rust/src/librustc_metadata/decoder.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// Decoding metadata from a single crate's metadata

use astencode::decode_inlined_item;
use cstore::{self, CrateMetadata, MetadataBlob, NativeLibraryKind};
use index::Index;
use schema::*;

use rustc::hir::map as hir_map;
use rustc::hir::map::{DefKey, DefPathData};
use rustc::util::nodemap::FnvHashMap;
use rustc::hir;
use rustc::hir::intravisit::IdRange;

use rustc::middle::cstore::{InlinedItem, LinkagePreference};
use rustc::hir::def::{self, Def};
use rustc::hir::def_id::{CrateNum, DefId, DefIndex, LOCAL_CRATE};
use rustc::middle::lang_items;
use rustc::ty::{self, Ty, TyCtxt};
use rustc::ty::subst::Substs;

use rustc_const_math::ConstInt;

use rustc::mir::repr::Mir;

use std::cell::Ref;
use std::io;
use std::mem;
use std::rc::Rc;
use std::str;
use std::u32;

use rustc_serialize::{Decodable, Decoder, SpecializedDecoder, opaque};
use syntax::attr;
use syntax::ast::{self, NodeId};
use syntax::codemap;
use syntax_pos::{self, Span, BytePos, Pos};

pub struct DecodeContext<'a, 'tcx: 'a> {
    opaque: opaque::Decoder<'a>,
    tcx: Option<TyCtxt<'a, 'tcx, 'tcx>>,
    cdata: Option<&'a CrateMetadata>,
    from_id_range: IdRange,
    to_id_range: IdRange,

    // Cache the last used filemap for translating spans as an optimization.
    last_filemap_index: usize,

    lazy_state: LazyState
}

/// Abstract over the various ways one can create metadata decoders.
pub trait Metadata<'a, 'tcx>: Copy {
    fn raw_bytes(self) -> &'a [u8];
    fn cdata(self) -> Option<&'a CrateMetadata> { None }
    fn tcx(self) -> Option<TyCtxt<'a, 'tcx, 'tcx>> { None }

    fn decoder(self, pos: usize) -> DecodeContext<'a, 'tcx> {
        let id_range = IdRange {
            min: NodeId::from_u32(u32::MIN),
            max: NodeId::from_u32(u32::MAX)
        };
        DecodeContext {
            opaque: opaque::Decoder::new(self.raw_bytes(), pos),
            cdata: self.cdata(),
            tcx: self.tcx(),
            from_id_range: id_range,
            to_id_range: id_range,
            last_filemap_index: 0,
            lazy_state: LazyState::NoNode
        }
    }
}

impl<'a, 'tcx> Metadata<'a, 'tcx> for &'a MetadataBlob {
    fn raw_bytes(self) -> &'a [u8] {
        match *self {
            MetadataBlob::Inflated(ref vec) => &vec[..],
            MetadataBlob::Archive(ref ar) => ar.as_slice(),
        }
    }
}

impl<'a, 'tcx> Metadata<'a, 'tcx> for &'a CrateMetadata {
    fn raw_bytes(self) -> &'a [u8] { self.blob.raw_bytes() }
    fn cdata(self) -> Option<&'a CrateMetadata> { Some(self) }
}

impl<'a, 'tcx> Metadata<'a, 'tcx> for (&'a CrateMetadata, TyCtxt<'a, 'tcx, 'tcx>) {
    fn raw_bytes(self) -> &'a [u8] { self.0.raw_bytes() }
    fn cdata(self) -> Option<&'a CrateMetadata> { Some(self.0) }
    fn tcx(self) -> Option<TyCtxt<'a, 'tcx, 'tcx>> { Some(self.1) }
}

// HACK(eddyb) Only used by astencode to customize the from/to IdRange's.
impl<'a, 'tcx> Metadata<'a, 'tcx> for (&'a CrateMetadata, TyCtxt<'a, 'tcx, 'tcx>, [IdRange; 2]) {
    fn raw_bytes(self) -> &'a [u8] { self.0.raw_bytes() }
    fn cdata(self) -> Option<&'a CrateMetadata> { Some(self.0) }
    fn tcx(self) -> Option<TyCtxt<'a, 'tcx, 'tcx>> { Some(self.1) }

    fn decoder(self, pos: usize) -> DecodeContext<'a, 'tcx> {
        let mut dcx = (self.0, self.1).decoder(pos);
        dcx.from_id_range = self.2[0];
        dcx.to_id_range = self.2[1];
        dcx
    }
}

impl<'a, 'tcx: 'a, T: Decodable> Lazy<T> {
    pub fn decode<M: Metadata<'a, 'tcx>>(self, meta: M) -> T {
        let mut dcx = meta.decoder(self.position);
        dcx.lazy_state = LazyState::NodeStart(self.position);
        T::decode(&mut dcx).unwrap()
    }
}

impl<'a, 'tcx: 'a, T: Decodable> LazySeq<T> {
    pub fn decode<M: Metadata<'a, 'tcx>>(self, meta: M) -> impl Iterator<Item=T> + 'a {
        let mut dcx = meta.decoder(self.position);
        dcx.lazy_state = LazyState::NodeStart(self.position);
        (0..self.len).map(move |_| {
            T::decode(&mut dcx).unwrap()
        })
    }
}

impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
    pub fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> {
        self.tcx.expect("missing TyCtxt in DecodeContext")
    }

    pub fn cdata(&self) -> &'a CrateMetadata {
        self.cdata.expect("missing CrateMetadata in DecodeContext")
    }

    fn with_position<F: FnOnce(&mut Self) -> R, R>(&mut self, pos: usize, f: F) -> R {
        let new_opaque = opaque::Decoder::new(self.opaque.data, pos);
        let old_opaque = mem::replace(&mut self.opaque, new_opaque);
        let old_state = mem::replace(&mut self.lazy_state, LazyState::NoNode);
        let r = f(self);
        self.opaque = old_opaque;
        self.lazy_state = old_state;
        r
    }

    fn read_lazy_distance(&mut self, min_size: usize)
                          -> Result<usize, <Self as Decoder>::Error> {
        let distance = self.read_usize()?;
        let position = match self.lazy_state {
            LazyState::NoNode => {
                bug!("read_lazy_distance: outside of a metadata node")
            }
            LazyState::NodeStart(start) => {
                assert!(distance + min_size <= start);
                start - distance - min_size
            }
            LazyState::Previous(last_min_end) => {
                last_min_end + distance
            }
        };
        self.lazy_state = LazyState::Previous(position + min_size);
        Ok(position)
    }
}

macro_rules! decoder_methods {
    ($($name:ident -> $ty:ty;)*) => {
        $(fn $name(&mut self) -> Result<$ty, Self::Error> {
            self.opaque.$name()
        })*
    }
}

impl<'doc, 'tcx> Decoder for DecodeContext<'doc, 'tcx> {
    type Error = <opaque::Decoder<'doc> as Decoder>::Error;

    decoder_methods! {
        read_nil -> ();

        read_u64 -> u64;
        read_u32 -> u32;
        read_u16 -> u16;
        read_u8 -> u8;
        read_usize -> usize;

        read_i64 -> i64;
        read_i32 -> i32;
        read_i16 -> i16;
        read_i8 -> i8;
        read_isize -> isize;

        read_bool -> bool;
        read_f64 -> f64;
        read_f32 -> f32;
        read_char -> char;
        read_str -> String;
    }

    fn error(&mut self, err: &str) -> Self::Error {
        self.opaque.error(err)
    }
}

impl<'a, 'tcx, T> SpecializedDecoder<Lazy<T>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<Lazy<T>, Self::Error> {
        Ok(Lazy::with_position(self.read_lazy_distance(Lazy::<T>::min_size())?))
    }
}

impl<'a, 'tcx, T> SpecializedDecoder<LazySeq<T>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<LazySeq<T>, Self::Error> {
        let len = self.read_usize()?;
        let position = if len == 0 {
            0
        } else {
            self.read_lazy_distance(LazySeq::<T>::min_size(len))?
        };
        Ok(LazySeq::with_position_and_length(position, len))
    }
}

impl<'a, 'tcx> SpecializedDecoder<NodeId> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<NodeId, Self::Error> {
        let id = u32::decode(self)?;

        // from_id_range should be non-empty
        assert!(!self.from_id_range.empty());
        // Make sure that translating the NodeId will actually yield a
        // meaningful result
        if !self.from_id_range.contains(NodeId::from_u32(id)) {
            bug!("NodeId::decode: {} out of DecodeContext range ({:?} -> {:?})",
                 id, self.from_id_range, self.to_id_range);
        }

        // Use wrapping arithmetic because otherwise it introduces control flow.
        // Maybe we should just have the control flow? -- aatch
        Ok(NodeId::from_u32(id.wrapping_sub(self.from_id_range.min.as_u32())
                              .wrapping_add(self.to_id_range.min.as_u32())))
    }
}

impl<'a, 'tcx> SpecializedDecoder<CrateNum> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<CrateNum, Self::Error> {
        let cnum = CrateNum::from_u32(u32::decode(self)?);
        if cnum == LOCAL_CRATE {
            Ok(self.cdata().cnum)
        } else {
            Ok(self.cdata().cnum_map.borrow()[cnum])
        }
    }
}

impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<Span, Self::Error> {
        let lo = BytePos::decode(self)?;
        let hi = BytePos::decode(self)?;

        let tcx = if let Some(tcx) = self.tcx {
            tcx
        } else {
            return Ok(syntax_pos::mk_sp(lo, hi));
        };

        let (lo, hi) = if lo > hi {
            // Currently macro expansion sometimes produces invalid Span values
            // where lo > hi. In order not to crash the compiler when trying to
            // translate these values, let's transform them into something we
            // can handle (and which will produce useful debug locations at
            // least some of the time).
            // This workaround is only necessary as long as macro expansion is
            // not fixed. FIXME(#23480)
            (lo, lo)
        } else {
            (lo, hi)
        };

