rust/src/interpreter.rs

use arena::TypedArena;
use rustc::middle::const_eval;
use rustc::middle::def_id::DefId;
use rustc::middle::infer;
use rustc::middle::subst::{self, Subst, Substs};
use rustc::middle::traits;
use rustc::middle::ty::{self, TyCtxt};
use rustc::mir::mir_map::MirMap;
use rustc::mir::repr as mir;
use rustc::util::nodemap::DefIdMap;
use rustc_data_structures::fnv::FnvHashMap;
use std::cell::RefCell;
use std::iter;
use std::ops::Deref;
use std::rc::Rc;
use syntax::ast;
use syntax::codemap::DUMMY_SP;

use error::EvalResult;
use memory::{self, FieldRepr, Memory, Pointer, Repr};
use primval;

const TRACE_EXECUTION: bool = true;

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

    /// 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>>>>,

    /// An arena allocator for type representations.
    repr_arena: &'arena TypedArena<Repr>,

    /// A cache for in-memory representations of types.
    repr_cache: RefCell<FnvHashMap<ty::Ty<'tcx>, &'arena Repr>>,

    /// The virtual memory system.
    memory: Memory,

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

    /// Another stack containing the type substitutions for the current function invocation. It
    /// exists separately from `stack` because it must contain the `Substs` for a function while
    /// *creating* the `Frame` for that same function.
    substs_stack: Vec<&'tcx Substs<'tcx>>,
}

/// A stack frame.
struct Frame<'a, 'tcx: 'a> {
    /// The MIR for the function called on this frame.
    mir: CachedMir<'a, 'tcx>,

    /// The block this frame will execute when a function call returns back to this frame.
    next_block: mir::BasicBlock,

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

    /// 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`.
    locals: Vec<Pointer>,

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

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

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

/// Represents the action to be taken in the main loop as a result of executing a terminator.
enum TerminatorTarget {
    /// Make a local jump to the given block.
    Block(mir::BasicBlock),

    /// Start executing from the new current frame. (For function calls.)
    Call,

    /// Stop executing the current frame and resume the previous frame.
    Return,
}

impl<'a, 'tcx: 'a, 'arena> Interpreter<'a, 'tcx, 'arena> {
    fn new(tcx: &'a TyCtxt<'tcx>, mir_map: &'a MirMap<'tcx>, repr_arena: &'arena TypedArena<Repr>)
        -> Self
    {
        Interpreter {
            tcx: tcx,
            mir_map: mir_map,
            mir_cache: RefCell::new(DefIdMap()),
            repr_arena: repr_arena,
            repr_cache: RefCell::new(FnvHashMap()),
            memory: Memory::new(),
            stack: Vec::new(),
            substs_stack: Vec::new(),
        }
    }

    fn run(&mut self) -> EvalResult<()> {
        use std::fmt::Debug;
        fn print_trace<T: Debug>(t: &T, suffix: &'static str, indent: usize) {
            if !TRACE_EXECUTION { return; }
            for _ in 0..indent { print!("  "); }
            println!("{:?}{}", t, suffix);
        }

        'outer: while !self.stack.is_empty() {
            let mut current_block = self.current_frame().next_block;

            loop {
                print_trace(&current_block, ":", self.stack.len());
                let current_mir = self.current_frame().mir.clone(); // Cloning a reference.
                let block_data = current_mir.basic_block_data(current_block);

                for stmt in &block_data.statements {
                    print_trace(stmt, "", self.stack.len() + 1);
                    let mir::StatementKind::Assign(ref lvalue, ref rvalue) = stmt.kind;
                    try!(self.eval_assignment(lvalue, rvalue));
                }

                let terminator = block_data.terminator();
                print_trace(terminator, "", self.stack.len() + 1);

                match try!(self.eval_terminator(terminator)) {
                    TerminatorTarget::Block(block) => current_block = block,
                    TerminatorTarget::Return => {
                        self.pop_stack_frame();
                        self.substs_stack.pop();
                        continue 'outer;
                    }
                    TerminatorTarget::Call => continue 'outer,
                }
            }
        }

