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.
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.
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.
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.
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.
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".
33 use middle::ty::{self, Ty, TypeFoldable};
34 use middle::ty::fold::TypeFolder;
35 use std::collections::hash_map::{self, Entry};
38 use super::unify_key::ToType;
40 pub struct TypeFreshener<'a, 'tcx:'a> {
41 infcx: &'a InferCtxt<'a, 'tcx>,
43 freshen_map: hash_map::HashMap<ty::InferTy, Ty<'tcx>>,
46 impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
47 pub fn new(infcx: &'a InferCtxt<'a, 'tcx>) -> TypeFreshener<'a, 'tcx> {
51 freshen_map: hash_map::HashMap::new(),
55 fn freshen<F>(&mut self,
56 opt_ty: Option<Ty<'tcx>>,
60 F: FnOnce(u32) -> ty::InferTy,
63 Some(ty) => { return ty.fold_with(self); }
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));
80 impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
81 fn tcx<'b>(&'b self) -> &'b ty::ctxt<'tcx> {
85 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
87 ty::ReEarlyBound(..) |
88 ty::ReLateBound(..) => {
89 // leave bound regions alone
97 ty::ReSkolemized(..) |
99 // replace all free regions with 'static
105 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
106 if !t.needs_infer() && !t.has_erasable_regions() {
110 let tcx = self.infcx.tcx;
113 ty::TyInfer(ty::TyVar(v)) => {
115 self.infcx.type_variables.borrow().probe(v),
120 ty::TyInfer(ty::IntVar(v)) => {
122 self.infcx.int_unification_table.borrow_mut()
124 .map(|v| v.to_type(tcx)),
129 ty::TyInfer(ty::FloatVar(v)) => {
131 self.infcx.float_unification_table.borrow_mut()
133 .map(|v| v.to_type(tcx)),
138 ty::TyInfer(ty::FreshTy(c)) |
139 ty::TyInfer(ty::FreshIntTy(c)) |
140 ty::TyInfer(ty::FreshFloatTy(c)) => {
141 if c >= self.freshen_count {
143 &format!("Encountered a freshend type with id {} \
144 but our counter is only at {}",
146 self.freshen_count));
169 ty::TyProjection(..) |
171 t.super_fold_with(self)