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 relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
  53                                                variance: ty::Variance,
  54                                                a: &T,
  55                                                b: &T)
  56                                                -> RelateResult<'tcx, T>
  57     {
  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),
  71         }
  72     }
  73
  74     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  75         debug!("{}.tys({:?}, {:?})", self.tag(),
  76                a, b);
  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) {
  83             (&ty::TyInfer(TyVar(a_id)), &ty::TyInfer(TyVar(b_id))) => {
  84                 infcx.type_variables.borrow_mut().relate_vars(a_id, BiTo, b_id);
  85                 Ok(a)
  86             }
  87
  88             (&ty::TyInfer(TyVar(a_id)), _) => {
  89                 try!(self.fields.instantiate(b, BiTo, a_id));
  90                 Ok(a)
  91             }
  92
  93             (_, &ty::TyInfer(TyVar(b_id))) => {
  94                 try!(self.fields.instantiate(a, BiTo, b_id));
  95                 Ok(a)
  96             }
  97
  98             _ => {
  99                 combine::super_combine_tys(self.fields.infcx, self, a, b)
 100             }
 101         }
 102     }
 103
 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>
 111     {
 112         let a1 = self.tcx().erase_late_bound_regions(a);
 113         let b1 = self.tcx().erase_late_bound_regions(b);
 114         let c = try!(self.relate(&a1, &b1));
 115         Ok(ty::Binder(c))
 116     }
 117 }