src/librustc_infer/infer/sub.rs

   1 use super::combine::{CombineFields, RelationDir};
   2 use super::SubregionOrigin;
   3
   4 use crate::infer::combine::ConstEquateRelation;
   5 use crate::traits::Obligation;
   6 use rustc_middle::ty::fold::TypeFoldable;
   7 use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
   8 use rustc_middle::ty::TyVar;
   9 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
  10 use std::mem;
  11
  12 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
  13 pub struct Sub<'combine, 'infcx, 'tcx> {
  14     fields: &'combine mut CombineFields<'infcx, 'tcx>,
  15     a_is_expected: bool,
  16 }
  17
  18 impl<'combine, 'infcx, 'tcx> Sub<'combine, 'infcx, 'tcx> {
  19     pub fn new(
  20         f: &'combine mut CombineFields<'infcx, 'tcx>,
  21         a_is_expected: bool,
  22     ) -> Sub<'combine, 'infcx, 'tcx> {
  23         Sub { fields: f, a_is_expected }
  24     }
  25
  26     fn with_expected_switched<R, F: FnOnce(&mut Self) -> R>(&mut self, f: F) -> R {
  27         self.a_is_expected = !self.a_is_expected;
  28         let result = f(self);
  29         self.a_is_expected = !self.a_is_expected;
  30         result
  31     }
  32 }
  33
  34 impl TypeRelation<'tcx> for Sub<'combine, 'infcx, 'tcx> {
  35     fn tag(&self) -> &'static str {
  36         "Sub"
  37     }
  38     fn tcx(&self) -> TyCtxt<'tcx> {
  39         self.fields.infcx.tcx
  40     }
  41
  42     fn param_env(&self) -> ty::ParamEnv<'tcx> {
  43         self.fields.param_env
  44     }
  45
  46     fn a_is_expected(&self) -> bool {
  47         self.a_is_expected
  48     }
  49
  50     fn with_cause<F, R>(&mut self, cause: Cause, f: F) -> R
  51     where
  52         F: FnOnce(&mut Self) -> R,
  53     {
  54         debug!("sub with_cause={:?}", cause);
  55         let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
  56         let r = f(self);
  57         debug!("sub old_cause={:?}", old_cause);
  58         self.fields.cause = old_cause;
  59         r
  60     }
  61
  62     fn relate_with_variance<T: Relate<'tcx>>(
  63         &mut self,
  64         variance: ty::Variance,
  65         a: T,
  66         b: T,
  67     ) -> RelateResult<'tcx, T> {
  68         match variance {
  69             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  70             ty::Covariant => self.relate(a, b),
  71             ty::Bivariant => Ok(a),
  72             ty::Contravariant => self.with_expected_switched(|this| this.relate(b, a)),
  73         }
  74     }
  75
  76     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  77         debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
  78
  79         if a == b {
  80             return Ok(a);
  81         }
  82
  83         let infcx = self.fields.infcx;
  84         let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
  85         let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
  86         match (&a.kind, &b.kind) {
  87             (&ty::Infer(TyVar(a_vid)), &ty::Infer(TyVar(b_vid))) => {
  88                 // Shouldn't have any LBR here, so we can safely put
  89                 // this under a binder below without fear of accidental
  90                 // capture.
  91                 assert!(!a.has_escaping_bound_vars());
  92                 assert!(!b.has_escaping_bound_vars());
  93
  94                 // can't make progress on `A <: B` if both A and B are
  95                 // type variables, so record an obligation. We also
  96                 // have to record in the `type_variables` tracker that
  97                 // the two variables are equal modulo subtyping, which
  98                 // is important to the occurs check later on.
  99                 infcx.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
 100                 self.fields.obligations.push(Obligation::new(
 101                     self.fields.trace.cause.clone(),
 102                     self.fields.param_env,
 103                     ty::PredicateAtom::Subtype(ty::SubtypePredicate {
 104                         a_is_expected: self.a_is_expected,
 105                         a,
 106                         b,
 107                     })
 108                     .to_predicate(self.tcx()),
 109                 ));
 110
 111                 Ok(a)
 112             }
 113             (&ty::Infer(TyVar(a_id)), _) => {
 114                 self.fields.instantiate(b, RelationDir::SupertypeOf, a_id, !self.a_is_expected)?;
 115                 Ok(a)
 116             }
 117             (_, &ty::Infer(TyVar(b_id))) => {
 118                 self.fields.instantiate(a, RelationDir::SubtypeOf, b_id, self.a_is_expected)?;
 119                 Ok(a)
 120             }
 121
 122             (&ty::Error(_), _) | (_, &ty::Error(_)) => {
 123                 infcx.set_tainted_by_errors();
 124                 Ok(self.tcx().ty_error())
 125             }
 126
 127             _ => {
 128                 self.fields.infcx.super_combine_tys(self, a, b)?;
 129                 Ok(a)
 130             }
 131         }
 132     }
 133
 134     fn regions(
 135         &mut self,
 136         a: ty::Region<'tcx>,
 137         b: ty::Region<'tcx>,
 138     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 139         debug!("{}.regions({:?}, {:?}) self.cause={:?}", self.tag(), a, b, self.fields.cause);
 140
 141         // FIXME -- we have more fine-grained information available
 142         // from the "cause" field, we could perhaps give more tailored
 143         // error messages.
 144         let origin = SubregionOrigin::Subtype(box self.fields.trace.clone());
 145         self.fields
 146             .infcx
 147             .inner
 148             .borrow_mut()
 149             .unwrap_region_constraints()
 150             .make_subregion(origin, a, b);
 151
 152         Ok(a)
 153     }
 154
 155     fn consts(
 156         &mut self,
 157         a: &'tcx ty::Const<'tcx>,
 158         b: &'tcx ty::Const<'tcx>,
 159     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
 160         self.fields.infcx.super_combine_consts(self, a, b)
 161     }
 162
 163     fn binders<T>(
 164         &mut self,
 165         a: ty::Binder<T>,
 166         b: ty::Binder<T>,
 167     ) -> RelateResult<'tcx, ty::Binder<T>>
 168     where
 169         T: Relate<'tcx>,
 170     {
 171         self.fields.higher_ranked_sub(a, b, self.a_is_expected)
 172     }
 173 }
 174
 175 impl<'tcx> ConstEquateRelation<'tcx> for Sub<'_, '_, 'tcx> {
 176     fn const_equate_obligation(&mut self, a: &'tcx ty::Const<'tcx>, b: &'tcx ty::Const<'tcx>) {
 177         self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
 178     }
 179 }