptr_size);
                        self.memory.write_ptr(dest_extra, vtable)?;
                    },

                    _ => bug!("invalid unsizing {:?} -> {:?}", src_ty, dest_ty),
                }
            }
            (&ty::TyAdt(def_a, substs_a), &ty::TyAdt(def_b, substs_b)) => {
                // unsizing of generic struct with pointer fields
                // Example: `Arc<T>` -> `Arc<Trait>`
                // here we need to increase the size of every &T thin ptr field to a fat ptr

                assert_eq!(def_a, def_b);

                let src_fields = def_a.variants[0].fields.iter();
                let dst_fields = def_b.variants[0].fields.iter();

                //let src = adt::MaybeSizedValue::sized(src);
                //let dst = adt::MaybeSizedValue::sized(dst);
                let src_ptr = match src {
                    Value::ByRef(ptr) => ptr,
                    _ => panic!("expected pointer, got {:?}", src),
                };

                let iter = src_fields.zip(dst_fields).enumerate();
                for (i, (src_f, dst_f)) in iter {
                    let src_fty = self.monomorphize_field_ty(src_f, substs_a);
                    let dst_fty = self.monomorphize_field_ty(dst_f, substs_b);
                    if self.type_size(dst_fty) == 0 {
                        continue;
                    }
                    let src_field_offset = self.get_field_offset(src_ty, i)?.bytes() as isize;
                    let dst_field_offset = self.get_field_offset(dest_ty, i)?.bytes() as isize;
                    let src_f_ptr = src_ptr.offset(src_field_offset);
                    let dst_f_ptr = dest.offset(dst_field_offset);
                    if src_fty == dst_fty {
                        self.move_(src_f_ptr, dst_f_ptr, src_fty)?;
                    } else {
                        self.unsize_into(Value::ByRef(src_f_ptr), src_fty, dst_f_ptr, dst_fty)?;
                    }
                }
            }
            _ => bug!("unsize_into: invalid conversion: {:?} -> {:?}",
                      src_ty,
                      dest_ty),
        }
        Ok(())
    }
}

impl Lvalue {
    fn to_ptr(self) -> Pointer {
        assert_eq!(self.extra, LvalueExtra::None);
        self.ptr
    }
}

impl<'mir, 'tcx: 'mir> Deref for CachedMir<'mir, 'tcx> {
    type Target = mir::Mir<'tcx>;
    fn deref(&self) -> &mir::Mir<'tcx> {
        match *self {
            CachedMir::Ref(r) => r,
            CachedMir::Owned(ref rc) => rc,
        }
    }
}

pub fn eval_main<'a, 'tcx: 'a>(
    tcx: TyCtxt<'a, 'tcx, 'tcx>,
    mir_map: &'a MirMap<'tcx>,
    def_id: DefId,
    memory_size: usize,
    step_limit: u64,
    stack_limit: usize,
    session: &'a Session,
) {
    let mir = mir_map.map.get(&def_id).expect("no mir for main function");
    let mut ecx = EvalContext::new(tcx, mir_map, memory_size, stack_limit, session);
    let substs = subst::Substs::empty(tcx);
    let return_ptr = ecx.alloc_ret_ptr(mir.return_ty, substs)
        .expect("should at least be able to allocate space for the main function's return value");

    ecx.push_stack_frame(def_id, mir.span, CachedMir::Ref(mir), substs, Some(return_ptr), StackPopCleanup::None)
        .expect("could not allocate first stack frame");

    if mir.arg_decls.len() == 2 {
        // start function
        let ptr_size = ecx.memory().pointer_size();
        let nargs = ecx.memory_mut().allocate(ptr_size, ptr_size).expect("can't allocate memory for nargs");
        ecx.memory_mut().write_usize(nargs, 0).unwrap();
        let args = ecx.memory_mut().allocate(ptr_size, ptr_size).expect("can't allocate memory for arg pointer");
        ecx.memory_mut().write_usize(args, 0).unwrap();
        ecx.frame_mut().locals[0] = nargs;
        ecx.frame_mut().locals[1] = args;
    }

    for _ in 0..step_limit {
        match ecx.step() {
            Ok(true) => {}
            Ok(false) => return,
            // FIXME: diverging functions can end up here in some future miri
            Err(e) => {
                report(tcx, &ecx, e);
                return;
            }
        }
    }
    report(tcx, &ecx, EvalError::ExecutionTimeLimitReached);
}

fn report(tcx: TyCtxt, ecx: &EvalContext, e: EvalError) {
    let frame = ecx.stack().last().expect("stackframe was empty");
    let block = &frame.mir.basic_blocks()[frame.block];
    let span = if frame.stmt < block.statements.len() {
        block.statements[frame.stmt].source_info.span
    } else {
        block.terminator().source_info.span
    };
    let mut err = tcx.sess.struct_span_err(span, &e.to_string());
    for &Frame { def_id, substs, span, .. } in ecx.stack().iter().rev() {
        // FIXME(solson): Find a way to do this without this Display impl hack.
        use rustc::util::ppaux;
        use std::fmt;
        struct Instance<'tcx>(DefId, &'tcx subst::Substs<'tcx>);
        impl<'tcx> ::std::panic::UnwindSafe for Instance<'tcx> {}
        impl<'tcx> ::std::panic::RefUnwindSafe for Instance<'tcx> {}
        impl<'tcx> fmt::Display for Instance<'tcx> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                ppaux::parameterized(f, self.1, self.0, ppaux::Ns::Value, &[])
            }
        }
        err.span_note(span, &format!("inside call to {}", Instance(def_id, substs)));
    }
    err.emit();
}

pub fn run_mir_passes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, mir_map: &mut MirMap<'tcx>) {
    let mut passes = ::rustc::mir::transform::Passes::new();
    passes.push_hook(Box::new(::rustc_mir::transform::dump_mir::DumpMir));
    passes.push_pass(Box::new(::rustc_mir::transform::no_landing_pads::NoLandingPads));
    passes.push_pass(Box::new(::rustc_mir::transform::simplify_cfg::SimplifyCfg::new("no-landing-pads")));

    passes.push_pass(Box::new(::rustc_mir::transform::erase_regions::EraseRegions));

    passes.push_pass(Box::new(::rustc_borrowck::ElaborateDrops));
    passes.push_pass(Box::new(::rustc_mir::transform::no_landing_pads::NoLandingPads));
    passes.push_pass(Box::new(::rustc_mir::transform::simplify_cfg::SimplifyCfg::new("elaborate-drops")));
    passes.push_pass(Box::new(::rustc_mir::transform::dump_mir::Marker("PreMiri")));

    passes.run_passes(tcx, mir_map);
}

// TODO(solson): Upstream these methods into rustc::ty::layout.

trait IntegerExt {
    fn size(self) -> Size;
}

impl IntegerExt for layout::Integer {
    fn size(self) -> Size {
        use rustc::ty::layout::Integer::*;
        match self {
            I1 | I8 => Size::from_bits(8),
            I16 => Size::from_bits(16),
            I32 => Size::from_bits(32),
            I64 => Size::from_bits(64),
        }
    }
}

trait StructExt {
    fn field_offset(&self, index: usize) -> Size;
}

impl StructExt for layout::Struct {
    fn field_offset(&self, index: usize) -> Size {
        if index == 0 {
            Size::from_bytes(0)
        } else {
            self.offset_after_field[index - 1]
        }
    }
}