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