src/librustc/infer/freshen.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 //! Freshening is the process of replacing unknown variables with fresh types. The idea is that
  12 //! the type, after freshening, contains no inference variables but instead contains either a
  13 //! value for each variable or fresh "arbitrary" types wherever a variable would have been.
  14 //!
  15 //! Freshening is used primarily to get a good type for inserting into a cache. The result
  16 //! summarizes what the type inferencer knows "so far". The primary place it is used right now is
  17 //! in the trait matching algorithm, which needs to be able to cache whether an `impl` self type
  18 //! matches some other type X -- *without* affecting `X`. That means if that if the type `X` is in
  19 //! fact an unbound type variable, we want the match to be regarded as ambiguous, because depending
  20 //! on what type that type variable is ultimately assigned, the match may or may not succeed.
  21 //!
  22 //! To handle closures, freshened types also have to contain the signature and kind of any
  23 //! closure in the local inference context, as otherwise the cache key might be invalidated.
  24 //! The way this is done is somewhat hacky - the closure signature is appended to the substs,
  25 //! as well as the closure kind "encoded" as a type. Also, special handling is needed when
  26 //! the closure signature contains a reference to the original closure.
  27 //!
  28 //! Note that you should be careful not to allow the output of freshening to leak to the user in
  29 //! error messages or in any other form. Freshening is only really useful as an internal detail.
  30 //!
  31 //! Because of the manipulation required to handle closures, doing arbitrary operations on
  32 //! freshened types is not recommended. However, in addition to doing equality/hash
  33 //! comparisons (for caching), it is possible to do a `ty::_match` operation between
  34 //! 2 freshened types - this works even with the closure encoding.
  35 //!
  36 //! __An important detail concerning regions.__ The freshener also replaces *all* free regions with
  37 //! 'erased. The reason behind this is that, in general, we do not take region relationships into
  38 //! account when making type-overloaded decisions. This is important because of the design of the
  39 //! region inferencer, which is not based on unification but rather on accumulating and then
  40 //! solving a set of constraints. In contrast, the type inferencer assigns a value to each type
  41 //! variable only once, and it does so as soon as it can, so it is reasonable to ask what the type
  42 //! inferencer knows "so far".
  43
  44 use ty::{self, Ty, TyCtxt, TypeFoldable};
  45 use ty::fold::TypeFolder;
  46 use util::nodemap::FxHashMap;
  47
  48 use std::collections::hash_map::Entry;
  49
  50 use super::InferCtxt;
  51 use super::unify_key::ToType;
  52
  53 pub struct TypeFreshener<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
  54     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
  55     freshen_count: u32,
  56     freshen_map: FxHashMap<ty::InferTy, Ty<'tcx>>,
  57 }
  58
  59 impl<'a, 'gcx, 'tcx> TypeFreshener<'a, 'gcx, 'tcx> {
  60     pub fn new(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>)
  61                -> TypeFreshener<'a, 'gcx, 'tcx> {
  62         TypeFreshener {
  63             infcx,
  64             freshen_count: 0,
  65             freshen_map: FxHashMap(),
  66         }
  67     }
  68
  69     fn freshen<F>(&mut self,
  70                   opt_ty: Option<Ty<'tcx>>,
  71                   key: ty::InferTy,
  72                   freshener: F)
  73                   -> Ty<'tcx> where
  74         F: FnOnce(u32) -> ty::InferTy,
  75     {
  76         if let Some(ty) = opt_ty {
  77             return ty.fold_with(self);
  78         }
  79
  80         match self.freshen_map.entry(key) {
  81             Entry::Occupied(entry) => *entry.get(),
  82             Entry::Vacant(entry) => {
  83                 let index = self.freshen_count;
  84                 self.freshen_count += 1;
  85                 let t = self.infcx.tcx.mk_infer(freshener(index));
  86                 entry.insert(t);
  87                 t
  88             }
  89         }
  90     }
  91 }
  92
  93 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
  94     fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> {
  95         self.infcx.tcx
  96     }
  97
  98     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
  99         match *r {
 100             ty::ReLateBound(..) => {
 101                 // leave bound regions alone
 102                 r
 103             }
 104
 105             ty::ReStatic |
 106             ty::ReEarlyBound(..) |
 107             ty::ReFree(_) |
 108             ty::ReScope(_) |
 109             ty::ReVar(_) |
 110             ty::ReSkolemized(..) |
 111             ty::ReEmpty |
 112             ty::ReErased => {
 113                 // replace all free regions with 'erased
 114                 self.tcx().types.re_erased
 115             }
 116
 117             ty::ReClosureBound(..) => {
 118                 bug!(
 119                     "encountered unexpected ReClosureBound: {:?}",
 120                     r,
 121                 );
 122             }
 123         }
 124     }
 125
 126     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 127         if !t.needs_infer() && !t.has_erasable_regions() &&
 128             !(t.has_closure_types() && self.infcx.in_progress_tables.is_some()) {
 129             return t;
 130         }
 131
 132         let tcx = self.infcx.tcx;
 133
 134         match t.sty {
 135             ty::TyInfer(ty::TyVar(v)) => {
 136                 let opt_ty = self.infcx.type_variables.borrow_mut().probe(v);
 137                 self.freshen(
 138                     opt_ty,
 139                     ty::TyVar(v),
 140                     ty::FreshTy)
 141             }
 142
 143             ty::TyInfer(ty::IntVar(v)) => {
 144                 self.freshen(
 145                     self.infcx.int_unification_table.borrow_mut()
 146                                                     .probe(v)
 147                                                     .map(|v| v.to_type(tcx)),
 148                     ty::IntVar(v),
 149                     ty::FreshIntTy)
 150             }
 151
 152             ty::TyInfer(ty::FloatVar(v)) => {
 153                 self.freshen(
 154                     self.infcx.float_unification_table.borrow_mut()
 155                                                       .probe(v)
 156                                                       .map(|v| v.to_type(tcx)),
 157                     ty::FloatVar(v),
 158                     ty::FreshFloatTy)
 159             }
 160
 161             ty::TyInfer(ty::FreshTy(c)) |
 162             ty::TyInfer(ty::FreshIntTy(c)) |
 163             ty::TyInfer(ty::FreshFloatTy(c)) => {
 164                 if c >= self.freshen_count {
 165                     bug!("Encountered a freshend type with id {} \
 166                           but our counter is only at {}",
 167                          c,
 168                          self.freshen_count);
 169                 }
 170                 t
 171             }
 172
 173             ty::TyGenerator(..) |
 174             ty::TyBool |
 175             ty::TyChar |
 176             ty::TyInt(..) |
 177             ty::TyUint(..) |
 178             ty::TyFloat(..) |
 179             ty::TyAdt(..) |
 180             ty::TyStr |
 181             ty::TyError |
 182             ty::TyArray(..) |
 183             ty::TySlice(..) |
 184             ty::TyRawPtr(..) |
 185             ty::TyRef(..) |
 186             ty::TyFnDef(..) |
 187             ty::TyFnPtr(_) |
 188             ty::TyDynamic(..) |
 189             ty::TyNever |
 190             ty::TyTuple(..) |
 191             ty::TyProjection(..) |
 192             ty::TyForeign(..) |
 193             ty::TyParam(..) |
 194             ty::TyClosure(..) |
 195             ty::TyGeneratorWitness(..) |
 196             ty::TyAnon(..) => {
 197                 t.super_fold_with(self)
 198             }
 199         }
 200     }
 201 }