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