        let imported_filemaps = self.cdata().imported_filemaps(&tcx.sess.codemap());
        let filemap = {
            // Optimize for the case that most spans within a translated item
            // originate from the same filemap.
            let last_filemap = &imported_filemaps[self.last_filemap_index];

            if lo >= last_filemap.original_start_pos &&
            lo <= last_filemap.original_end_pos &&
            hi >= last_filemap.original_start_pos &&
            hi <= last_filemap.original_end_pos {
                last_filemap
            } else {
                let mut a = 0;
                let mut b = imported_filemaps.len();

                while b - a > 1 {
                    let m = (a + b) / 2;
                    if imported_filemaps[m].original_start_pos > lo {
                        b = m;
                    } else {
                        a = m;
                    }
                }

                self.last_filemap_index = a;
                &imported_filemaps[a]
            }
        };

        let lo = (lo - filemap.original_start_pos) +
                  filemap.translated_filemap.start_pos;
        let hi = (hi - filemap.original_start_pos) +
                  filemap.translated_filemap.start_pos;

        Ok(syntax_pos::mk_sp(lo, hi))
    }
}

// FIXME(#36588) These impls are horribly unsound as they allow
// the caller to pick any lifetime for 'tcx, including 'static,
// by using the unspecialized proxies to them.

impl<'a, 'tcx> SpecializedDecoder<Ty<'tcx>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<Ty<'tcx>, Self::Error> {
        let tcx = self.tcx();

        // Handle shorthands first, if we have an usize > 0x80.
        if self.opaque.data[self.opaque.position()] & 0x80 != 0 {
            let pos = self.read_usize()?;
            assert!(pos >= SHORTHAND_OFFSET);
            let key = ty::CReaderCacheKey {
                cnum: self.cdata().cnum,
                pos: pos - SHORTHAND_OFFSET
            };
            if let Some(ty) = tcx.rcache.borrow().get(&key).cloned() {
                return Ok(ty);
            }

            let ty = self.with_position(key.pos, Ty::decode)?;
            tcx.rcache.borrow_mut().insert(key, ty);
            Ok(ty)
        } else {
            Ok(tcx.mk_ty(ty::TypeVariants::decode(self)?))
        }
    }
}


impl<'a, 'tcx> SpecializedDecoder<ty::GenericPredicates<'tcx>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<ty::GenericPredicates<'tcx>, Self::Error> {
        Ok(ty::GenericPredicates {
            parent: Decodable::decode(self)?,
            predicates: (0..self.read_usize()?).map(|_| {
                // Handle shorthands first, if we have an usize > 0x80.
                if self.opaque.data[self.opaque.position()] & 0x80 != 0 {
                    let pos = self.read_usize()?;
                    assert!(pos >= SHORTHAND_OFFSET);
                    let pos = pos - SHORTHAND_OFFSET;

                    self.with_position(pos, ty::Predicate::decode)
                } else {
                    ty::Predicate::decode(self)
                }
            }).collect()?
        })
    }
}

impl<'a, 'tcx> SpecializedDecoder<&'tcx Substs<'tcx>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<&'tcx Substs<'tcx>, Self::Error> {
        Ok(self.tcx().mk_substs(Decodable::decode(self)?))
    }
}

impl<'a, 'tcx> SpecializedDecoder<&'tcx ty::Region> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<&'tcx ty::Region, Self::Error> {
        Ok(self.tcx().mk_region(Decodable::decode(self)?))
    }
}

impl<'a, 'tcx> SpecializedDecoder<&'tcx ty::Slice<Ty<'tcx>>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<&'tcx ty::Slice<Ty<'tcx>>, Self::Error> {
        Ok(self.tcx().mk_type_list(Decodable::decode(self)?))
    }
}

impl<'a, 'tcx> SpecializedDecoder<&'tcx ty::BareFnTy<'tcx>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<&'tcx ty::BareFnTy<'tcx>, Self::Error> {
        Ok(self.tcx().mk_bare_fn(Decodable::decode(self)?))
    }
}

impl<'a, 'tcx> SpecializedDecoder<ty::AdtDef<'tcx>> for DecodeContext<'a, 'tcx> {
    fn specialized_decode(&mut self) -> Result<ty::AdtDef<'tcx>, Self::Error> {
        let def_id = DefId::decode(self)?;
        Ok(self.tcx().lookup_adt_def(def_id))
    }
}

impl<'a, 'tcx> MetadataBlob {
    pub fn is_compatible(&self) -> bool {
        self.raw_bytes().starts_with(METADATA_HEADER)
    }

    pub fn get_root(&self) -> CrateRoot {
        let slice = self.raw_bytes();
        let offset = METADATA_HEADER.len();
        let pos = (((slice[offset + 0] as u32) << 24) |
                   ((slice[offset + 1] as u32) << 16) |
                   ((slice[offset + 2] as u32) << 8) |
                   ((slice[offset + 3] as u32) << 0)) as usize;
        Lazy::with_position(pos).decode(self)
    }