        Ok(())
    }

    fn push_stack_frame(&mut self, mir: CachedMir<'a, 'tcx>, return_ptr: Option<Pointer>)
        -> EvalResult<()>
    {
        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 locals: Vec<Pointer> = arg_tys.chain(var_tys).chain(temp_tys).map(|ty| {
            let size = self.ty_size(ty);
            self.memory.allocate(size)
        }).collect();

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

        self.stack.push(Frame {
            mir: mir.clone(),
            next_block: mir::START_BLOCK,
            return_ptr: return_ptr,
            locals: locals,
            var_offset: num_args,
            temp_offset: num_args + num_vars,
        });

        Ok(())
    }

    fn pop_stack_frame(&mut self) {
        let _frame = self.stack.pop().expect("tried to pop a stack frame, but there were none");
        // TODO(tsion): Deallocate local variables.
    }

    fn eval_terminator(&mut self, terminator: &mir::Terminator<'tcx>)
            -> EvalResult<TerminatorTarget> {
        use rustc::mir::repr::Terminator::*;
        let target = match *terminator {
            Return => TerminatorTarget::Return,

            Goto { target } => TerminatorTarget::Block(target),

            If { ref cond, targets: (then_target, else_target) } => {
                let cond_ptr = try!(self.eval_operand(cond));
                let cond_val = try!(self.memory.read_bool(cond_ptr));
                TerminatorTarget::Block(if cond_val { then_target } else { else_target })
            }

            SwitchInt { ref discr, ref values, ref targets, .. } => {
                let discr_ptr = try!(self.eval_lvalue(discr));
                let discr_size = self.lvalue_repr(discr).size();
                let discr_val = try!(self.memory.read_uint(discr_ptr, discr_size));

                // Branch to the `otherwise` case by default, if no match is found.
                let mut target_block = targets[targets.len() - 1];

                for (index, val_const) in values.iter().enumerate() {
                    let ptr = try!(self.const_to_ptr(val_const));
                    let val = try!(self.memory.read_uint(ptr, discr_size));
                    if discr_val == val {
                        target_block = targets[index];
                        break;
                    }
                }

                TerminatorTarget::Block(target_block)
            }

            Switch { ref discr, ref targets, .. } => {
                let adt_ptr = try!(self.eval_lvalue(discr));
                let adt_repr = self.lvalue_repr(discr);
                let discr_size = match *adt_repr {
                    Repr::Aggregate { discr_size, .. } => discr_size,
                    _ => panic!("attmpted to switch on non-aggregate type"),
                };
                let discr_val = try!(self.memory.read_uint(adt_ptr, discr_size));
                TerminatorTarget::Block(targets[discr_val as usize])
            }

            Call { ref func, ref args, ref destination, .. } => {
                let mut return_ptr = None;
                if let Some((ref lv, target)) = *destination {
                    self.current_frame_mut().next_block = target;
                    return_ptr = Some(try!(self.eval_lvalue(lv)));
                }

                let func_ty = self.operand_ty(func);
                match func_ty.sty {
                    ty::TyFnDef(def_id, substs, fn_ty) => {
                        use syntax::abi::Abi;
                        match fn_ty.abi {
                            Abi::RustIntrinsic => {
                                let name = self.tcx.item_name(def_id).as_str();
                                try!(self.call_intrinsic(&name, substs, args))
                            }

                            Abi::Rust | Abi::RustCall => {
                                // TODO(tsion): Adjust the first argument when calling a Fn or
                                // FnMut closure via FnOnce::call_once.

