compiler/rustc_infer/src/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::relate::{Cause, Relate, RelateResult, TypeRelation};
   7 use rustc_middle::ty::visit::TypeVisitable;
   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, 'a, 'tcx> {
  14     fields: &'combine mut CombineFields<'a, '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<'tcx> TypeRelation<'tcx> for Sub<'_, '_, '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         _info: ty::VarianceDiagInfo<'tcx>,
  66         a: T,
  67         b: T,
  68     ) -> RelateResult<'tcx, T> {
  69         match variance {
  70             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  71             ty::Covariant => self.relate(a, b),
  72             ty::Bivariant => Ok(a),
  73             ty::Contravariant => self.with_expected_switched(|this| this.relate(b, a)),
  74         }
  75     }
  76
  77     #[instrument(skip(self), level = "debug")]
  78     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  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
  87         match (a.kind(), b.kind()) {
  88             (&ty::Infer(TyVar(_)), &ty::Infer(TyVar(_))) => {
  89                 // Shouldn't have any LBR here, so we can safely put
  90                 // this under a binder below without fear of accidental
  91                 // capture.
  92                 assert!(!a.has_escaping_bound_vars());
  93                 assert!(!b.has_escaping_bound_vars());
  94
  95                 // can't make progress on `A <: B` if both A and B are
  96                 // type variables, so record an obligation.
  97                 self.fields.obligations.push(Obligation::new(
  98                     self.fields.trace.cause.clone(),
  99                     self.fields.param_env,
 100                     ty::Binder::dummy(ty::PredicateKind::Subtype(ty::SubtypePredicate {
 101                         a_is_expected: self.a_is_expected,
 102                         a,
 103                         b,
 104                     }))
 105                     .to_predicate(self.tcx()),
 106                 ));
 107
 108                 Ok(a)
 109             }
 110             (&ty::Infer(TyVar(a_id)), _) => {
 111                 self.fields.instantiate(b, RelationDir::SupertypeOf, a_id, !self.a_is_expected)?;
 112                 Ok(a)
 113             }
 114             (_, &ty::Infer(TyVar(b_id))) => {
 115                 self.fields.instantiate(a, RelationDir::SubtypeOf, b_id, self.a_is_expected)?;
 116                 Ok(a)
 117             }
 118
 119             (&ty::Error(_), _) | (_, &ty::Error(_)) => {
 120                 infcx.set_tainted_by_errors();
 121                 Ok(self.tcx().ty_error())
 122             }
 123
 124             (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
 125                 self.fields.infcx.super_combine_tys(self, a, b)?;
 126                 Ok(a)
 127             }
 128             (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..))
 129                 if self.fields.define_opaque_types && did.is_local() =>
 130             {
 131                 self.fields.obligations.extend(
 132                     infcx
 133                         .handle_opaque_type(
 134                             a,
 135                             b,
 136                             self.a_is_expected,
 137                             &self.fields.trace.cause,
 138                             self.param_env(),
 139                         )?
 140                         .obligations,
 141                 );
 142                 Ok(a)
 143             }
 144             // Optimization of GeneratorWitness relation since we know that all
 145             // free regions are replaced with bound regions during construction.
 146             // This greatly speeds up subtyping of GeneratorWitness.
 147             (&ty::GeneratorWitness(a_types), &ty::GeneratorWitness(b_types)) => {
 148                 let a_types = infcx.tcx.anonymize_bound_vars(a_types);
 149                 let b_types = infcx.tcx.anonymize_bound_vars(b_types);
 150                 if a_types.bound_vars() == b_types.bound_vars() {
 151                     let (a_types, b_types) = infcx.replace_bound_vars_with_placeholders(
 152                         a_types.map_bound(|a_types| (a_types, b_types.skip_binder())),
 153                     );
 154                     for (a, b) in std::iter::zip(a_types, b_types) {
 155                         self.relate(a, b)?;
 156                     }
 157                     Ok(a)
 158                 } else {
 159                     Err(ty::error::TypeError::Sorts(ty::relate::expected_found(self, a, b)))
 160                 }
 161             }
 162
 163             _ => {
 164                 self.fields.infcx.super_combine_tys(self, a, b)?;
 165                 Ok(a)
 166             }
 167         }
 168     }
 169
 170     fn regions(
 171         &mut self,
 172         a: ty::Region<'tcx>,
 173         b: ty::Region<'tcx>,
 174     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 175         debug!("{}.regions({:?}, {:?}) self.cause={:?}", self.tag(), a, b, self.fields.cause);
 176
 177         // FIXME -- we have more fine-grained information available
 178         // from the "cause" field, we could perhaps give more tailored
 179         // error messages.
 180         let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
 181         self.fields
 182             .infcx
 183             .inner
 184             .borrow_mut()
 185             .unwrap_region_constraints()
 186             .make_subregion(origin, a, b);
 187
 188         Ok(a)
 189     }
 190
 191     fn consts(
 192         &mut self,
 193         a: ty::Const<'tcx>,
 194         b: ty::Const<'tcx>,
 195     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 196         self.fields.infcx.super_combine_consts(self, a, b)
 197     }
 198
 199     fn binders<T>(
 200         &mut self,
 201         a: ty::Binder<'tcx, T>,
 202         b: ty::Binder<'tcx, T>,
 203     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 204     where
 205         T: Relate<'tcx>,
 206     {
 207         self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
 208         Ok(a)
 209     }
 210 }
 211
 212 impl<'tcx> ConstEquateRelation<'tcx> for Sub<'_, '_, 'tcx> {
 213     fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
 214         self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
 215     }
 216 }