src/librustc/middle/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 middle::ty::{self, Ty};
  34 use middle::ty_fold;
  35 use middle::ty_fold::TypeFoldable;
  36 use middle::ty_fold::TypeFolder;
  37 use std::collections::hash_map::{self, Entry};
  38
  39 use super::InferCtxt;
  40 use super::unify::InferCtxtMethodsForSimplyUnifiableTypes;
  41
  42 pub struct TypeFreshener<'a, 'tcx:'a> {
  43     infcx: &'a InferCtxt<'a, 'tcx>,
  44     freshen_count: u32,
  45     freshen_map: hash_map::HashMap<ty::InferTy, Ty<'tcx>>,
  46 }
  47
  48 impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
  49     pub fn new(infcx: &'a InferCtxt<'a, 'tcx>) -> TypeFreshener<'a, 'tcx> {
  50         TypeFreshener {
  51             infcx: infcx,
  52             freshen_count: 0,
  53             freshen_map: hash_map::HashMap::new(),
  54         }
  55     }
  56
  57     fn freshen<F>(&mut self,
  58                   opt_ty: Option<Ty<'tcx>>,
  59                   key: ty::InferTy,
  60                   freshener: F)
  61                   -> Ty<'tcx> where
  62         F: FnOnce(u32) -> ty::InferTy,
  63     {
  64         match opt_ty {
  65             Some(ty) => { return ty.fold_with(self); }
  66             None => { }
  67         }
  68
  69         match self.freshen_map.entry(key) {
  70             Entry::Occupied(entry) => *entry.get(),
  71             Entry::Vacant(entry) => {
  72                 let index = self.freshen_count;
  73                 self.freshen_count += 1;
  74                 let t = ty::mk_infer(self.infcx.tcx, freshener(index));
  75                 entry.insert(t);
  76                 t
  77             }
  78         }
  79     }
  80 }
  81
  82 impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
  83     fn tcx<'b>(&'b self) -> &'b ty::ctxt<'tcx> {
  84         self.infcx.tcx
  85     }
  86
  87     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
  88         match r {
  89             ty::ReEarlyBound(..) |
  90             ty::ReLateBound(..) => {
  91                 // leave bound regions alone
  92                 r
  93             }
  94
  95             ty::ReStatic |
  96             ty::ReFree(_) |
  97             ty::ReScope(_) |
  98             ty::ReInfer(_) |
  99             ty::ReEmpty => {
 100                 // replace all free regions with 'static
 101                 ty::ReStatic
 102             }
 103         }
 104     }
 105
 106     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 107         match t.sty {
 108             ty::ty_infer(ty::TyVar(v)) => {
 109                 self.freshen(self.infcx.type_variables.borrow().probe(v),
 110                                ty::TyVar(v),
 111                                ty::FreshTy)
 112             }
 113
 114             ty::ty_infer(ty::IntVar(v)) => {
 115                 self.freshen(self.infcx.probe_var(v),
 116                              ty::IntVar(v),
 117                              ty::FreshIntTy)
 118             }
 119
 120             ty::ty_infer(ty::FloatVar(v)) => {
 121                 self.freshen(self.infcx.probe_var(v),
 122                              ty::FloatVar(v),
 123                              ty::FreshIntTy)
 124             }
 125
 126             ty::ty_infer(ty::FreshTy(c)) |
 127             ty::ty_infer(ty::FreshIntTy(c)) => {
 128                 if c >= self.freshen_count {
 129                     self.tcx().sess.bug(
 130                         &format!("Encountered a freshend type with id {} \
 131                                   but our counter is only at {}",
 132                                  c,
 133                                  self.freshen_count));
 134                 }
 135                 t
 136             }
 137
 138             ty::ty_open(..) |
 139             ty::ty_bool |
 140             ty::ty_char |
 141             ty::ty_int(..) |
 142             ty::ty_uint(..) |
 143             ty::ty_float(..) |
 144             ty::ty_enum(..) |
 145             ty::ty_uniq(..) |
 146             ty::ty_str |
 147             ty::ty_err |
 148             ty::ty_vec(..) |
 149             ty::ty_ptr(..) |
 150             ty::ty_rptr(..) |
 151             ty::ty_bare_fn(..) |
 152             ty::ty_trait(..) |
 153             ty::ty_struct(..) |
 154             ty::ty_closure(..) |
 155             ty::ty_tup(..) |
 156             ty::ty_projection(..) |
 157             ty::ty_param(..) => {
 158                 ty_fold::super_fold_ty(self, t)
 159             }
 160         }
 161     }
 162 }