    /// Go through each item in the metadata and create a map from that
    /// item's def-key to the item's DefIndex.
    pub fn load_key_map(&self, index: LazySeq<Index>) -> FnvHashMap<DefKey, DefIndex> {
        index.iter_enumerated(self.raw_bytes()).map(|(index, item)| {
            (item.decode(self).def_key.decode(self), index)
        }).collect()
    }

    pub fn list_crate_metadata(&self, out: &mut io::Write) -> io::Result<()> {
        write!(out, "=External Dependencies=\n")?;
        let root = self.get_root();
        for (i, dep) in root.crate_deps.decode(self).enumerate() {
            write!(out, "{} {}-{}\n", i + 1, dep.name, dep.hash)?;
        }
        write!(out, "\n")?;
        Ok(())
    }
}

impl<'tcx> EntryKind<'tcx> {
    fn to_def(&self, did: DefId) -> Option<Def> {
        Some(match *self {
            EntryKind::Const  => Def::Const(did),
            EntryKind::AssociatedConst(_) => Def::AssociatedConst(did),
            EntryKind::ImmStatic |
            EntryKind::ForeignImmStatic => Def::Static(did, false),
            EntryKind::MutStatic |
            EntryKind::ForeignMutStatic => Def::Static(did, true),
            EntryKind::Struct(_) => Def::Struct(did),
            EntryKind::Union(_) => Def::Union(did),
            EntryKind::Fn(_) |
            EntryKind::ForeignFn(_) => Def::Fn(did),
            EntryKind::Method(_) => Def::Method(did),
            EntryKind::Type => Def::TyAlias(did),
            EntryKind::AssociatedType(_) => Def::AssociatedTy(did),
            EntryKind::Mod(_) => Def::Mod(did),
            EntryKind::Variant(_) => Def::Variant(did),
            EntryKind::Trait(_) => Def::Trait(did),
            EntryKind::Enum => Def::Enum(did),

            EntryKind::ForeignMod |
            EntryKind::Impl(_) |
            EntryKind::DefaultImpl(_) |
            EntryKind::Field |
            EntryKind::Closure (_) => {
                return None
            }
        })
    }
}

impl<'a, 'tcx> CrateMetadata {
    fn maybe_entry(&self, item_id: DefIndex) -> Option<Lazy<Entry<'tcx>>> {
        self.root.index.lookup(self.blob.raw_bytes(), item_id)
    }

    fn entry(&self, item_id: DefIndex) -> Entry<'tcx> {
        match self.maybe_entry(item_id) {
            None => bug!("entry: id not found: {:?} in crate {:?} with number {}",
                         item_id,
                         self.name,
                         self.cnum),
            Some(d) => d.decode(self)
        }
    }

    fn local_def_id(&self, index: DefIndex) -> DefId {
        DefId {
            krate: self.cnum,
            index: index
        }
    }

    fn item_name(&self, item: &Entry<'tcx>) -> ast::Name {
        item.def_key.decode(self).disambiguated_data.data.get_opt_name()
            .expect("no name in item_name")
    }

    pub fn get_def(&self, index: DefIndex) -> Option<Def> {
        self.entry(index).kind.to_def(self.local_def_id(index))
    }

    pub fn get_trait_def(&self,
                         item_id: DefIndex,
                         tcx: TyCtxt<'a, 'tcx, 'tcx>) -> ty::TraitDef<'tcx> {
        let data = match self.entry(item_id).kind {
            EntryKind::Trait(data) => data.decode(self),
            _ => bug!()
        };

        ty::TraitDef::new(data.unsafety, data.paren_sugar,
                          tcx.lookup_generics(self.local_def_id(item_id)),
                          data.trait_ref.decode((self, tcx)),
                          self.def_path(item_id).unwrap().deterministic_hash(tcx))
    }

    fn get_variant(&self, item: &Entry<'tcx>, index: DefIndex)
                   -> (ty::VariantDefData<'tcx, 'tcx>, Option<DefIndex>) {
        let data = match item.kind {
            EntryKind::Variant(data) |
            EntryKind::Struct(data) |
            EntryKind::Union(data) => data.decode(self),
            _ => bug!()
        };

        let fields = item.children.decode(self).map(|index| {
            let f = self.entry(index);
            ty::FieldDefData::new(self.local_def_id(index),
                                  self.item_name(&f),
                                  f.visibility)
        }).collect();

        (ty::VariantDefData {
            did: self.local_def_id(data.struct_ctor.unwrap_or(index)),
            name: self.item_name(item),
            fields: fields,
            disr_val: ConstInt::Infer(data.disr),
            kind: data.kind,
        }, data.struct_ctor)
    }

    pub fn get_adt_def(&self, item_id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>)
                       -> ty::AdtDefMaster<'tcx> {
        let item = self.entry(item_id);
        let did = self.local_def_id(item_id);
        let mut ctor_index = None;
        let variants = if let EntryKind::Enum = item.kind {
            item.children.decode(self).map(|index| {
                let (variant, struct_ctor) = self.get_variant(&self.entry(index), index);
                assert_eq!(struct_ctor, None);
                variant
            }).collect()
        } else{
            let (variant, struct_ctor) = self.get_variant(&item, item_id);
            ctor_index = struct_ctor;
            vec![variant]
        };
        let kind = match item.kind {
            EntryKind::Enum => ty::AdtKind::Enum,
            EntryKind::Struct(_) => ty::AdtKind::Struct,
            EntryKind::Union(_) => ty::AdtKind::Union,
            _ => bug!("get_adt_def called on a non-ADT {:?}", did)
        };

        let adt = tcx.intern_adt_def(did, kind, variants);
        if let Some(ctor_index) = ctor_index {
            // Make adt definition available through constructor id as well.
            tcx.insert_adt_def(self.local_def_id(ctor_index), adt);
        }

