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 //! Note that you should be careful not to allow the output of freshening to leak to the user in
  23 //! error messages or in any other form. Freshening is only really useful as an internal detail.
  24 //!
  25 //! __An important detail concerning regions.__ The freshener also replaces *all* regions with
  26 //! 'static. The reason behind this is that, in general, we do not take region relationships into
  27 //! account when making type-overloaded decisions. This is important because of the design of the
  28 //! region inferencer, which is not based on unification but rather on accumulating and then
  29 //! solving a set of constraints. In contrast, the type inferencer assigns a value to each type
  30 //! variable only once, and it does so as soon as it can, so it is reasonable to ask what the type
  31 //! inferencer knows "so far".
  32
  33 use ty::{self, Ty, TyCtxt, TypeFoldable};
  34 use ty::fold::TypeFolder;
  35 use std::collections::hash_map::{self, Entry};
  36
  37 use super::InferCtxt;
  38 use super::unify_key::ToType;
  39
  40 pub struct TypeFreshener<'a, 'tcx:'a> {
  41     infcx: &'a InferCtxt<'a, 'tcx>,
  42     freshen_count: u32,
  43     freshen_map: hash_map::HashMap<ty::InferTy, Ty<'tcx>>,
  44 }
  45
  46 impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
  47     pub fn new(infcx: &'a InferCtxt<'a, 'tcx>) -> TypeFreshener<'a, 'tcx> {
  48         TypeFreshener {
  49             infcx: infcx,
  50             freshen_count: 0,
  51             freshen_map: hash_map::HashMap::new(),
  52         }
  53     }
  54
  55     fn freshen<F>(&mut self,
  56                   opt_ty: Option<Ty<'tcx>>,
  57                   key: ty::InferTy,
  58                   freshener: F)
  59                   -> Ty<'tcx> where
  60         F: FnOnce(u32) -> ty::InferTy,
  61     {
  62         match opt_ty {
  63             Some(ty) => { return ty.fold_with(self); }
  64             None => { }
  65         }
  66
  67         match self.freshen_map.entry(key) {
  68             Entry::Occupied(entry) => *entry.get(),
  69             Entry::Vacant(entry) => {
  70                 let index = self.freshen_count;
  71                 self.freshen_count += 1;
  72                 let t = self.infcx.tcx.mk_infer(freshener(index));
  73                 entry.insert(t);
  74                 t
  75             }
  76         }
  77     }
  78 }
  79
  80 impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
  81     fn tcx<'b>(&'b self) -> &'b TyCtxt<'tcx> {
  82         self.infcx.tcx
  83     }
  84
  85     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
  86         match r {
  87             ty::ReEarlyBound(..) |
  88             ty::ReLateBound(..) => {
  89                 // leave bound regions alone
  90                 r
  91             }
  92
  93             ty::ReStatic |
  94             ty::ReFree(_) |
  95             ty::ReScope(_) |
  96             ty::ReVar(_) |
  97             ty::ReSkolemized(..) |
  98             ty::ReEmpty => {
  99                 // replace all free regions with 'static
 100                 ty::ReStatic
 101             }
 102         }
 103     }
 104
 105     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 106         if !t.needs_infer() && !t.has_erasable_regions() {
 107             return t;
 108         }
 109
 110         let tcx = self.infcx.tcx;
 111
 112         match t.sty {
 113             ty::TyInfer(ty::TyVar(v)) => {
 114                 let opt_ty = self.infcx.type_variables.borrow_mut().probe(v);
 115                 self.freshen(
 116                     opt_ty,
 117                     ty::TyVar(v),
 118                     ty::FreshTy)
 119             }
 120
 121             ty::TyInfer(ty::IntVar(v)) => {
 122                 self.freshen(
 123                     self.infcx.int_unification_table.borrow_mut()
 124                                                     .probe(v)
 125                                                     .map(|v| v.to_type(tcx)),
 126                     ty::IntVar(v),
 127                     ty::FreshIntTy)
 128             }
 129
 130             ty::TyInfer(ty::FloatVar(v)) => {
 131                 self.freshen(
 132                     self.infcx.float_unification_table.borrow_mut()
 133                                                       .probe(v)
 134                                                       .map(|v| v.to_type(tcx)),
 135                     ty::FloatVar(v),
 136                     ty::FreshFloatTy)
 137             }
 138
 139             ty::TyInfer(ty::FreshTy(c)) |
 140             ty::TyInfer(ty::FreshIntTy(c)) |
 141             ty::TyInfer(ty::FreshFloatTy(c)) => {
 142                 if c >= self.freshen_count {
 143                     bug!("Encountered a freshend type with id {} \
 144                           but our counter is only at {}",
 145                          c,
 146                          self.freshen_count);
 147                 }
 148                 t
 149             }
 150
 151             ty::TyBool |
 152             ty::TyChar |
 153             ty::TyInt(..) |
 154             ty::TyUint(..) |
 155             ty::TyFloat(..) |
 156             ty::TyEnum(..) |
 157             ty::TyBox(..) |
 158             ty::TyStr |
 159             ty::TyError |
 160             ty::TyArray(..) |
 161             ty::TySlice(..) |
 162             ty::TyRawPtr(..) |
 163             ty::TyRef(..) |
 164             ty::TyFnDef(..) |
 165             ty::TyFnPtr(_) |
 166             ty::TyTrait(..) |
 167             ty::TyStruct(..) |
 168             ty::TyClosure(..) |
 169             ty::TyTuple(..) |
 170             ty::TyProjection(..) |
 171             ty::TyParam(..) => {
 172                 t.super_fold_with(self)
 173             }
 174         }
 175     }
 176 }