src/librustc/infer/sub.rs

   1 use super::SubregionOrigin;
   2 use super::combine::{CombineFields, RelationDir};
   3
   4 use crate::traits::Obligation;
   5 use crate::ty::{self, Ty, TyCtxt};
   6 use crate::ty::TyVar;
   7 use crate::ty::fold::TypeFoldable;
   8 use crate::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
   9 use std::mem;
  10
  11 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
  12 pub struct Sub<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  13     fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
  14     a_is_expected: bool,
  15 }
  16
  17 impl<'combine, 'infcx, 'gcx, 'tcx> Sub<'combine, 'infcx, 'gcx, 'tcx> {
  18     pub fn new(f: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
  19         -> Sub<'combine, 'infcx, 'gcx, 'tcx>
  20     {
  21         Sub { fields: f, a_is_expected: a_is_expected }
  22     }
  23
  24     fn with_expected_switched<R, F: FnOnce(&mut Self) -> R>(&mut self, f: F) -> R {
  25         self.a_is_expected = !self.a_is_expected;
  26         let result = f(self);
  27         self.a_is_expected = !self.a_is_expected;
  28         result
  29     }
  30 }
  31
  32 impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
  33     for Sub<'combine, 'infcx, 'gcx, 'tcx>
  34 {
  35     fn tag(&self) -> &'static str { "Sub" }
  36     fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.infcx.tcx }
  37     fn a_is_expected(&self) -> bool { self.a_is_expected }
  38
  39     fn with_cause<F,R>(&mut self, cause: Cause, f: F) -> R
  40         where F: FnOnce(&mut Self) -> R
  41     {
  42         debug!("sub with_cause={:?}", cause);
  43         let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
  44         let r = f(self);
  45         debug!("sub old_cause={:?}", old_cause);
  46         self.fields.cause = old_cause;
  47         r
  48     }
  49
  50     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
  51                                              variance: ty::Variance,
  52                                              a: &T,
  53                                              b: &T)
  54                                              -> RelateResult<'tcx, T>
  55     {
  56         match variance {
  57             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  58             ty::Covariant => self.relate(a, b),
  59             ty::Bivariant => Ok(a.clone()),
  60             ty::Contravariant => self.with_expected_switched(|this| { this.relate(b, a) }),
  61         }
  62     }
  63
  64     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  65         debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
  66
  67         if a == b { return Ok(a); }
  68
  69         let infcx = self.fields.infcx;
  70         let a = infcx.type_variables.borrow_mut().replace_if_possible(a);
  71         let b = infcx.type_variables.borrow_mut().replace_if_possible(b);
  72         match (&a.sty, &b.sty) {
  73             (&ty::Infer(TyVar(a_vid)), &ty::Infer(TyVar(b_vid))) => {
  74                 // Shouldn't have any LBR here, so we can safely put
  75                 // this under a binder below without fear of accidental
  76                 // capture.
  77                 assert!(!a.has_escaping_bound_vars());
  78                 assert!(!b.has_escaping_bound_vars());
  79
  80                 // can't make progress on `A <: B` if both A and B are
  81                 // type variables, so record an obligation. We also
  82                 // have to record in the `type_variables` tracker that
  83                 // the two variables are equal modulo subtyping, which
  84                 // is important to the occurs check later on.
  85                 infcx.type_variables.borrow_mut().sub(a_vid, b_vid);
  86                 self.fields.obligations.push(
  87                     Obligation::new(
  88                         self.fields.trace.cause.clone(),
  89                         self.fields.param_env,
  90                         ty::Predicate::Subtype(
  91                             ty::Binder::dummy(ty::SubtypePredicate {
  92                                 a_is_expected: self.a_is_expected,
  93                                 a,
  94                                 b,
  95                             }))));
  96
  97                 Ok(a)
  98             }
  99             (&ty::Infer(TyVar(a_id)), _) => {
 100                 self.fields
 101                     .instantiate(b, RelationDir::SupertypeOf, a_id, !self.a_is_expected)?;
 102                 Ok(a)
 103             }
 104             (_, &ty::Infer(TyVar(b_id))) => {
 105                 self.fields.instantiate(a, RelationDir::SubtypeOf, b_id, self.a_is_expected)?;
 106                 Ok(a)
 107             }
 108
 109             (&ty::Error, _) | (_, &ty::Error) => {
 110                 infcx.set_tainted_by_errors();
 111                 Ok(self.tcx().types.err)
 112             }
 113
 114             _ => {
 115                 self.fields.infcx.super_combine_tys(self, a, b)?;
 116                 Ok(a)
 117             }
 118         }
 119     }
 120
 121     fn regions(&mut self, a: ty::Region<'tcx>, b: ty::Region<'tcx>)
 122                -> RelateResult<'tcx, ty::Region<'tcx>> {
 123         debug!("{}.regions({:?}, {:?}) self.cause={:?}",
 124                self.tag(), a, b, self.fields.cause);
 125
 126         // FIXME -- we have more fine-grained information available
 127         // from the "cause" field, we could perhaps give more tailored
 128         // error messages.
 129         let origin = SubregionOrigin::Subtype(self.fields.trace.clone());
 130         self.fields.infcx.borrow_region_constraints()
 131                          .make_subregion(origin, a, b);
 132
 133         Ok(a)
 134     }
 135
 136     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 137                   -> RelateResult<'tcx, ty::Binder<T>>
 138         where T: Relate<'tcx>
 139     {
 140         self.fields.higher_ranked_sub(a, b, self.a_is_expected)
 141     }
 142 }