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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//! Applies the "bivariance relationship" to two types and/or regions.
12//! If (A,B) are bivariant then either A <: B or B <: A. It occurs
13//! when type/lifetime parameters are unconstrained. Usually this is
14//! an error, but we permit it in the specific case where a type
15//! parameter is constrained in a where-clause via an associated type.
16//!
17//! There are several ways one could implement bivariance. You could
18//! just do nothing at all, for example, or you could fully verify
19//! that one of the two subtyping relationships hold. We choose to
20//! thread a middle line: we relate types up to regions, but ignore
21//! all region relationships.
22//!
23//! At one point, handling bivariance in this fashion was necessary
24//! for inference, but I'm actually not sure if that is true anymore.
25//! In particular, it might be enough to say (A,B) are bivariant for
26//! all (A,B).
27
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28use super::combine::{self, CombineFields};
29use super::type_variable::{BiTo};
30
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31use middle::ty::{self, Ty};
32use middle::ty::TyVar;
e9174d1e 33use middle::ty::relate::{Relate, RelateResult, TypeRelation};
85aaf69f 34
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35pub struct Bivariate<'a, 'tcx: 'a> {
36 fields: CombineFields<'a, 'tcx>
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37}
38
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39impl<'a, 'tcx> Bivariate<'a, 'tcx> {
40 pub fn new(fields: CombineFields<'a, 'tcx>) -> Bivariate<'a, 'tcx> {
41 Bivariate { fields: fields }
85aaf69f 42 }
c34b1796 43}
85aaf69f 44
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45impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Bivariate<'a, 'tcx> {
46 fn tag(&self) -> &'static str { "Bivariate" }
85aaf69f 47
c34b1796 48 fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
85aaf69f 49
c34b1796 50 fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
85aaf69f 51
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52 fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
53 variance: ty::Variance,
54 a: &T,
55 b: &T)
56 -> RelateResult<'tcx, T>
85aaf69f 57 {
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58 match variance {
59 // If we have Foo<A> and Foo is invariant w/r/t A,
60 // and we want to assert that
61 //
62 // Foo<A> <: Foo<B> ||
63 // Foo<B> <: Foo<A>
64 //
65 // then still A must equal B.
66 ty::Invariant => self.relate(a, b),
67
68 ty::Covariant => self.relate(a, b),
69 ty::Bivariant => self.relate(a, b),
70 ty::Contravariant => self.relate(a, b),
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71 }
72 }
73
c34b1796 74 fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
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75 debug!("{}.tys({:?}, {:?})", self.tag(),
76 a, b);
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77 if a == b { return Ok(a); }
78
79 let infcx = self.fields.infcx;
80 let a = infcx.type_variables.borrow().replace_if_possible(a);
81 let b = infcx.type_variables.borrow().replace_if_possible(b);
82 match (&a.sty, &b.sty) {
62682a34 83 (&ty::TyInfer(TyVar(a_id)), &ty::TyInfer(TyVar(b_id))) => {
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84 infcx.type_variables.borrow_mut().relate_vars(a_id, BiTo, b_id);
85 Ok(a)
86 }
87
62682a34 88 (&ty::TyInfer(TyVar(a_id)), _) => {
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89 try!(self.fields.instantiate(b, BiTo, a_id));
90 Ok(a)
91 }
92
62682a34 93 (_, &ty::TyInfer(TyVar(b_id))) => {
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94 try!(self.fields.instantiate(a, BiTo, b_id));
95 Ok(a)
96 }
97
98 _ => {
c34b1796 99 combine::super_combine_tys(self.fields.infcx, self, a, b)
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100 }
101 }
102 }
103
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104 fn regions(&mut self, a: ty::Region, _: ty::Region) -> RelateResult<'tcx, ty::Region> {
105 Ok(a)
106 }
107
108 fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
109 -> RelateResult<'tcx, ty::Binder<T>>
110 where T: Relate<'a,'tcx>
85aaf69f 111 {
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112 let a1 = self.tcx().erase_late_bound_regions(a);
113 let b1 = self.tcx().erase_late_bound_regions(b);
c34b1796 114 let c = try!(self.relate(&a1, &b1));
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115 Ok(ty::Binder(c))
116 }
117}