src/librustc/infer/glb.rs

   1 use super::combine::CombineFields;
   2 use super::InferCtxt;
   3 use super::lattice::{self, LatticeDir};
   4 use super::Subtype;
   5
   6 use crate::traits::ObligationCause;
   7 use crate::ty::{self, Ty, TyCtxt};
   8 use crate::ty::relate::{Relate, RelateResult, TypeRelation};
   9
  10 /// "Greatest lower bound" (common subtype)
  11 pub struct Glb<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  12     fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
  13     a_is_expected: bool,
  14 }
  15
  16 impl<'combine, 'infcx, 'gcx, 'tcx> Glb<'combine, 'infcx, 'gcx, 'tcx> {
  17     pub fn new(fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
  18         -> Glb<'combine, 'infcx, 'gcx, 'tcx>
  19     {
  20         Glb { fields: fields, a_is_expected: a_is_expected }
  21     }
  22 }
  23
  24 impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
  25     for Glb<'combine, 'infcx, 'gcx, 'tcx>
  26 {
  27     fn tag(&self) -> &'static str { "Glb" }
  28
  29     fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.tcx() }
  30
  31     fn a_is_expected(&self) -> bool { self.a_is_expected }
  32
  33     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
  34                                              variance: ty::Variance,
  35                                              a: &T,
  36                                              b: &T)
  37                                              -> RelateResult<'tcx, T>
  38     {
  39         match variance {
  40             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
  41             ty::Covariant => self.relate(a, b),
  42             // FIXME(#41044) -- not correct, need test
  43             ty::Bivariant => Ok(a.clone()),
  44             ty::Contravariant => self.fields.lub(self.a_is_expected).relate(a, b),
  45         }
  46     }
  47
  48     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  49         lattice::super_lattice_tys(self, a, b)
  50     }
  51
  52     fn regions(&mut self, a: ty::Region<'tcx>, b: ty::Region<'tcx>)
  53                -> RelateResult<'tcx, ty::Region<'tcx>> {
  54         debug!("{}.regions({:?}, {:?})",
  55                self.tag(),
  56                a,
  57                b);
  58
  59         let origin = Subtype(self.fields.trace.clone());
  60         Ok(self.fields.infcx.borrow_region_constraints().glb_regions(self.tcx(), origin, a, b))
  61     }
  62
  63     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
  64                   -> RelateResult<'tcx, ty::Binder<T>>
  65         where T: Relate<'tcx>
  66     {
  67         debug!("binders(a={:?}, b={:?})", a, b);
  68
  69         // When higher-ranked types are involved, computing the LUB is
  70         // very challenging, switch to invariance. This is obviously
  71         // overly conservative but works ok in practice.
  72         self.relate_with_variance(ty::Variance::Invariant, a, b)?;
  73         Ok(a.clone())
  74     }
  75 }
  76
  77 impl<'combine, 'infcx, 'gcx, 'tcx> LatticeDir<'infcx, 'gcx, 'tcx>
  78     for Glb<'combine, 'infcx, 'gcx, 'tcx>
  79 {
  80     fn infcx(&self) -> &'infcx InferCtxt<'infcx, 'gcx, 'tcx> {
  81         self.fields.infcx
  82     }
  83
  84     fn cause(&self) -> &ObligationCause<'tcx> {
  85         &self.fields.trace.cause
  86     }
  87
  88     fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
  89         let mut sub = self.fields.sub(self.a_is_expected);
  90         sub.relate(&v, &a)?;
  91         sub.relate(&v, &b)?;
  92         Ok(())
  93     }
  94 }