        // this needs to be done *after* the variant is interned,
        // to support recursive structures
        for variant in &adt.variants {
            for field in &variant.fields {
                debug!("evaluating the type of {:?}::{:?}", variant.name, field.name);
                let ty = self.get_type(field.did.index, tcx);
                field.fulfill_ty(ty);
                debug!("evaluating the type of {:?}::{:?}: {:?}",
                       variant.name, field.name, ty);
            }
        }

        adt
    }

    pub fn get_predicates(&self, item_id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>)
                          -> ty::GenericPredicates<'tcx> {
        self.entry(item_id).predicates.unwrap().decode((self, tcx))
    }

    pub fn get_super_predicates(&self, item_id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>)
                                -> ty::GenericPredicates<'tcx> {
        match self.entry(item_id).kind {
            EntryKind::Trait(data) => {
                data.decode(self).super_predicates.decode((self, tcx))
            }
            _ => bug!()
        }
    }

    pub fn get_generics(&self, item_id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>)
                        -> ty::Generics<'tcx> {
        self.entry(item_id).generics.unwrap().decode((self, tcx))
    }

    pub fn get_type(&self, id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> Ty<'tcx> {
        self.entry(id).ty.unwrap().decode((self, tcx))
    }

    pub fn get_stability(&self, id: DefIndex) -> Option<attr::Stability> {
        self.entry(id).stability.map(|stab| stab.decode(self))
    }

    pub fn get_deprecation(&self, id: DefIndex) -> Option<attr::Deprecation> {
        self.entry(id).deprecation.map(|depr| depr.decode(self))
    }

    pub fn get_visibility(&self, id: DefIndex) -> ty::Visibility {
        self.entry(id).visibility
    }

    fn get_impl_data(&self, id: DefIndex) -> ImplData<'tcx> {
        match self.entry(id).kind {
            EntryKind::Impl(data) => data.decode(self),
            _ => bug!()
        }
    }

    pub fn get_parent_impl(&self, id: DefIndex) -> Option<DefId> {
        self.get_impl_data(id).parent_impl
    }

    pub fn get_impl_polarity(&self, id: DefIndex) -> hir::ImplPolarity {
        self.get_impl_data(id).polarity
    }

    pub fn get_custom_coerce_unsized_kind(&self, id: DefIndex)
                                          -> Option<ty::adjustment::CustomCoerceUnsized> {
        self.get_impl_data(id).coerce_unsized_kind
    }

    pub fn get_impl_trait(&self,
                          id: DefIndex,
                          tcx: TyCtxt<'a, 'tcx, 'tcx>)
                          -> Option<ty::TraitRef<'tcx>> {
        self.get_impl_data(id).trait_ref.map(|tr| tr.decode((self, tcx)))
    }

    /// Iterates over the language items in the given crate.
    pub fn get_lang_items(&self) -> Vec<(DefIndex, usize)> {
        self.root.lang_items.decode(self).collect()
    }

    /// Iterates over each child of the given item.
    pub fn each_child_of_item<F>(&self, id: DefIndex, mut callback: F)
        where F: FnMut(def::Export)
    {
        // Find the item.
        let item = match self.maybe_entry(id) {
            None => return,
            Some(item) => item.decode(self),
        };

        // Iterate over all children.
        for child_index in item.children.decode(self) {
            // Get the item.
            if let Some(child) = self.maybe_entry(child_index) {
                let child = child.decode(self);
                // Hand off the item to the callback.
                match child.kind {
                    // FIXME(eddyb) Don't encode these in children.
                    EntryKind::ForeignMod => {
                        for child_index in child.children.decode(self) {
                            callback(def::Export {
                                def_id: self.local_def_id(child_index),
                                name: self.item_name(&self.entry(child_index))
                            });
                        }
                        continue;
                    }
                    EntryKind::Impl(_) | EntryKind::DefaultImpl(_) => continue,

