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 //! Skolemization is the process of replacing unknown variables with fresh types. The idea is that
12 //! the type, after skolemization, 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 //! Skolemization 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 skolemization to leak to the user in
23 //! error messages or in any other form. Skolemization is only really useful as an internal detail.
25 //! __An important detail concerning regions.__ The skolemizer 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::{mod, Ty};
35 use middle::ty_fold::TypeFoldable;
36 use middle::ty_fold::TypeFolder;
37 use std::collections::hash_map;
40 use super::unify::InferCtxtMethodsForSimplyUnifiableTypes;
42 pub struct TypeSkolemizer<'a, 'tcx:'a> {
43 infcx: &'a InferCtxt<'a, 'tcx>,
44 skolemization_count: uint,
45 skolemization_map: hash_map::HashMap<ty::InferTy, Ty<'tcx>>,
48 impl<'a, 'tcx> TypeSkolemizer<'a, 'tcx> {
49 pub fn new<'tcx>(infcx: &'a InferCtxt<'a, 'tcx>) -> TypeSkolemizer<'a, 'tcx> {
52 skolemization_count: 0,
53 skolemization_map: hash_map::HashMap::new(),
57 fn skolemize(&mut self,
58 opt_ty: Option<Ty<'tcx>>,
60 skolemizer: |uint| -> ty::InferTy)
64 Some(ty) => { return ty.fold_with(self); }
68 match self.skolemization_map.entry(key) {
69 hash_map::Occupied(entry) => *entry.get(),
70 hash_map::Vacant(entry) => {
71 let index = self.skolemization_count;
72 self.skolemization_count += 1;
73 let t = ty::mk_infer(self.infcx.tcx, skolemizer(index));
81 impl<'a, 'tcx> TypeFolder<'tcx> for TypeSkolemizer<'a, 'tcx> {
82 fn tcx<'b>(&'b self) -> &'b ty::ctxt<'tcx> {
86 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
88 ty::ReEarlyBound(..) |
89 ty::ReLateBound(..) => {
90 // leave bound regions alone
99 // replace all free regions with 'static
105 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
107 ty::ty_infer(ty::TyVar(v)) => {
108 self.skolemize(self.infcx.type_variables.borrow().probe(v),
113 ty::ty_infer(ty::IntVar(v)) => {
114 self.skolemize(self.infcx.probe_var(v),
119 ty::ty_infer(ty::FloatVar(v)) => {
120 self.skolemize(self.infcx.probe_var(v),
125 ty::ty_infer(ty::SkolemizedTy(c)) |
126 ty::ty_infer(ty::SkolemizedIntTy(c)) => {
127 if c >= self.skolemization_count {
129 format!("Encountered a skolemized type with id {} \
130 but our counter is only at {}",
132 self.skolemization_count).as_slice());
138 self.tcx().sess.bug("Cannot skolemize an open existential type");
157 ty::ty_unboxed_closure(..) |
159 ty::ty_param(..) => {
160 ty_fold::super_fold_ty(self, t)