src/librustc/middle/infer/sub.rs

   1 // Copyright 2012 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 use super::combine::{self, CombineFields};
  12 use super::higher_ranked::HigherRankedRelations;
  13 use super::Subtype;
  14 use super::type_variable::{SubtypeOf, SupertypeOf};
  15
  16 use middle::ty::{self, Ty};
  17 use middle::ty::TyVar;
  18 use middle::ty_relate::{Relate, RelateResult, TypeRelation};
  19 use util::ppaux::{Repr};
  20
  21 /// "Greatest lower bound" (common subtype)
  22 pub struct Sub<'a, 'tcx: 'a> {
  23     fields: CombineFields<'a, 'tcx>
  24 }
  25
  26 impl<'a, 'tcx> Sub<'a, 'tcx> {
  27     pub fn new(f: CombineFields<'a, 'tcx>) -> Sub<'a, 'tcx> {
  28         Sub { fields: f }
  29     }
  30 }
  31
  32 impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Sub<'a, 'tcx> {
  33     fn tag(&self) -> &'static str { "Sub" }
  34     fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.infcx.tcx }
  35     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
  36
  37     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
  38                                                variance: ty::Variance,
  39                                                a: &T,
  40                                                b: &T)
  41                                                -> RelateResult<'tcx, T>
  42     {
  43         match variance {
  44             ty::Invariant => self.fields.equate().relate(a, b),
  45             ty::Covariant => self.relate(a, b),
  46             ty::Bivariant => self.fields.bivariate().relate(a, b),
  47             ty::Contravariant => self.fields.switch_expected().sub().relate(b, a),
  48         }
  49     }
  50
  51     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  52         debug!("{}.tys({}, {})", self.tag(), a.repr(self.tcx()), b.repr(self.tcx()));
  53
  54         if a == b { return Ok(a); }
  55
  56         let infcx = self.fields.infcx;
  57         let a = infcx.type_variables.borrow().replace_if_possible(a);
  58         let b = infcx.type_variables.borrow().replace_if_possible(b);
  59         match (&a.sty, &b.sty) {
  60             (&ty::ty_infer(TyVar(a_id)), &ty::ty_infer(TyVar(b_id))) => {
  61                 infcx.type_variables
  62                     .borrow_mut()
  63                     .relate_vars(a_id, SubtypeOf, b_id);
  64                 Ok(a)
  65             }
  66             (&ty::ty_infer(TyVar(a_id)), _) => {
  67                 try!(self.fields
  68                          .switch_expected()
  69                          .instantiate(b, SupertypeOf, a_id));
  70                 Ok(a)
  71             }
  72             (_, &ty::ty_infer(TyVar(b_id))) => {
  73                 try!(self.fields.instantiate(a, SubtypeOf, b_id));
  74                 Ok(a)
  75             }
  76
  77             (&ty::ty_err, _) | (_, &ty::ty_err) => {
  78                 Ok(self.tcx().types.err)
  79             }
  80
  81             _ => {
  82                 combine::super_combine_tys(self.fields.infcx, self, a, b)
  83             }
  84         }
  85     }
  86
  87     fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
  88         debug!("{}.regions({}, {})",
  89                self.tag(),
  90                a.repr(self.tcx()),
  91                b.repr(self.tcx()));
  92         let origin = Subtype(self.fields.trace.clone());
  93         self.fields.infcx.region_vars.make_subregion(origin, a, b);
  94         Ok(a)
  95     }
  96
  97     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
  98                   -> RelateResult<'tcx, ty::Binder<T>>
  99         where T: Relate<'a,'tcx>
 100     {
 101         self.fields.higher_ranked_sub(a, b)
 102     }
 103 }