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, 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
  39     fn intercrate(&self) -> bool {
  40         self.fields.infcx.intercrate
  41     }
  42
  43     fn tcx(&self) -> TyCtxt<'tcx> {
  44         self.fields.infcx.tcx
  45     }
  46
  47     fn param_env(&self) -> ty::ParamEnv<'tcx> {
  48         self.fields.param_env
  49     }
  50
  51     fn a_is_expected(&self) -> bool {
  52         self.a_is_expected
  53     }
  54
  55     fn mark_ambiguous(&mut self) {
  56         self.fields.mark_ambiguous()
  57     }
  58
  59     fn with_cause<F, R>(&mut self, cause: Cause, f: F) -> R
  60     where
  61         F: FnOnce(&mut Self) -> R,
  62     {
  63         debug!("sub with_cause={:?}", cause);
  64         let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
  65         let r = f(self);
  66         debug!("sub old_cause={:?}", old_cause);
  67         self.fields.cause = old_cause;
  68         r
  69     }
  70
  71     fn relate_with_variance<T: Relate<'tcx>>(
  72         &mut self,
  73         variance: ty::Variance,
  74         _info: ty::VarianceDiagInfo<'tcx>,
  75         a: T,
  76         b: T,
  77     ) -> RelateResult<'tcx, T> {
  78         match variance {
  79             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  80             ty::Covariant => self.relate(a, b),
  81             ty::Bivariant => Ok(a),
  82             ty::Contravariant => self.with_expected_switched(|this| this.relate(b, a)),
  83         }
  84     }
  85
  86     #[instrument(skip(self), level = "debug")]
  87     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  88         if a == b {
  89             return Ok(a);
  90         }
  91
  92         let infcx = self.fields.infcx;
  93         let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
  94         let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
  95
  96         match (a.kind(), b.kind()) {
  97             (&ty::Infer(TyVar(_)), &ty::Infer(TyVar(_))) => {
  98                 // Shouldn't have any LBR here, so we can safely put
  99                 // this under a binder below without fear of accidental
 100                 // capture.
 101                 assert!(!a.has_escaping_bound_vars());
 102                 assert!(!b.has_escaping_bound_vars());
 103
 104                 // can't make progress on `A <: B` if both A and B are
 105                 // type variables, so record an obligation.
 106                 self.fields.obligations.push(Obligation::new(
 107                     self.tcx(),
 108                     self.fields.trace.cause.clone(),
 109                     self.fields.param_env,
 110                     ty::Binder::dummy(ty::PredicateKind::Subtype(ty::SubtypePredicate {
 111                         a_is_expected: self.a_is_expected,
 112                         a,
 113                         b,
 114                     })),
 115                 ));
 116
 117                 Ok(a)
 118             }
 119             (&ty::Infer(TyVar(a_id)), _) => {
 120                 self.fields.instantiate(b, RelationDir::SupertypeOf, a_id, !self.a_is_expected)?;
 121                 Ok(a)
 122             }
 123             (_, &ty::Infer(TyVar(b_id))) => {
 124                 self.fields.instantiate(a, RelationDir::SubtypeOf, b_id, self.a_is_expected)?;
 125                 Ok(a)
 126             }
 127
 128             (&ty::Error(e), _) | (_, &ty::Error(e)) => {
 129                 infcx.set_tainted_by_errors(e);
 130                 Ok(self.tcx().ty_error_with_guaranteed(e))
 131             }
 132
 133             (
 134                 &ty::Alias(ty::Opaque, ty::AliasTy { def_id: a_def_id, .. }),
 135                 &ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }),
 136             ) if a_def_id == b_def_id => {
 137                 self.fields.infcx.super_combine_tys(self, a, b)?;
 138                 Ok(a)
 139             }
 140             (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
 141             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
 142                 if self.fields.define_opaque_types && def_id.is_local() =>
 143             {
 144                 self.fields.obligations.extend(
 145                     infcx
 146                         .handle_opaque_type(
 147                             a,
 148                             b,
 149                             self.a_is_expected,
 150                             &self.fields.trace.cause,
 151                             self.param_env(),
 152                         )?
 153                         .obligations,
 154                 );
 155                 Ok(a)
 156             }
 157             // Optimization of GeneratorWitness relation since we know that all
 158             // free regions are replaced with bound regions during construction.
 159             // This greatly speeds up subtyping of GeneratorWitness.
 160             (&ty::GeneratorWitness(a_types), &ty::GeneratorWitness(b_types)) => {
 161                 let a_types = infcx.tcx.anonymize_bound_vars(a_types);
 162                 let b_types = infcx.tcx.anonymize_bound_vars(b_types);
 163                 if a_types.bound_vars() == b_types.bound_vars() {
 164                     let (a_types, b_types) = infcx.instantiate_binder_with_placeholders(
 165                         a_types.map_bound(|a_types| (a_types, b_types.skip_binder())),
 166                     );
 167                     for (a, b) in std::iter::zip(a_types, b_types) {
 168                         self.relate(a, b)?;
 169                     }
 170                     Ok(a)
 171                 } else {
 172                     Err(ty::error::TypeError::Sorts(ty::relate::expected_found(self, a, b)))
 173                 }
 174             }
 175
 176             _ => {
 177                 self.fields.infcx.super_combine_tys(self, a, b)?;
 178                 Ok(a)
 179             }
 180         }
 181     }
 182
 183     fn regions(
 184         &mut self,
 185         a: ty::Region<'tcx>,
 186         b: ty::Region<'tcx>,
 187     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 188         debug!("{}.regions({:?}, {:?}) self.cause={:?}", self.tag(), a, b, self.fields.cause);
 189
 190         // FIXME -- we have more fine-grained information available
 191         // from the "cause" field, we could perhaps give more tailored
 192         // error messages.
 193         let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
 194         // Subtype(&'a u8, &'b u8) => Outlives('a: 'b) => SubRegion('b, 'a)
 195         self.fields
 196             .infcx
 197             .inner
 198             .borrow_mut()
 199             .unwrap_region_constraints()
 200             .make_subregion(origin, b, a);
 201
 202         Ok(a)
 203     }
 204
 205     fn consts(
 206         &mut self,
 207         a: ty::Const<'tcx>,
 208         b: ty::Const<'tcx>,
 209     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 210         self.fields.infcx.super_combine_consts(self, a, b)
 211     }
 212
 213     fn binders<T>(
 214         &mut self,
 215         a: ty::Binder<'tcx, T>,
 216         b: ty::Binder<'tcx, T>,
 217     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 218     where
 219         T: Relate<'tcx>,
 220     {
 221         // A binder is always a subtype of itself if it's structually equal to itself
 222         if a == b {
 223             return Ok(a);
 224         }
 225
 226         self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
 227         Ok(a)
 228     }
 229 }
 230
 231 impl<'tcx> ConstEquateRelation<'tcx> for Sub<'_, '_, 'tcx> {
 232     fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
 233         self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
 234     }
 235 }