compiler/rustc_trait_selection/src/traits/relationships.rs

   1 use crate::infer::InferCtxt;
   2 use crate::traits::query::evaluate_obligation::InferCtxtExt;
   3 use crate::traits::PredicateObligation;
   4 use rustc_infer::traits::TraitEngine;
   5 use rustc_middle::ty;
   6
   7 pub(crate) fn update<'tcx, T>(
   8     engine: &mut T,
   9     infcx: &InferCtxt<'tcx>,
  10     obligation: &PredicateObligation<'tcx>,
  11 ) where
  12     T: TraitEngine<'tcx>,
  13 {
  14     // (*) binder skipped
  15     if let ty::PredicateKind::Trait(tpred) = obligation.predicate.kind().skip_binder()
  16         && let Some(ty) = infcx.shallow_resolve(tpred.self_ty()).ty_vid().map(|t| infcx.root_var(t))
  17         && infcx.tcx.lang_items().sized_trait().map_or(false, |st| st != tpred.trait_ref.def_id)
  18     {
  19         let new_self_ty = infcx.tcx.types.unit;
  20
  21         let trait_ref = ty::TraitRef {
  22             substs: infcx.tcx.mk_substs_trait(new_self_ty, &tpred.trait_ref.substs[1..]),
  23             ..tpred.trait_ref
  24         };
  25
  26         // Then construct a new obligation with Self = () added
  27         // to the ParamEnv, and see if it holds.
  28         let o = obligation.with(infcx.tcx,
  29             obligation
  30                 .predicate
  31                 .kind()
  32                 .rebind(
  33                     // (*) binder moved here
  34                     ty::PredicateKind::Trait(ty::TraitPredicate {
  35                         trait_ref,
  36                         constness: tpred.constness,
  37                         polarity: tpred.polarity,
  38                     })
  39                 ),
  40         );
  41         // Don't report overflow errors. Otherwise equivalent to may_hold.
  42         if let Ok(result) = infcx.probe(|_| infcx.evaluate_obligation(&o)) && result.may_apply() {
  43             engine.relationships().entry(ty).or_default().self_in_trait = true;
  44         }
  45     }
  46
  47     if let ty::PredicateKind::Projection(predicate) = obligation.predicate.kind().skip_binder() {
  48         // If the projection predicate (Foo::Bar == X) has X as a non-TyVid,
  49         // we need to make it into one.
  50         if let Some(vid) = predicate.term.ty().and_then(|ty| ty.ty_vid()) {
  51             debug!("relationship: {:?}.output = true", vid);
  52             engine.relationships().entry(vid).or_default().output = true;
  53         }
  54     }
  55 }