                                // Only trait methods can have a Self parameter.
                                let (def_id, substs) = if substs.self_ty().is_some() {
                                    self.trait_method(def_id, substs)
                                } else {
                                    (def_id, substs)
                                };

                                let mut arg_srcs = Vec::new();
                                for arg in args {
                                    let (src, repr) = try!(self.eval_operand_and_repr(arg));
                                    arg_srcs.push((src, repr.size()));
                                }

                                if fn_ty.abi == Abi::RustCall && !args.is_empty() {
                                    arg_srcs.pop();
                                    let last_arg = args.last().unwrap();
                                    let (last_src, last_repr) =
                                        try!(self.eval_operand_and_repr(last_arg));
                                    match *last_repr {
                                        Repr::Aggregate { discr_size: 0, ref variants, .. } => {
                                            assert_eq!(variants.len(), 1);
                                            for field in &variants[0] {
                                                let src = last_src.offset(field.offset as isize);
                                                arg_srcs.push((src, field.size));
                                            }
                                        }

                                        _ => panic!("expected tuple as last argument in function with 'rust-call' ABI"),
                                    }
                                }

                                let mir = self.load_mir(def_id);
                                self.substs_stack.push(substs);
                                try!(self.push_stack_frame(mir, return_ptr));

                                for (i, (src, size)) in arg_srcs.into_iter().enumerate() {
                                    let dest = self.current_frame().locals[i];
                                    try!(self.memory.copy(src, dest, size));
                                }

                                TerminatorTarget::Call
                            }

                            abi => panic!("can't handle function with ABI {:?}", abi),
                        }
                    }

                    _ => panic!("can't handle callee of type {:?}", func_ty),
                }
            }

            Drop { target, .. } => {
                // TODO: Handle destructors and dynamic drop.
                TerminatorTarget::Block(target)
            }

            Resume => unimplemented!(),
        };

        Ok(target)
    }

    fn call_intrinsic(&mut self, name: &str, substs: &'tcx Substs<'tcx>,
        args: &[mir::Operand<'tcx>]) -> EvalResult<TerminatorTarget>
    {
        let ret_ptr = &mir::Lvalue::ReturnPointer;
        let dest = try!(self.eval_lvalue(ret_ptr));
        let dest_size = self.lvalue_repr(ret_ptr).size();

        match name {
            "copy_nonoverlapping" => {
                let elem_ty = *substs.types.get(subst::FnSpace, 0);
                let elem_size = self.ty_size(elem_ty);

                let src_arg   = try!(self.eval_operand(&args[0]));
                let dest_arg  = try!(self.eval_operand(&args[1]));
                let count_arg = try!(self.eval_operand(&args[2]));

                let src   = try!(self.memory.read_ptr(src_arg));
                let dest  = try!(self.memory.read_ptr(dest_arg));
                let count = try!(self.memory.read_int(count_arg, self.memory.pointer_size));

                try!(self.memory.copy(src, dest, count as usize * elem_size));
            }

            "forget" => {}

            "offset" => {
                let pointee_ty = *substs.types.get(subst::FnSpace, 0);
                let pointee_size = self.ty_size(pointee_ty) as isize;

                let ptr_arg    = try!(self.eval_operand(&args[0]));
                let offset_arg = try!(self.eval_operand(&args[1]));

                let ptr    = try!(self.memory.read_ptr(ptr_arg));
                let offset = try!(self.memory.read_int(offset_arg, self.memory.pointer_size));

                let result_ptr = ptr.offset(offset as isize * pointee_size);
                try!(self.memory.write_ptr(dest, result_ptr));
            }

            "size_of" => {
                let ty = *substs.types.get(subst::FnSpace, 0);
                let size = self.ty_size(ty) as u64;
                try!(self.memory.write_uint(dest, size, dest_size));
            }

