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 //! 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.
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.
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
28 use super::combine::CombineFields;
29 use super::type_variable::{BiTo};
31 use ty::{self, Ty, TyCtxt};
33 use ty::relate::{Relate, RelateResult, TypeRelation};
35 pub struct Bivariate<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
36 fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
40 impl<'combine, 'infcx, 'gcx, 'tcx> Bivariate<'combine, 'infcx, 'gcx, 'tcx> {
41 pub fn new(fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
42 -> Bivariate<'combine, 'infcx, 'gcx, 'tcx>
44 Bivariate { fields: fields, a_is_expected: a_is_expected }
48 impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
49 for Bivariate<'combine, 'infcx, 'gcx, 'tcx>
51 fn tag(&self) -> &'static str { "Bivariate" }
53 fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.tcx() }
55 fn a_is_expected(&self) -> bool { self.a_is_expected }
57 fn relate_with_variance<T: Relate<'tcx>>(&mut self,
58 variance: ty::Variance,
61 -> RelateResult<'tcx, T>
64 // If we have Foo<A> and Foo is invariant w/r/t A,
65 // and we want to assert that
67 // Foo<A> <: Foo<B> ||
70 // then still A must equal B.
71 ty::Invariant => self.relate(a, b),
73 ty::Covariant => self.relate(a, b),
74 ty::Bivariant => self.relate(a, b),
75 ty::Contravariant => self.relate(a, b),
79 fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
80 debug!("{}.tys({:?}, {:?})", self.tag(),
82 if a == b { return Ok(a); }
84 let infcx = self.fields.infcx;
85 let a = infcx.type_variables.borrow_mut().replace_if_possible(a);
86 let b = infcx.type_variables.borrow_mut().replace_if_possible(b);
87 match (&a.sty, &b.sty) {
88 (&ty::TyInfer(TyVar(a_id)), &ty::TyInfer(TyVar(b_id))) => {
89 infcx.type_variables.borrow_mut().relate_vars(a_id, BiTo, b_id);
93 (&ty::TyInfer(TyVar(a_id)), _) => {
94 self.fields.instantiate(b, BiTo, a_id, self.a_is_expected)?;
98 (_, &ty::TyInfer(TyVar(b_id))) => {
99 self.fields.instantiate(a, BiTo, b_id, self.a_is_expected)?;
104 self.fields.infcx.super_combine_tys(self, a, b)
109 fn regions(&mut self, a: &'tcx ty::Region, _: &'tcx ty::Region)
110 -> RelateResult<'tcx, &'tcx ty::Region> {
114 fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
115 -> RelateResult<'tcx, ty::Binder<T>>
116 where T: Relate<'tcx>
118 let a1 = self.tcx().erase_late_bound_regions(a);
119 let b1 = self.tcx().erase_late_bound_regions(b);
120 let c = self.relate(&a1, &b1)?;