src/librustc/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::SubregionOrigin;
  14 use super::type_variable::{SubtypeOf, SupertypeOf};
  15
  16 use ty::{self, Ty, TyCtxt};
  17 use ty::TyVar;
  18 use ty::relate::{Cause, Relate, RelateResult, TypeRelation};
  19 use traits::PredicateObligations;
  20 use std::mem;
  21
  22 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
  23 pub struct Sub<'a, 'tcx: 'a> {
  24     fields: CombineFields<'a, 'tcx>,
  25 }
  26
  27 impl<'a, 'tcx> Sub<'a, 'tcx> {
  28     pub fn new(f: CombineFields<'a, 'tcx>) -> Sub<'a, 'tcx> {
  29         Sub { fields: f }
  30     }
  31
  32     pub fn obligations(self) -> PredicateObligations<'tcx> {
  33         self.fields.obligations
  34     }
  35 }
  36
  37 impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Sub<'a, 'tcx> {
  38     fn tag(&self) -> &'static str { "Sub" }
  39     fn tcx(&self) -> &'a TyCtxt<'tcx> { self.fields.infcx.tcx }
  40     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
  41
  42     fn with_cause<F,R>(&mut self, cause: Cause, f: F) -> R
  43         where F: FnOnce(&mut Self) -> R
  44     {
  45         debug!("sub with_cause={:?}", cause);
  46         let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
  47         let r = f(self);
  48         debug!("sub old_cause={:?}", old_cause);
  49         self.fields.cause = old_cause;
  50         r
  51     }
  52
  53     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
  54                                                variance: ty::Variance,
  55                                                a: &T,
  56                                                b: &T)
  57                                                -> RelateResult<'tcx, T>
  58     {
  59         match variance {
  60             ty::Invariant => self.fields.equate().relate(a, b),
  61             ty::Covariant => self.relate(a, b),
  62             ty::Bivariant => self.fields.bivariate().relate(a, b),
  63             ty::Contravariant => self.fields.switch_expected().sub().relate(b, a),
  64         }
  65     }
  66
  67     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  68         debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
  69
  70         if a == b { return Ok(a); }
  71
  72         let infcx = self.fields.infcx;
  73         let a = infcx.type_variables.borrow_mut().replace_if_possible(a);
  74         let b = infcx.type_variables.borrow_mut().replace_if_possible(b);
  75         match (&a.sty, &b.sty) {
  76             (&ty::TyInfer(TyVar(a_id)), &ty::TyInfer(TyVar(b_id))) => {
  77                 infcx.type_variables
  78                     .borrow_mut()
  79                     .relate_vars(a_id, SubtypeOf, b_id);
  80                 Ok(a)
  81             }
  82             (&ty::TyInfer(TyVar(a_id)), _) => {
  83                 self.fields
  84                     .switch_expected()
  85                     .instantiate(b, SupertypeOf, a_id)?;
  86                 Ok(a)
  87             }
  88             (_, &ty::TyInfer(TyVar(b_id))) => {
  89                 self.fields.instantiate(a, SubtypeOf, b_id)?;
  90                 Ok(a)
  91             }
  92
  93             (&ty::TyError, _) | (_, &ty::TyError) => {
  94                 Ok(self.tcx().types.err)
  95             }
  96
  97             _ => {
  98                 combine::super_combine_tys(self.fields.infcx, self, a, b)?;
  99                 Ok(a)
 100             }
 101         }
 102     }
 103
 104     fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
 105         debug!("{}.regions({:?}, {:?}) self.cause={:?}",
 106                self.tag(), a, b, self.fields.cause);
 107         // FIXME -- we have more fine-grained information available
 108         // from the "cause" field, we could perhaps give more tailored
 109         // error messages.
 110         let origin = SubregionOrigin::Subtype(self.fields.trace.clone());
 111         self.fields.infcx.region_vars.make_subregion(origin, a, b);
 112         Ok(a)
 113     }
 114
 115     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 116                   -> RelateResult<'tcx, ty::Binder<T>>
 117         where T: Relate<'a,'tcx>
 118     {
 119         self.fields.higher_ranked_sub(a, b)
 120     }
 121 }