            "uninit" => {}

            name => panic!("can't handle intrinsic: {}", name),
        }

        // Since we pushed no stack frame, the main loop will act
        // as if the call just completed and it's returning to the
        // current frame.
        Ok(TerminatorTarget::Call)
    }

    fn assign_to_aggregate(&mut self, dest: Pointer, dest_repr: &Repr, variant: usize,
                         operands: &[mir::Operand<'tcx>]) -> EvalResult<()> {
        match *dest_repr {
            Repr::Aggregate { discr_size, ref variants, .. } => {
                if discr_size > 0 {
                    let discr = variant as u64;
                    try!(self.memory.write_uint(dest, discr, discr_size));
                }
                let after_discr = dest.offset(discr_size as isize);
                for (field, operand) in variants[variant].iter().zip(operands) {
                    let src = try!(self.eval_operand(operand));
                    let field_dest = after_discr.offset(field.offset as isize);
                    try!(self.memory.copy(src, field_dest, field.size));
                }
            }
            _ => panic!("expected Repr::Aggregate target"),
        }
        Ok(())
    }

    fn eval_assignment(&mut self, lvalue: &mir::Lvalue<'tcx>, rvalue: &mir::Rvalue<'tcx>)
        -> EvalResult<()>
    {
        let dest = try!(self.eval_lvalue(lvalue));
        let dest_repr = self.lvalue_repr(lvalue);

        use rustc::mir::repr::Rvalue::*;
        match *rvalue {
            Use(ref operand) => {
                let src = try!(self.eval_operand(operand));
                self.memory.copy(src, dest, dest_repr.size())
            }

            BinaryOp(bin_op, ref left, ref right) => {
                let left_ptr = try!(self.eval_operand(left));
                let left_ty = self.operand_ty(left);
                let left_val = try!(self.memory.read_primval(left_ptr, left_ty));

                let right_ptr = try!(self.eval_operand(right));
                let right_ty = self.operand_ty(right);
                let right_val = try!(self.memory.read_primval(right_ptr, right_ty));

                self.memory.write_primval(dest, primval::binary_op(bin_op, left_val, right_val))
            }

            UnaryOp(un_op, ref operand) => {
                let ptr = try!(self.eval_operand(operand));
                let ty = self.operand_ty(operand);
                let val = try!(self.memory.read_primval(ptr, ty));
                self.memory.write_primval(dest, primval::unary_op(un_op, val))
            }

            Aggregate(ref kind, ref operands) => {
                use rustc::mir::repr::AggregateKind::*;
                match *kind {
                    Tuple => self.assign_to_aggregate(dest, &dest_repr, 0, operands),

                    Adt(_, variant_idx, _) =>
                        self.assign_to_aggregate(dest, &dest_repr, variant_idx, operands),

                    Vec => match *dest_repr {
                        Repr::Array { elem_size, length } => {
                            assert_eq!(length, operands.len());
                            for (i, operand) in operands.iter().enumerate() {
                                let src = try!(self.eval_operand(operand));
                                let offset = i * elem_size;
                                let elem_dest = dest.offset(offset as isize);
                                try!(self.memory.copy(src, elem_dest, elem_size));
                            }
                            Ok(())
                        }
                        _ => panic!("expected Repr::Array target"),
                    },

                    Closure(..) => self.assign_to_aggregate(dest, &dest_repr, 0, operands),
                }
            }

            Ref(_, _, ref lvalue) => {
                let ptr = try!(self.eval_lvalue(lvalue));
                self.memory.write_ptr(dest, ptr)
            }

            Box(ty) => {
                let size = self.ty_size(ty);
                let ptr = self.memory.allocate(size);
                self.memory.write_ptr(dest, ptr)
            }

            Cast(kind, ref operand, dest_ty) => {
                fn pointee_type<'tcx>(ptr_ty: ty::Ty<'tcx>) -> Option<ty::Ty<'tcx>> {
                    match ptr_ty.sty {
                        ty::TyRef(_, ty::TypeAndMut { ty, .. }) |
                        ty::TyRawPtr(ty::TypeAndMut { ty, .. }) |
                        ty::TyBox(ty) => {
                            Some(ty)
                        }

