compiler/rustc_trait_selection/src/solve/project_goals.rs

   1 use crate::traits::{specialization_graph, translate_substs};
   2
   3 use super::assembly::{self, AssemblyCtxt};
   4 use super::{EvalCtxt, Goal, QueryResult};
   5 use rustc_errors::ErrorGuaranteed;
   6 use rustc_hir::def::DefKind;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_infer::infer::{InferCtxt, InferOk};
   9 use rustc_infer::traits::query::NoSolution;
  10 use rustc_infer::traits::specialization_graph::LeafDef;
  11 use rustc_infer::traits::{ObligationCause, Reveal};
  12 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  13 use rustc_middle::ty::ProjectionPredicate;
  14 use rustc_middle::ty::TypeVisitable;
  15 use rustc_middle::ty::{self, Ty, TyCtxt};
  16 use rustc_span::DUMMY_SP;
  17 use std::iter;
  18
  19 #[allow(dead_code)] // FIXME: implement and use all variants.
  20 #[derive(Debug, Clone, Copy)]
  21 pub(super) enum CandidateSource {
  22     Impl(DefId),
  23     ParamEnv(usize),
  24     Builtin,
  25 }
  26
  27 type Candidate<'tcx> = assembly::Candidate<'tcx, ProjectionPredicate<'tcx>>;
  28
  29 impl<'tcx> EvalCtxt<'_, 'tcx> {
  30     pub(super) fn compute_projection_goal(
  31         &mut self,
  32         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
  33     ) -> QueryResult<'tcx> {
  34         let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
  35         self.merge_project_candidates(candidates)
  36     }
  37
  38     fn merge_project_candidates(
  39         &mut self,
  40         mut candidates: Vec<Candidate<'tcx>>,
  41     ) -> QueryResult<'tcx> {
  42         match candidates.len() {
  43             0 => return Err(NoSolution),
  44             1 => return Ok(candidates.pop().unwrap().result),
  45             _ => {}
  46         }
  47
  48         if candidates.len() > 1 {
  49             let mut i = 0;
  50             'outer: while i < candidates.len() {
  51                 for j in (0..candidates.len()).filter(|&j| i != j) {
  52                     if self.project_candidate_should_be_dropped_in_favor_of(
  53                         &candidates[i],
  54                         &candidates[j],
  55                     ) {
  56                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
  57                         candidates.swap_remove(i);
  58                         continue 'outer;
  59                     }
  60                 }
  61
  62                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
  63                 // If there are *STILL* multiple candidates, give up
  64                 // and report ambiguity.
  65                 i += 1;
  66                 if i > 1 {
  67                     debug!("multiple matches, ambig");
  68                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
  69                     unimplemented!();
  70                 }
  71             }
  72         }
  73
  74         Ok(candidates.pop().unwrap().result)
  75     }
  76
  77     fn project_candidate_should_be_dropped_in_favor_of(
  78         &self,
  79         candidate: &Candidate<'tcx>,
  80         other: &Candidate<'tcx>,
  81     ) -> bool {
  82         // FIXME: implement this
  83         match (candidate.source, other.source) {
  84             (CandidateSource::Impl(_), _)
  85             | (CandidateSource::ParamEnv(_), _)
  86             | (CandidateSource::Builtin, _) => unimplemented!(),
  87         }
  88     }
  89 }
  90
  91 impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
  92     type CandidateSource = CandidateSource;
  93
  94     fn self_ty(self) -> Ty<'tcx> {
  95         self.self_ty()
  96     }
  97
  98     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
  99         self.with_self_ty(tcx, self_ty)
 100     }
 101
 102     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
 103         self.trait_def_id(tcx)
 104     }
 105
 106     fn consider_impl_candidate(
 107         acx: &mut AssemblyCtxt<'_, '_, 'tcx, ProjectionPredicate<'tcx>>,
 108         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
 109         impl_def_id: DefId,
 110     ) {
 111         let tcx = acx.cx.tcx();
 112         let infcx = acx.cx.infcx;
 113
 114         let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
 115         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 116         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
 117         if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
 118             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
 119         {
 120             return;
 121         }
 122
 123         infcx.probe(|_| {
 124             let impl_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
 125             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
 126
 127             let Ok(InferOk { obligations, .. }) = infcx
 128                 .at(&ObligationCause::dummy(), goal.param_env)
 129                 .define_opaque_types(false)
 130                 .eq(goal_trait_ref, impl_trait_ref)
 131                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
 132             else {
 133                 return
 134             };
 135             let where_clause_bounds = tcx
