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::CombineFields;
  12 use super::SubregionOrigin;
  13 use super::type_variable::{SubtypeOf, SupertypeOf};
  14
  15 use ty::{self, Ty, TyCtxt};
  16 use ty::TyVar;
  17 use ty::relate::{Cause, Relate, RelateResult, TypeRelation};
  18 use std::mem;
  19
  20 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
  21 pub struct Sub<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  22     fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
  23     a_is_expected: bool,
  24 }
  25
  26 impl<'combine, 'infcx, 'gcx, 'tcx> Sub<'combine, 'infcx, 'gcx, 'tcx> {
  27     pub fn new(f: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
  28         -> Sub<'combine, 'infcx, 'gcx, 'tcx>
  29     {
  30         Sub { fields: f, a_is_expected: a_is_expected }
  31     }
  32
  33     fn with_expected_switched<R, F: FnOnce(&mut Self) -> R>(&mut self, f: F) -> R {
  34         self.a_is_expected = !self.a_is_expected;
  35         let result = f(self);
  36         self.a_is_expected = !self.a_is_expected;
  37         result
  38     }
  39 }
  40
  41 impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
  42     for Sub<'combine, 'infcx, 'gcx, 'tcx>
  43 {
  44     fn tag(&self) -> &'static str { "Sub" }
  45     fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.infcx.tcx }
  46     fn a_is_expected(&self) -> bool { self.a_is_expected }
  47
  48     fn with_cause<F,R>(&mut self, cause: Cause, f: F) -> R
  49         where F: FnOnce(&mut Self) -> R
  50     {
  51         debug!("sub with_cause={:?}", cause);
  52         let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
  53         let r = f(self);
  54         debug!("sub old_cause={:?}", old_cause);
  55         self.fields.cause = old_cause;
  56         r
  57     }
  58
  59     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
  60                                              variance: ty::Variance,
  61                                              a: &T,
  62                                              b: &T)
  63                                              -> RelateResult<'tcx, T>
  64     {
  65         match variance {
  66             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  67             ty::Covariant => self.relate(a, b),
  68             ty::Bivariant => self.fields.bivariate(self.a_is_expected).relate(a, b),
  69             ty::Contravariant => self.with_expected_switched(|this| { this.relate(b, a) }),
  70         }
  71     }
  72
  73     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  74         debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
  75
  76         if a == b { return Ok(a); }
  77
  78         let infcx = self.fields.infcx;
  79         let a = infcx.type_variables.borrow_mut().replace_if_possible(a);
  80         let b = infcx.type_variables.borrow_mut().replace_if_possible(b);
  81         match (&a.sty, &b.sty) {
  82             (&ty::TyInfer(TyVar(a_id)), &ty::TyInfer(TyVar(b_id))) => {
  83                 infcx.type_variables
  84                     .borrow_mut()
  85                     .relate_vars(a_id, SubtypeOf, b_id);
  86                 Ok(a)
  87             }
  88             (&ty::TyInfer(TyVar(a_id)), _) => {
  89                 self.fields
  90                     .instantiate(b, SupertypeOf, a_id, !self.a_is_expected)?;
  91                 Ok(a)
  92             }
  93             (_, &ty::TyInfer(TyVar(b_id))) => {
  94                 self.fields.instantiate(a, SubtypeOf, b_id, self.a_is_expected)?;
  95                 Ok(a)
  96             }
  97
  98             (&ty::TyError, _) | (_, &ty::TyError) => {
  99                 infcx.set_tainted_by_errors();
 100                 Ok(self.tcx().types.err)
 101             }
 102
 103             _ => {
 104                 self.fields.infcx.super_combine_tys(self, a, b)?;
 105                 Ok(a)
 106             }
 107         }
 108     }
 109
 110     fn regions(&mut self, a: &'tcx ty::Region, b: &'tcx ty::Region)
 111                -> RelateResult<'tcx, &'tcx ty::Region> {
 112         debug!("{}.regions({:?}, {:?}) self.cause={:?}",
 113                self.tag(), a, b, self.fields.cause);
 114         // FIXME -- we have more fine-grained information available
 115         // from the "cause" field, we could perhaps give more tailored
 116         // error messages.
 117         let origin = SubregionOrigin::Subtype(self.fields.trace.clone());
 118         self.fields.infcx.region_vars.make_subregion(origin, a, b);
 119         Ok(a)
 120     }
 121
 122     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 123                   -> RelateResult<'tcx, ty::Binder<T>>
 124         where T: Relate<'tcx>
 125     {
 126         self.fields.higher_ranked_sub(a, b, self.a_is_expected)
 127     }
 128 }