                        _ => None,
                    }
                }

                let src = try!(self.eval_operand(operand));
                let src_ty = self.operand_ty(operand);

                use rustc::mir::repr::CastKind::*;
                match kind {
                    Unsize => {
                        try!(self.memory.copy(src, dest, 8));
                        let src_pointee_ty = pointee_type(src_ty).unwrap();
                        let dest_pointee_ty = pointee_type(dest_ty).unwrap();

                        match (&src_pointee_ty.sty, &dest_pointee_ty.sty) {
                            (&ty::TyArray(_, length), &ty::TySlice(_)) => {
                                let size = self.memory.pointer_size;
                                self.memory.write_uint(
                                    dest.offset(size as isize),
                                    length as u64,
                                    size,
                                )
                            }

                            _ => panic!("can't handle cast: {:?}", rvalue),
                        }
                    }

                    Misc => {
                        if pointee_type(src_ty).is_some() && pointee_type(dest_ty).is_some() {
                            // FIXME(tsion): Wrong for fat pointers.
                            self.memory.copy(src, dest, 8)
                        } else {
                            // FIXME(tsion): Wrong for almost everything.
                            self.memory.copy(src, dest, 8)
                            // panic!("can't handle cast: {:?}", rvalue);
                        }
                    }

                    _ => panic!("can't handle cast: {:?}", rvalue),
                }
            }

            ref r => panic!("can't handle rvalue: {:?}", r),
        }
    }

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

    fn eval_operand(&mut self, op: &mir::Operand<'tcx>) -> EvalResult<Pointer> {
        self.eval_operand_and_repr(op).map(|(p, _)| p)
    }

    fn eval_operand_and_repr(&mut self, op: &mir::Operand<'tcx>)
        -> EvalResult<(Pointer, &'arena Repr)>
    {
        use rustc::mir::repr::Operand::*;
        match *op {
            Consume(ref lvalue) => Ok((try!(self.eval_lvalue(lvalue)), self.lvalue_repr(lvalue))),
            Constant(mir::Constant { ref literal, ty, .. }) => {
                use rustc::mir::repr::Literal::*;
                match *literal {
                    Value { ref value } => Ok((
                        try!(self.const_to_ptr(value)),
                        self.ty_to_repr(ty),
                    )),
                    ref l => panic!("can't handle item literal: {:?}", l),
                }
            }
        }
    }

    // TODO(tsion): Replace this inefficient hack with a wrapper like LvalueTy (e.g. LvalueRepr).
    fn lvalue_repr(&self, lvalue: &mir::Lvalue<'tcx>) -> &'arena Repr {
        use rustc::mir::tcx::LvalueTy;
        match self.current_frame().mir.lvalue_ty(self.tcx, lvalue) {
            LvalueTy::Ty { ty } => self.ty_to_repr(ty),
            LvalueTy::Downcast { ref adt_def, substs, variant_index } => {
                let field_tys = adt_def.variants[variant_index].fields.iter()
                    .map(|f| f.ty(self.tcx, substs));
                self.repr_arena.alloc(self.make_aggregate_repr(iter::once(field_tys)))
            }
        }
    }

    fn eval_lvalue(&self, lvalue: &mir::Lvalue<'tcx>) -> EvalResult<Pointer> {
        let frame = self.current_frame();

        use rustc::mir::repr::Lvalue::*;
        let ptr = match *lvalue {
            ReturnPointer =>
                frame.return_ptr.expect("ReturnPointer used in a function with no return value"),
            Arg(i) => frame.locals[i as usize],
            Var(i) => frame.locals[frame.var_offset + i as usize],
            Temp(i) => frame.locals[frame.temp_offset + i as usize],