                    _ => {}
                }

                let def_key = child.def_key.decode(self);
                if let Some(name) = def_key.disambiguated_data.data.get_opt_name() {
                    callback(def::Export {
                        def_id: self.local_def_id(child_index),
                        name: name
                    });
                }
            }
        }

        if let EntryKind::Mod(data) = item.kind {
            for exp in data.decode(self).reexports.decode(self) {
                callback(exp);
            }
        }
    }

    pub fn maybe_get_item_ast(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefIndex)
                              -> Option<&'tcx InlinedItem> {
        debug!("Looking up item: {:?}", id);
        let item_doc = self.entry(id);
        let item_did = self.local_def_id(id);
        let parent_def_id = self.local_def_id(self.def_key(id).parent.unwrap());
        let mut parent_def_path = self.def_path(id).unwrap();
        parent_def_path.data.pop();
        item_doc.ast.map(|ast| {
            let ast = ast.decode(self);
            decode_inlined_item(self, tcx, parent_def_path, parent_def_id, ast, item_did)
        })
    }

    pub fn is_item_mir_available(&self, id: DefIndex) -> bool {
        self.maybe_entry(id).and_then(|item| item.decode(self).mir).is_some()
    }

    pub fn maybe_get_item_mir(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefIndex)
                              -> Option<Mir<'tcx>> {
        self.entry(id).mir.map(|mir| mir.decode((self, tcx)))
    }

    pub fn get_impl_or_trait_item(&self, id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>)
                                  -> Option<ty::ImplOrTraitItem<'tcx>> {
        let item = self.entry(id);
        let parent_and_name = || {
            let def_key = item.def_key.decode(self);
            (self.local_def_id(def_key.parent.unwrap()),
             def_key.disambiguated_data.data.get_opt_name().unwrap())
        };

        Some(match item.kind {
            EntryKind::AssociatedConst(container) => {
                let (parent, name) = parent_and_name();
                ty::ConstTraitItem(Rc::new(ty::AssociatedConst {
                    name: name,
                    ty: item.ty.unwrap().decode((self, tcx)),
                    vis: item.visibility,
                    defaultness: container.defaultness(),
                    def_id: self.local_def_id(id),
                    container: container.with_def_id(parent),
                    has_value: container.has_body(),
                }))
            }
            EntryKind::Method(data) => {
                let (parent, name) = parent_and_name();
                let ity = item.ty.unwrap().decode((self, tcx));
                let fty = match ity.sty {
                    ty::TyFnDef(.., fty) => fty,
                    _ => bug!(
                        "the type {:?} of the method {:?} is not a function?",
                        ity, name)
                };

                let data = data.decode(self);
                ty::MethodTraitItem(Rc::new(ty::Method {
                    name: name,
                    generics: tcx.lookup_generics(self.local_def_id(id)),
                    predicates: item.predicates.unwrap().decode((self, tcx)),
                    fty: fty,
                    explicit_self: data.explicit_self.decode((self, tcx)),
                    vis: item.visibility,
                    defaultness: data.container.defaultness(),
                    has_body: data.container.has_body(),
                    def_id: self.local_def_id(id),
                    container: data.container.with_def_id(parent),
                }))
            }
            EntryKind::AssociatedType(container) => {
                let (parent, name) = parent_and_name();
                ty::TypeTraitItem(Rc::new(ty::AssociatedType {
                    name: name,
                    ty: item.ty.map(|ty| ty.decode((self, tcx))),
                    vis: item.visibility,
                    defaultness: container.defaultness(),
                    def_id: self.local_def_id(id),
                    container: container.with_def_id(parent),
                }))
            }
            _ => return None
        })
    }

    pub fn get_item_variances(&self, id: DefIndex) -> Vec<ty::Variance> {
        self.entry(id).variances.decode(self).collect()
    }

    pub fn get_struct_ctor_def_id(&self, node_id: DefIndex) -> Option<DefId> {
        match self.entry(node_id).kind {
            EntryKind::Struct(data) => {
                data.decode(self).struct_ctor.map(|index| self.local_def_id(index))
            }
            _ => None
        }
    }

    pub fn get_item_attrs(&self, node_id: DefIndex) -> Vec<ast::Attribute> {
        // The attributes for a tuple struct are attached to the definition, not the ctor;
        // we assume that someone passing in a tuple struct ctor is actually wanting to
        // look at the definition
        let mut item = self.entry(node_id);
        let def_key = item.def_key.decode(self);
        if def_key.disambiguated_data.data == DefPathData::StructCtor {
            item = self.entry(def_key.parent.unwrap());
        }
        self.get_attributes(&item)
    }

    pub fn get_struct_field_names(&self, id: DefIndex) -> Vec<ast::Name> {
        self.entry(id).children.decode(self).map(|index| {
            self.item_name(&self.entry(index))
        }).collect()
    }

    fn get_attributes(&self, item: &Entry<'tcx>) -> Vec<ast::Attribute> {
        item.attributes.decode(self).map(|mut attr| {
            // Need new unique IDs: old thread-local IDs won't map to new threads.
            attr.node.id = attr::mk_attr_id();
            attr
        }).collect()
    }

    // Translate a DefId from the current compilation environment to a DefId
    // for an external crate.
    fn reverse_translate_def_id(&self, did: DefId) -> Option<DefId> {
        for (local, &global) in self.cnum_map.borrow().iter_enumerated() {
            if global == did.krate {
                return Some(DefId { krate: local, index: did.index });
            }
        }

