src/librustc/middle/infer/bivariate.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 //! 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
  28 use super::combine::{self, CombineFields};
  29 use super::type_variable::{BiTo};
  30
  31 use middle::ty::{self, Ty};
  32 use middle::ty::TyVar;
  33 use middle::ty_relate::{Relate, RelateResult, TypeRelation};
  34
  35 pub struct Bivariate<'a, 'tcx: 'a> {
  36     fields: CombineFields<'a, 'tcx>
  37 }
  38
  39 impl<'a, 'tcx> Bivariate<'a, 'tcx> {
  40     pub fn new(fields: CombineFields<'a, 'tcx>) -> Bivariate<'a, 'tcx> {
  41         Bivariate { fields: fields }
  42     }
  43 }
  44
  45 impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Bivariate<'a, 'tcx> {
  46     fn tag(&self) -> &'static str { "Bivariate" }
  47
  48     fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
  49
  50     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
  51
  52     fn will_change(&mut self, _: bool, _: bool) -> bool {
  53         // since we are not comparing regions, we don't care
  54         false
  55     }
  56
  57     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
  58                                                variance: ty::Variance,
  59                                                a: &T,
  60                                                b: &T)
  61                                                -> RelateResult<'tcx, T>
  62     {
  63         match variance {
  64             // If we have Foo<A> and Foo is invariant w/r/t A,
  65             // and we want to assert that
  66             //
  67             //     Foo<A> <: Foo<B> ||
  68             //     Foo<B> <: Foo<A>
  69             //
  70             // then still A must equal B.
  71             ty::Invariant => self.relate(a, b),
  72
  73             ty::Covariant => self.relate(a, b),
  74             ty::Bivariant => self.relate(a, b),
  75             ty::Contravariant => self.relate(a, b),
  76         }
  77     }
  78
  79     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  80         debug!("{}.tys({:?}, {:?})", self.tag(),
  81                a, b);
  82         if a == b { return Ok(a); }
  83
  84         let infcx = self.fields.infcx;
  85         let a = infcx.type_variables.borrow().replace_if_possible(a);
  86         let b = infcx.type_variables.borrow().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);
  90                 Ok(a)
  91             }
  92
  93             (&ty::TyInfer(TyVar(a_id)), _) => {
  94                 try!(self.fields.instantiate(b, BiTo, a_id));
  95                 Ok(a)
  96             }
  97
  98             (_, &ty::TyInfer(TyVar(b_id))) => {
  99                 try!(self.fields.instantiate(a, BiTo, b_id));
 100                 Ok(a)
 101             }
 102
 103             _ => {
 104                 combine::super_combine_tys(self.fields.infcx, self, a, b)
 105             }
 106         }
 107     }
 108
 109     fn regions(&mut self, a: ty::Region, _: ty::Region) -> RelateResult<'tcx, ty::Region> {
 110         Ok(a)
 111     }
 112
 113     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 114                   -> RelateResult<'tcx, ty::Binder<T>>
 115         where T: Relate<'a,'tcx>
 116     {
 117         let a1 = ty::erase_late_bound_regions(self.tcx(), a);
 118         let b1 = ty::erase_late_bound_regions(self.tcx(), b);
 119         let c = try!(self.relate(&a1, &b1));
 120         Ok(ty::Binder(c))
 121     }
 122 }