            Projection(ref proj) => {
                let base_ptr = try!(self.eval_lvalue(&proj.base));
                let base_repr = self.lvalue_repr(&proj.base);
                use rustc::mir::repr::ProjectionElem::*;
                match proj.elem {
                    Field(field, _) => match *base_repr {
                        Repr::Aggregate { discr_size: 0, ref variants, .. } => {
                            let fields = &variants[0];
                            base_ptr.offset(fields[field.index()].offset as isize)
                        }
                        _ => panic!("field access on non-product type: {:?}", base_repr),
                    },

                    Downcast(..) => match *base_repr {
                        Repr::Aggregate { discr_size, .. } => base_ptr.offset(discr_size as isize),
                        _ => panic!("variant downcast on non-aggregate type: {:?}", base_repr),
                    },

                    Deref => try!(self.memory.read_ptr(base_ptr)),

                    _ => unimplemented!(),
                }
            }

            ref l => panic!("can't handle lvalue: {:?}", l),
        };

        Ok(ptr)
    }

    fn const_to_ptr(&mut self, const_val: &const_eval::ConstVal) -> EvalResult<Pointer> {
        use rustc::middle::const_eval::ConstVal::*;
        match *const_val {
            Float(_f) => unimplemented!(),
            Integral(int) => {
                // TODO(tsion): Check int constant type.
                let ptr = self.memory.allocate(8);
                try!(self.memory.write_uint(ptr, int.to_u64_unchecked(), 8));
                Ok(ptr)
            }
            Str(ref _s) => unimplemented!(),
            ByteStr(ref _bs) => unimplemented!(),
            Bool(b) => {
                let ptr = self.memory.allocate(1);
                try!(self.memory.write_bool(ptr, b));
                Ok(ptr)
            }
            Char(_c)          => unimplemented!(),
            Struct(_node_id)  => unimplemented!(),
            Tuple(_node_id)   => unimplemented!(),
            Function(_def_id) => unimplemented!(),
            Array(_, _)       => unimplemented!(),
            Repeat(_, _)      => unimplemented!(),
            Dummy             => unimplemented!(),
        }
    }

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

    fn ty_size(&self, ty: ty::Ty<'tcx>) -> usize {
        self.ty_to_repr(ty).size()
    }

    fn ty_to_repr(&self, ty: ty::Ty<'tcx>) -> &'arena Repr {
        let ty = self.monomorphize(ty);

        if let Some(repr) = self.repr_cache.borrow().get(ty) {
            return repr;
        }

        use syntax::ast::{IntTy, UintTy};
        let repr = match ty.sty {
            ty::TyBool => Repr::Primitive { size: 1 },
            ty::TyInt(IntTy::Is)  => Repr::Primitive { size: self.memory.pointer_size },
            ty::TyInt(IntTy::I8)  => Repr::Primitive { size: 1 },
            ty::TyInt(IntTy::I16) => Repr::Primitive { size: 2 },
            ty::TyInt(IntTy::I32) => Repr::Primitive { size: 4 },
            ty::TyInt(IntTy::I64) => Repr::Primitive { size: 8 },

            ty::TyUint(UintTy::Us)  => Repr::Primitive { size: self.memory.pointer_size },
            ty::TyUint(UintTy::U8)  => Repr::Primitive { size: 1 },
            ty::TyUint(UintTy::U16) => Repr::Primitive { size: 2 },
            ty::TyUint(UintTy::U32) => Repr::Primitive { size: 4 },
            ty::TyUint(UintTy::U64) => Repr::Primitive { size: 8 },

            ty::TyTuple(ref fields) =>
                self.make_aggregate_repr(iter::once(fields.iter().cloned())),

            ty::TyEnum(adt_def, substs) | ty::TyStruct(adt_def, substs) => {
                let variants = adt_def.variants.iter().map(|v| {
                    v.fields.iter().map(|f| f.ty(self.tcx, substs))
                });
                self.make_aggregate_repr(variants)
            }

            ty::TyArray(ref elem_ty, length) => Repr::Array {
                elem_size: self.ty_size(elem_ty),
                length: length,
            },