        None
    }

    pub fn get_inherent_implementations_for_type(&self, id: DefIndex) -> Vec<DefId> {
        self.entry(id).inherent_impls.decode(self).map(|index| {
            self.local_def_id(index)
        }).collect()
    }

    pub fn get_implementations_for_trait(&self, filter: Option<DefId>, result: &mut Vec<DefId>) {
        // Do a reverse lookup beforehand to avoid touching the crate_num
        // hash map in the loop below.
        let filter = match filter.map(|def_id| self.reverse_translate_def_id(def_id)) {
            Some(Some(def_id)) => Some((def_id.krate.as_u32(), def_id.index)),
            Some(None) => return,
            None => None
        };

        // FIXME(eddyb) Make this O(1) instead of O(n).
        for trait_impls in self.root.impls.decode(self) {
            if filter.is_some() && filter != Some(trait_impls.trait_id) {
                continue;
            }

            result.extend(trait_impls.impls.decode(self).map(|index| {
                self.local_def_id(index)
            }));

            if filter.is_some() {
                break;
            }
        }
    }

    pub fn get_trait_of_item(&self, id: DefIndex) -> Option<DefId> {
        self.entry(id).def_key.decode(self).parent.and_then(|parent_index| {
            match self.entry(parent_index).kind {
                EntryKind::Trait(_) => Some(self.local_def_id(parent_index)),
                _ => None
            }
        })
    }


    pub fn get_native_libraries(&self) -> Vec<(NativeLibraryKind, String)> {
        self.root.native_libraries.decode(self).collect()
    }

    pub fn get_dylib_dependency_formats(&self) -> Vec<(CrateNum, LinkagePreference)> {
        self.root.dylib_dependency_formats.decode(self).enumerate().flat_map(|(i, link)| {
            let cnum = CrateNum::new(i + 1);
            link.map(|link| (self.cnum_map.borrow()[cnum], link))
        }).collect()
    }

    pub fn get_missing_lang_items(&self) -> Vec<lang_items::LangItem> {
        self.root.lang_items_missing.decode(self).collect()
    }

    pub fn get_fn_arg_names(&self, id: DefIndex) -> Vec<ast::Name> {
        let arg_names = match self.entry(id).kind {
            EntryKind::Fn(data) |
            EntryKind::ForeignFn(data) => data.decode(self).arg_names,
            EntryKind::Method(data) => data.decode(self).fn_data.arg_names,
            _ => LazySeq::empty()
        };
        arg_names.decode(self).collect()
    }

    pub fn get_reachable_ids(&self) -> Vec<DefId> {
        self.root.reachable_ids.decode(self).map(|index| self.local_def_id(index)).collect()
    }

    pub fn is_const_fn(&self, id: DefIndex) -> bool {
        let constness = match self.entry(id).kind {
            EntryKind::Method(data) => data.decode(self).fn_data.constness,
            EntryKind::Fn(data) => data.decode(self).constness,
            _ => hir::Constness::NotConst
        };
        constness == hir::Constness::Const
    }

    pub fn is_extern_item(&self, id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> bool {
        let item = match self.maybe_entry(id) {
            Some(item) => item.decode(self),
            None => return false,
        };
        let applicable = match item.kind {
            EntryKind::ImmStatic |
            EntryKind::MutStatic |
            EntryKind::ForeignImmStatic |
            EntryKind::ForeignMutStatic => true,

            EntryKind::Fn(_) | EntryKind::ForeignFn(_) => {
                self.get_generics(id, tcx).types.is_empty()
            }

            _ => false,
        };

        if applicable {
            attr::contains_extern_indicator(tcx.sess.diagnostic(),
                                            &self.get_attributes(&item))
        } else {
            false
        }
    }

    pub fn is_foreign_item(&self, id: DefIndex) -> bool {
        match self.entry(id).kind {
            EntryKind::ForeignImmStatic |
            EntryKind::ForeignMutStatic |
            EntryKind::ForeignFn(_) => true,
            _ => false
        }
    }

    pub fn is_defaulted_trait(&self, trait_id: DefIndex) -> bool {
        match self.entry(trait_id).kind {
            EntryKind::Trait(data) => data.decode(self).has_default_impl,
            _ => bug!()
        }
    }

    pub fn is_default_impl(&self, impl_id: DefIndex) -> bool {
        match self.entry(impl_id).kind  {
            EntryKind::DefaultImpl(_) => true,
            _ => false
        }
    }

    pub fn closure_kind(&self, closure_id: DefIndex) -> ty::ClosureKind {
        match self.entry(closure_id).kind {
            EntryKind::Closure(data) => data.decode(self).kind,
            _ => bug!()
        }
    }

    pub fn closure_ty(&self, closure_id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>)
                      -> ty::ClosureTy<'tcx> {
        match self.entry(closure_id).kind {
            EntryKind::Closure(data) => data.decode(self).ty.decode((self, tcx)),
            _ => bug!()
        }
    }

    pub fn def_key(&self, id: DefIndex) -> hir_map::DefKey {
        debug!("def_key: id={:?}", id);
        self.entry(id).def_key.decode(self)
    }

