compiler/rustc_infer/src/infer/lub.rs

   1 use super::combine::CombineFields;
   2 use super::lattice::{self, LatticeDir};
   3 use super::InferCtxt;
   4 use super::Subtype;
   5
   6 use crate::infer::combine::ConstEquateRelation;
   7 use crate::traits::ObligationCause;
   8 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
   9 use rustc_middle::ty::{self, Ty, TyCtxt};
  10
  11 /// "Least upper bound" (common supertype)
  12 pub struct Lub<'combine, 'infcx, 'tcx> {
  13     fields: &'combine mut CombineFields<'infcx, 'tcx>,
  14     a_is_expected: bool,
  15 }
  16
  17 impl<'combine, 'infcx, 'tcx> Lub<'combine, 'infcx, 'tcx> {
  18     pub fn new(
  19         fields: &'combine mut CombineFields<'infcx, 'tcx>,
  20         a_is_expected: bool,
  21     ) -> Lub<'combine, 'infcx, 'tcx> {
  22         Lub { fields, a_is_expected }
  23     }
  24 }
  25
  26 impl TypeRelation<'tcx> for Lub<'combine, 'infcx, 'tcx> {
  27     fn tag(&self) -> &'static str {
  28         "Lub"
  29     }
  30
  31     fn tcx(&self) -> TyCtxt<'tcx> {
  32         self.fields.tcx()
  33     }
  34
  35     fn param_env(&self) -> ty::ParamEnv<'tcx> {
  36         self.fields.param_env
  37     }
  38
  39     fn a_is_expected(&self) -> bool {
  40         self.a_is_expected
  41     }
  42
  43     fn relate_with_variance<T: Relate<'tcx>>(
  44         &mut self,
  45         variance: ty::Variance,
  46         a: T,
  47         b: T,
  48     ) -> RelateResult<'tcx, T> {
  49         match variance {
  50             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  51             ty::Covariant => self.relate(a, b),
  52             // FIXME(#41044) -- not correct, need test
  53             ty::Bivariant => Ok(a),
  54             ty::Contravariant => self.fields.glb(self.a_is_expected).relate(a, b),
  55         }
  56     }
  57
  58     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  59         lattice::super_lattice_tys(self, a, b)
  60     }
  61
  62     fn regions(
  63         &mut self,
  64         a: ty::Region<'tcx>,
  65         b: ty::Region<'tcx>,
  66     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
  67         debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
  68
  69         let origin = Subtype(box self.fields.trace.clone());
  70         Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().lub_regions(
  71             self.tcx(),
  72             origin,
  73             a,
  74             b,
  75         ))
  76     }
  77
  78     fn consts(
  79         &mut self,
  80         a: &'tcx ty::Const<'tcx>,
  81         b: &'tcx ty::Const<'tcx>,
  82     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
  83         self.fields.infcx.super_combine_consts(self, a, b)
  84     }
  85
  86     fn binders<T>(
  87         &mut self,
  88         a: ty::Binder<'tcx, T>,
  89         b: ty::Binder<'tcx, T>,
  90     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
  91     where
  92         T: Relate<'tcx>,
  93     {
  94         debug!("binders(a={:?}, b={:?})", a, b);
  95
  96         // When higher-ranked types are involved, computing the LUB is
  97         // very challenging, switch to invariance. This is obviously
  98         // overly conservative but works ok in practice.
  99         self.relate_with_variance(ty::Variance::Invariant, a, b)?;
 100         Ok(a)
 101     }
 102 }
 103
 104 impl<'tcx> ConstEquateRelation<'tcx> for Lub<'_, '_, 'tcx> {
 105     fn const_equate_obligation(&mut self, a: &'tcx ty::Const<'tcx>, b: &'tcx ty::Const<'tcx>) {
 106         self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
 107     }
 108 }
 109
 110 impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx, 'tcx> {
 111     fn infcx(&self) -> &'infcx InferCtxt<'infcx, 'tcx> {
 112         self.fields.infcx
 113     }
 114
 115     fn cause(&self) -> &ObligationCause<'tcx> {
 116         &self.fields.trace.cause
 117     }
 118
 119     fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
 120         let mut sub = self.fields.sub(self.a_is_expected);
 121         sub.relate(a, v)?;
 122         sub.relate(b, v)?;
 123         Ok(())
 124     }
 125 }