            ty::TyRef(_, ty::TypeAndMut { ty, .. }) |
            ty::TyRawPtr(ty::TypeAndMut { ty, .. }) |
            ty::TyBox(ty) => {
                if ty.is_sized(&self.tcx.empty_parameter_environment(), DUMMY_SP) {
                    Repr::Primitive { size: self.memory.pointer_size }
                } else {
                    Repr::Primitive { size: self.memory.pointer_size * 2 }
                }
            }

            ty::TyClosure(_, ref closure_substs) =>
                self.make_aggregate_repr(iter::once(closure_substs.upvar_tys.iter().cloned())),

            ref t => panic!("can't convert type to repr: {:?}", t),
        };

        let repr_ref = self.repr_arena.alloc(repr);
        self.repr_cache.borrow_mut().insert(ty, repr_ref);
        repr_ref
    }

    fn make_aggregate_repr<V>(&self, variant_fields: V) -> Repr
        where V: IntoIterator, V::Item: IntoIterator<Item = ty::Ty<'tcx>>
    {
        let mut variants = Vec::new();
        let mut max_variant_size = 0;

        for field_tys in variant_fields {
            let mut fields = Vec::new();
            let mut size = 0;

            for ty in field_tys {
                let field_size = self.ty_size(ty);
                let offest = size;
                size += field_size;
                fields.push(FieldRepr { offset: offest, size: field_size });
            }

            if size > max_variant_size { max_variant_size = size; }
            variants.push(fields);
        }

        let discr_size = match variants.len() {
            n if n <= 1       => 0,
            n if n <= 1 << 8  => 1,
            n if n <= 1 << 16 => 2,
            n if n <= 1 << 32 => 4,
            _                 => 8,
        };
        Repr::Aggregate {
            discr_size: discr_size,
            size: max_variant_size + discr_size,
            variants: variants,
        }

    }

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

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

    fn current_substs(&self) -> &'tcx Substs<'tcx> {
        self.substs_stack.last().cloned().unwrap_or_else(|| self.tcx.mk_substs(Substs::empty()))
    }

    fn load_mir(&self, def_id: DefId) -> CachedMir<'a, 'tcx> {
        match self.tcx.map.as_local_node_id(def_id) {
            Some(node_id) => CachedMir::Ref(self.mir_map.map.get(&node_id).unwrap()),
            None => {
                let mut mir_cache = self.mir_cache.borrow_mut();
                if let Some(mir) = mir_cache.get(&def_id) {
                    return CachedMir::Owned(mir.clone());
                }

                use rustc::middle::cstore::CrateStore;
                let cs = &self.tcx.sess.cstore;
                let mir = cs.maybe_get_item_mir(self.tcx, def_id).unwrap_or_else(|| {
                    panic!("no mir for {:?}", def_id);
                });
                let cached = Rc::new(mir);
                mir_cache.insert(def_id, cached.clone());
                CachedMir::Owned(cached)
            }
        }
    }

    fn fulfill_obligation(&self, trait_ref: ty::PolyTraitRef<'tcx>) -> traits::Vtable<'tcx, ()> {
        // Do the initial selection for the obligation. This yields the shallow result we are
        // looking for -- that is, what specific impl.
        let infcx = infer::normalizing_infer_ctxt(self.tcx, &self.tcx.tables);
        let mut selcx = traits::SelectionContext::new(&infcx);

        let obligation = traits::Obligation::new(
            traits::ObligationCause::misc(DUMMY_SP, ast::DUMMY_NODE_ID),
            trait_ref.to_poly_trait_predicate(),
        );
        let selection = selcx.select(&obligation).unwrap().unwrap();