 136                 .predicates_of(impl_def_id)
 137                 .instantiate(tcx, impl_substs)
 138                 .predicates
 139                 .into_iter()
 140                 .map(|pred| goal.with(tcx, pred));
 141
 142             let nested_goals =
 143                 obligations.into_iter().map(|o| o.into()).chain(where_clause_bounds).collect();
 144             let Ok(trait_ref_certainty) = acx.cx.evaluate_all(nested_goals) else { return };
 145
 146             let Some(assoc_def) = fetch_eligible_assoc_item_def(
 147                 infcx,
 148                 goal.param_env,
 149                 goal_trait_ref,
 150                 goal.predicate.def_id(),
 151                 impl_def_id
 152             ) else {
 153                 return
 154             };
 155
 156             if !assoc_def.item.defaultness(tcx).has_value() {
 157                 tcx.sess.delay_span_bug(
 158                     tcx.def_span(assoc_def.item.def_id),
 159                     "missing value for assoc item in impl",
 160                 );
 161             }
 162
 163             // Getting the right substitutions here is complex, e.g. given:
 164             // - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
 165             // - the applicable impl `impl<T> Trait<i32> for Vec<T>`
 166             // - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
 167             //
 168             // We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
 169             // to `[u32, u64]`.
 170             //
 171             // And then map these substs to the substs of the defining impl of `Assoc`, going
 172             // from `[u32, u64]` to `[u32, i32, u64]`.
 173             let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
 174                 tcx,
 175                 goal_trait_ref.def_id,
 176                 impl_substs,
 177             );
 178             let substs = translate_substs(
 179                 infcx,
 180                 goal.param_env,
 181                 impl_def_id,
 182                 impl_substs_with_gat,
 183                 assoc_def.defining_node,
 184             );
 185
 186             // Finally we construct the actual value of the associated type.
 187             let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
 188             let ty = tcx.bound_type_of(assoc_def.item.def_id);
 189             let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
 190                 let identity_substs =
 191                     ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
 192                 let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
 193                 let kind =
 194                     ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
 195                 ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
 196             } else {
 197                 ty.map_bound(|ty| ty.into())
 198             };
 199
 200             let Ok(InferOk { obligations, .. }) = infcx
 201                 .at(&ObligationCause::dummy(), goal.param_env)
 202                 .define_opaque_types(false)
 203                 .eq(goal.predicate.term,  term.subst(tcx, substs))
 204                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
 205             else {
 206                 return
 207             };
 208
 209             let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
 210             let Ok(rhs_certainty) = acx.cx.evaluate_all(nested_goals) else { return };
 211
 212             let certainty = trait_ref_certainty.unify_and(rhs_certainty);
 213             acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
 214         })
 215     }
 216 }
 217
 218 /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
 219 ///
 220 /// FIXME: We should merge these 3 implementations as it's likely that they otherwise
 221 /// diverge.
 222 #[instrument(level = "debug", skip(infcx, param_env), ret)]
 223 fn fetch_eligible_assoc_item_def<'tcx>(
 224     infcx: &InferCtxt<'tcx>,
 225     param_env: ty::ParamEnv<'tcx>,
 226     goal_trait_ref: ty::TraitRef<'tcx>,
 227     trait_assoc_def_id: DefId,
 228     impl_def_id: DefId,
 229 ) -> Option<LeafDef> {
 230     let node_item = specialization_graph::assoc_def(infcx.tcx, impl_def_id, trait_assoc_def_id)
 231         .map_err(|ErrorGuaranteed { .. }| ())
 232         .ok()?;
 233
 234     let eligible = if node_item.is_final() {
 235         // Non-specializable items are always projectable.
 236         true
 237     } else {
 238         // Only reveal a specializable default if we're past type-checking
 239         // and the obligation is monomorphic, otherwise passes such as
 240         // transmute checking and polymorphic MIR optimizations could
 241         // get a result which isn't correct for all monomorphizations.
 242         if param_env.reveal() == Reveal::All {
 243             let poly_trait_ref = infcx.resolve_vars_if_possible(goal_trait_ref);
 244             !poly_trait_ref.still_further_specializable()
 245         } else {
 246             debug!(?node_item.item.def_id, "not eligible due to default");
 247             false
 248         }
 249     };
 250
 251     if eligible { Some(node_item) } else { None }
 252 }