    // Returns the path leading to the thing with this `id`. Note that
    // some def-ids don't wind up in the metadata, so `def_path` sometimes
    // returns `None`
    pub fn def_path(&self, id: DefIndex) -> Option<hir_map::DefPath> {
        debug!("def_path(id={:?})", id);
        if self.maybe_entry(id).is_some() {
            Some(hir_map::DefPath::make(self.cnum, id, |parent| self.def_key(parent)))
        } else {
            None
        }
    }

    /// Imports the codemap from an external crate into the codemap of the crate
    /// currently being compiled (the "local crate").
    ///
    /// The import algorithm works analogous to how AST items are inlined from an
    /// external crate's metadata:
    /// For every FileMap in the external codemap an 'inline' copy is created in the
    /// local codemap. The correspondence relation between external and local
    /// FileMaps is recorded in the `ImportedFileMap` objects returned from this
    /// function. When an item from an external crate is later inlined into this
    /// crate, this correspondence information is used to translate the span
    /// information of the inlined item so that it refers the correct positions in
    /// the local codemap (see `<decoder::DecodeContext as SpecializedDecoder<Span>>`).
    ///
    /// The import algorithm in the function below will reuse FileMaps already
    /// existing in the local codemap. For example, even if the FileMap of some
    /// source file of libstd gets imported many times, there will only ever be
    /// one FileMap object for the corresponding file in the local codemap.
    ///
    /// Note that imported FileMaps do not actually contain the source code of the
    /// file they represent, just information about length, line breaks, and
    /// multibyte characters. This information is enough to generate valid debuginfo
    /// for items inlined from other crates.
    pub fn imported_filemaps(&'a self, local_codemap: &codemap::CodeMap)
                             -> Ref<'a, Vec<cstore::ImportedFileMap>> {
        {
            let filemaps = self.codemap_import_info.borrow();
            if !filemaps.is_empty() {
                return filemaps;
            }
        }

        let external_codemap = self.root.codemap.decode(self);

        let imported_filemaps = external_codemap.map(|filemap_to_import| {
            // Try to find an existing FileMap that can be reused for the filemap to
            // be imported. A FileMap is reusable if it is exactly the same, just
            // positioned at a different offset within the codemap.
            let reusable_filemap = {
                local_codemap.files
                             .borrow()
                             .iter()
                             .find(|fm| are_equal_modulo_startpos(&fm, &filemap_to_import))
                             .map(|rc| rc.clone())
            };

            match reusable_filemap {
                Some(fm) => {
                    cstore::ImportedFileMap {
                        original_start_pos: filemap_to_import.start_pos,
                        original_end_pos: filemap_to_import.end_pos,
                        translated_filemap: fm
                    }
                }
                None => {
                    // We can't reuse an existing FileMap, so allocate a new one
                    // containing the information we need.
                    let syntax_pos::FileMap {
                        name,
                        abs_path,
                        start_pos,
                        end_pos,
                        lines,
                        multibyte_chars,
                        ..
                    } = filemap_to_import;

                    let source_length = (end_pos - start_pos).to_usize();

                    // Translate line-start positions and multibyte character
                    // position into frame of reference local to file.
                    // `CodeMap::new_imported_filemap()` will then translate those
                    // coordinates to their new global frame of reference when the
                    // offset of the FileMap is known.
                    let mut lines = lines.into_inner();
                    for pos in &mut lines {
                        *pos = *pos - start_pos;
                    }
                    let mut multibyte_chars = multibyte_chars.into_inner();
                    for mbc in &mut multibyte_chars {
                        mbc.pos = mbc.pos - start_pos;
                    }

                    let local_version = local_codemap.new_imported_filemap(name,
                                                                           abs_path,
                                                                           source_length,
                                                                           lines,
                                                                           multibyte_chars);
                    cstore::ImportedFileMap {
                        original_start_pos: start_pos,
                        original_end_pos: end_pos,
                        translated_filemap: local_version
                    }
                }
            }
        }).collect();

        // This shouldn't borrow twice, but there is no way to downgrade RefMut to Ref.
        *self.codemap_import_info.borrow_mut() = imported_filemaps;
        self.codemap_import_info.borrow()
    }
}

fn are_equal_modulo_startpos(fm1: &syntax_pos::FileMap, fm2: &syntax_pos::FileMap) -> bool {
    if fm1.name != fm2.name {
        return false;
    }

    let lines1 = fm1.lines.borrow();
    let lines2 = fm2.lines.borrow();

    if lines1.len() != lines2.len() {
        return false;
    }

    for (&line1, &line2) in lines1.iter().zip(lines2.iter()) {
        if (line1 - fm1.start_pos) != (line2 - fm2.start_pos) {
            return false;
        }
    }

    let multibytes1 = fm1.multibyte_chars.borrow();
    let multibytes2 = fm2.multibyte_chars.borrow();

    if multibytes1.len() != multibytes2.len() {
        return false;
    }

    for (mb1, mb2) in multibytes1.iter().zip(multibytes2.iter()) {
        if (mb1.bytes != mb2.bytes) ||
            ((mb1.pos - fm1.start_pos) != (mb2.pos - fm2.start_pos)) {
            return false;
        }
    }

    true
}