src/librustc/middle/infer/type_variable.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 pub use self::RelationDir::*;
  12 use self::TypeVariableValue::*;
  13 use self::UndoEntry::*;
  14
  15 use middle::ty::{self, Ty};
  16 use std::cmp::min;
  17 use std::marker::PhantomData;
  18 use std::mem;
  19 use std::u32;
  20 use rustc_data_structures::snapshot_vec as sv;
  21
  22 pub struct TypeVariableTable<'tcx> {
  23     values: sv::SnapshotVec<Delegate<'tcx>>,
  24 }
  25
  26 struct TypeVariableData<'tcx> {
  27     value: TypeVariableValue<'tcx>,
  28     diverging: bool
  29 }
  30
  31 enum TypeVariableValue<'tcx> {
  32     Known(Ty<'tcx>),
  33     Bounded(Vec<Relation>),
  34 }
  35
  36 pub struct Snapshot {
  37     snapshot: sv::Snapshot
  38 }
  39
  40 enum UndoEntry {
  41     // The type of the var was specified.
  42     SpecifyVar(ty::TyVid, Vec<Relation>),
  43     Relate(ty::TyVid, ty::TyVid),
  44 }
  45
  46 struct Delegate<'tcx>(PhantomData<&'tcx ()>);
  47
  48 type Relation = (RelationDir, ty::TyVid);
  49
  50 #[derive(Copy, Clone, PartialEq, Debug)]
  51 pub enum RelationDir {
  52     SubtypeOf, SupertypeOf, EqTo, BiTo
  53 }
  54
  55 impl RelationDir {
  56     fn opposite(self) -> RelationDir {
  57         match self {
  58             SubtypeOf => SupertypeOf,
  59             SupertypeOf => SubtypeOf,
  60             EqTo => EqTo,
  61             BiTo => BiTo,
  62         }
  63     }
  64 }
  65
  66 impl<'tcx> TypeVariableTable<'tcx> {
  67     pub fn new() -> TypeVariableTable<'tcx> {
  68         TypeVariableTable { values: sv::SnapshotVec::new() }
  69     }
  70
  71     fn relations<'a>(&'a mut self, a: ty::TyVid) -> &'a mut Vec<Relation> {
  72         relations(self.values.get_mut(a.index as usize))
  73     }
  74
  75     pub fn var_diverges<'a>(&'a self, vid: ty::TyVid) -> bool {
  76         self.values.get(vid.index as usize).diverging
  77     }
  78
  79     /// Records that `a <: b`, `a :> b`, or `a == b`, depending on `dir`.
  80     ///
  81     /// Precondition: neither `a` nor `b` are known.
  82     pub fn relate_vars(&mut self, a: ty::TyVid, dir: RelationDir, b: ty::TyVid) {
  83         if a != b {
  84             self.relations(a).push((dir, b));
  85             self.relations(b).push((dir.opposite(), a));
  86             self.values.record(Relate(a, b));
  87         }
  88     }
  89
  90     /// Instantiates `vid` with the type `ty` and then pushes an entry onto `stack` for each of the
  91     /// relations of `vid` to other variables. The relations will have the form `(ty, dir, vid1)`
  92     /// where `vid1` is some other variable id.