        // Currently, we use a fulfillment context to completely resolve all nested obligations.
        // This is because they can inform the inference of the impl's type parameters.
        let mut fulfill_cx = traits::FulfillmentContext::new();
        let vtable = selection.map(|predicate| {
            fulfill_cx.register_predicate_obligation(&infcx, predicate);
        });
        let vtable = infer::drain_fulfillment_cx_or_panic(
            DUMMY_SP, &infcx, &mut fulfill_cx, &vtable
        );

        vtable
    }

    /// Trait method, which has to be resolved to an impl method.
    pub fn trait_method(&self, def_id: DefId, substs: &'tcx Substs<'tcx>)
            -> (DefId, &'tcx Substs<'tcx>) {
        let method_item = self.tcx.impl_or_trait_item(def_id);
        let trait_id = method_item.container().id();
        let trait_ref = ty::Binder(substs.to_trait_ref(self.tcx, trait_id));
        match self.fulfill_obligation(trait_ref) {
            traits::VtableImpl(vtable_impl) => {
                let impl_did = vtable_impl.impl_def_id;
                let mname = self.tcx.item_name(def_id);
                // Create a concatenated set of substitutions which includes those from the impl
                // and those from the method:
                let impl_substs = vtable_impl.substs.with_method_from(substs);
                let substs = self.tcx.mk_substs(impl_substs);
                let mth = self.tcx.get_impl_method(impl_did, substs, mname);

                (mth.method.def_id, mth.substs)
            }

            traits::VtableClosure(vtable_closure) =>
                (vtable_closure.closure_def_id, vtable_closure.substs.func_substs),

            traits::VtableFnPointer(_fn_ty) => {
                let _trait_closure_kind = self.tcx.lang_items.fn_trait_kind(trait_id).unwrap();
                unimplemented!()
                // let llfn = trans_fn_pointer_shim(ccx, trait_closure_kind, fn_ty);

                // let method_ty = def_ty(tcx, def_id, substs);
                // let fn_ptr_ty = match method_ty.sty {
                //     ty::TyFnDef(_, _, fty) => tcx.mk_ty(ty::TyFnPtr(fty)),
                //     _ => unreachable!("expected fn item type, found {}",
                //                       method_ty)
                // };
                // Callee::ptr(immediate_rvalue(llfn, fn_ptr_ty))
            }

            traits::VtableObject(ref _data) => {
                unimplemented!()
                // Callee {
                //     data: Virtual(traits::get_vtable_index_of_object_method(
                //                   tcx, data, def_id)),
                //                   ty: def_ty(tcx, def_id, substs)
                // }
            }
            vtable => unreachable!("resolved vtable bad vtable {:?} in trans", vtable),
        }
    }
}

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 interpret_start_points<'tcx>(tcx: &TyCtxt<'tcx>, mir_map: &MirMap<'tcx>) {
    /// Print the given allocation and all allocations it depends on.
    fn print_allocation_tree(memory: &Memory, alloc_id: memory::AllocId) {
        let alloc = memory.get(alloc_id).unwrap();
        println!("  {:?}: {:?}", alloc_id, alloc);
        for &target_alloc in alloc.relocations.values() {
            print_allocation_tree(memory, target_alloc);
        }
    }

    for (&id, mir) in &mir_map.map {
        for attr in tcx.map.attrs(id) {
            use syntax::attr::AttrMetaMethods;
            if attr.check_name("miri_run") {
                let item = tcx.map.expect_item(id);

                println!("Interpreting: {}", item.name);

                let repr_arena = TypedArena::new();
                let mut miri = Interpreter::new(tcx, mir_map, &repr_arena);
                let return_ptr = match mir.return_ty {
                    ty::FnConverging(ty) => {
                        let size = miri.ty_size(ty);
                        Some(miri.memory.allocate(size))
                    }
                    ty::FnDiverging => None,
                };
                miri.push_stack_frame(CachedMir::Ref(mir), return_ptr).unwrap();
                miri.run().unwrap();

                if let Some(ret) = return_ptr {
                    println!("Result:");
                    print_allocation_tree(&miri.memory, ret.alloc_id);
                    println!("");
                }
            }
        }
    }
}