  93     pub fn instantiate_and_push(
  94         &mut self,
  95         vid: ty::TyVid,
  96         ty: Ty<'tcx>,
  97         stack: &mut Vec<(Ty<'tcx>, RelationDir, ty::TyVid)>)
  98     {
  99         let old_value = {
 100             let value_ptr = &mut self.values.get_mut(vid.index as usize).value;
 101             mem::replace(value_ptr, Known(ty))
 102         };
 103
 104         let relations = match old_value {
 105             Bounded(b) => b,
 106             Known(_) => panic!("Asked to instantiate variable that is \
 107                                already instantiated")
 108         };
 109
 110         for &(dir, vid) in &relations {
 111             stack.push((ty, dir, vid));
 112         }
 113
 114         self.values.record(SpecifyVar(vid, relations));
 115     }
 116
 117     pub fn new_var(&mut self, diverging: bool) -> ty::TyVid {
 118         let index = self.values.push(TypeVariableData {
 119             value: Bounded(vec![]),
 120             diverging: diverging
 121         });
 122         ty::TyVid { index: index as u32 }
 123     }
 124
 125     pub fn probe(&self, vid: ty::TyVid) -> Option<Ty<'tcx>> {
 126         match self.values.get(vid.index as usize).value {
 127             Bounded(..) => None,
 128             Known(t) => Some(t)
 129         }
 130     }
 131
 132     pub fn replace_if_possible(&self, t: Ty<'tcx>) -> Ty<'tcx> {
 133         match t.sty {
 134             ty::TyInfer(ty::TyVar(v)) => {
 135                 match self.probe(v) {
 136                     None => t,
 137                     Some(u) => u
 138                 }
 139             }
 140             _ => t,
 141         }
 142     }
 143
 144     pub fn snapshot(&mut self) -> Snapshot {
 145         Snapshot { snapshot: self.values.start_snapshot() }
 146     }
 147
 148     pub fn rollback_to(&mut self, s: Snapshot) {
 149         self.values.rollback_to(s.snapshot);
 150     }
 151
 152     pub fn commit(&mut self, s: Snapshot) {
 153         self.values.commit(s.snapshot);
 154     }
 155
 156     pub fn types_escaping_snapshot(&self, s: &Snapshot) -> Vec<Ty<'tcx>> {
 157         /*!
 158          * Find the set of type variables that existed *before* `s`
 159          * but which have only been unified since `s` started, and
 160          * return the types with which they were unified. So if we had
 161          * a type variable `V0`, then we started the snapshot, then we
 162          * created a type variable `V1`, unifed `V0` with `T0`, and
 163          * unified `V1` with `T1`, this function would return `{T0}`.
 164          */
 165
 166         let mut new_elem_threshold = u32::MAX;
 167         let mut escaping_types = Vec::new();
 168         let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot);
 169         debug!("actions_since_snapshot.len() = {}", actions_since_snapshot.len());
 170         for action in actions_since_snapshot {
 171             match *action {
 172                 sv::UndoLog::NewElem(index) => {
 173                     // if any new variables were created during the
 174                     // snapshot, remember the lower index (which will
 175                     // always be the first one we see). Note that this
 176                     // action must precede those variables being
 177                     // specified.
 178                     new_elem_threshold = min(new_elem_threshold, index as u32);
 179                     debug!("NewElem({}) new_elem_threshold={}", index, new_elem_threshold);
 180                 }
 181
 182                 sv::UndoLog::Other(SpecifyVar(vid, _)) => {
 183                     if vid.index < new_elem_threshold {
 184                         // quick check to see if this variable was
 185                         // created since the snapshot started or not.
 186                         let escaping_type = self.probe(vid).unwrap();
 187                         escaping_types.push(escaping_type);
 188                     }
 189                     debug!("SpecifyVar({:?}) new_elem_threshold={}", vid, new_elem_threshold);
 190                 }
 191
 192                 _ => { }
 193             }
 194         }
 195
 196         escaping_types
 197     }
 198 }
 199
 200 impl<'tcx> sv::SnapshotVecDelegate for Delegate<'tcx> {
 201     type Value = TypeVariableData<'tcx>;
 202     type Undo = UndoEntry;
 203
 204     fn reverse(values: &mut Vec<TypeVariableData<'tcx>>, action: UndoEntry) {
 205         match action {
 206             SpecifyVar(vid, relations) => {
 207                 values[vid.index as usize].value = Bounded(relations);
 208             }
 209
 210             Relate(a, b) => {
 211                 relations(&mut (*values)[a.index as usize]).pop();
 212                 relations(&mut (*values)[b.index as usize]).pop();
 213             }
 214         }
 215     }
 216 }
 217
 218 fn relations<'a>(v: &'a mut TypeVariableData) -> &'a mut Vec<Relation> {
 219     match v.value {
 220         Known(_) => panic!("var_sub_var: variable is known"),
 221         Bounded(ref mut relations) => relations
 222     }
 223 }