compiler/rustc_trait_selection/src/solve/project_goals.rs

   1 use crate::traits::{specialization_graph, translate_substs};
   2
   3 use super::infcx_ext::InferCtxtExt;
   4 use super::{
   5     fixme_instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty,
   6     EvalCtxt, Goal, QueryResult,
   7 };
   8 use rustc_errors::ErrorGuaranteed;
   9 use rustc_hir::def::DefKind;
  10 use rustc_hir::def_id::DefId;
  11 use rustc_infer::infer::canonical::{CanonicalVarValues, OriginalQueryValues};
  12 use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
  13 use rustc_infer::traits::query::NoSolution;
  14 use rustc_infer::traits::specialization_graph::LeafDef;
  15 use rustc_infer::traits::{ObligationCause, Reveal};
  16 use rustc_middle::ty;
  17 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  18 use rustc_middle::ty::ProjectionPredicate;
  19 use rustc_middle::ty::TypeVisitable;
  20 use rustc_span::DUMMY_SP;
  21 use std::iter;
  22
  23 // FIXME: Deduplicate the candidate code between projection and trait goal.
  24
  25 /// Similar to [super::trait_goals::Candidate] but for `Projection` goals.
  26 #[derive(Debug, Clone)]
  27 struct Candidate<'tcx> {
  28     source: CandidateSource,
  29     result: CanonicalResponse<'tcx>,
  30 }
  31
  32 #[allow(dead_code)] // FIXME: implement and use all variants.
  33 #[derive(Debug, Clone, Copy)]
  34 enum CandidateSource {
  35     Impl(DefId),
  36     ParamEnv(usize),
  37     Builtin,
  38 }
  39
  40 impl<'tcx> EvalCtxt<'tcx> {
  41     pub(super) fn compute_projection_goal(
  42         &mut self,
  43         goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
  44     ) -> QueryResult<'tcx> {
  45         let candidates = self.assemble_and_evaluate_project_candidates(goal);
  46         self.merge_project_candidates(candidates)
  47     }
  48
  49     fn assemble_and_evaluate_project_candidates(
  50         &mut self,
  51         goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
  52     ) -> Vec<Candidate<'tcx>> {
  53         let (ref infcx, goal, var_values) =
  54             self.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
  55         let mut acx = AssemblyCtxt { cx: self, infcx, var_values, candidates: Vec::new() };
  56
  57         acx.assemble_candidates_after_normalizing_self_ty(goal);
  58         acx.assemble_impl_candidates(goal);
  59         acx.candidates
  60     }
  61
  62     fn merge_project_candidates(
  63         &mut self,
  64         mut candidates: Vec<Candidate<'tcx>>,
  65     ) -> QueryResult<'tcx> {
  66         match candidates.len() {
  67             0 => return Err(NoSolution),
  68             1 => return Ok(candidates.pop().unwrap().result),
  69             _ => {}
  70         }
  71
  72         if candidates.len() > 1 {
  73             let mut i = 0;
  74             'outer: while i < candidates.len() {
  75                 for j in (0..candidates.len()).filter(|&j| i != j) {
  76                     if self.project_candidate_should_be_dropped_in_favor_of(
  77                         &candidates[i],
  78                         &candidates[j],
  79                     ) {
  80                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
  81                         candidates.swap_remove(i);
  82                         continue 'outer;
  83                     }
  84                 }
  85
  86                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
  87                 // If there are *STILL* multiple candidates, give up
  88                 // and report ambiguity.
  89                 i += 1;
  90                 if i > 1 {
  91                     debug!("multiple matches, ambig");
  92                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
  93                     unimplemented!();
  94                 }
  95             }
  96         }
  97
  98         Ok(candidates.pop().unwrap().result)
  99     }
 100
 101     fn project_candidate_should_be_dropped_in_favor_of(
 102         &self,
 103         candidate: &Candidate<'tcx>,
 104         other: &Candidate<'tcx>,
 105     ) -> bool {
 106         // FIXME: implement this
 107         match (candidate.source, other.source) {
 108             (CandidateSource::Impl(_), _)
 109             | (CandidateSource::ParamEnv(_), _)
 110             | (CandidateSource::Builtin, _) => unimplemented!(),
 111         }
 112     }
 113 }
 114
 115 /// Similar to [super::trait_goals::AssemblyCtxt] but for `Projection` goals.
 116 struct AssemblyCtxt<'a, 'tcx> {
 117     cx: &'a mut EvalCtxt<'tcx>,
 118     infcx: &'a InferCtxt<'tcx>,
 119     var_values: CanonicalVarValues<'tcx>,
 120     candidates: Vec<Candidate<'tcx>>,
 121 }
 122
 123 impl<'tcx> AssemblyCtxt<'_, 'tcx> {
 124     fn try_insert_candidate(&mut self, source: CandidateSource, certainty: Certainty) {
 125         match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
 126             Ok(result) => self.candidates.push(Candidate { source, result }),
 127             Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
 128         }
 129     }
 130
 131     fn assemble_candidates_after_normalizing_self_ty(
 132         &mut self,
 133         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
 134     ) {
 135         let tcx = self.cx.tcx;
 136         let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.projection_ty.self_ty().kind() else {
 137             return
 138         };
 139         self.infcx.probe(|_| {
 140             let normalized_ty = self.infcx.next_ty_infer();
 141             let normalizes_to_goal = goal.with(
 142                 tcx,
 143                 ty::Binder::dummy(ty::ProjectionPredicate {
 144                     projection_ty,
 145                     term: normalized_ty.into(),
 146                 }),
 147             );
 148             let normalization_certainty =
 149                 match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
 150                     Ok((_, certainty)) => certainty,
 151                     Err(NoSolution) => return,
 152                 };
 153
 154             // NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
 155             // This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
 156             let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
 157             let mut orig_values = OriginalQueryValues::default();
 158             let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 159             let normalized_candidates = self.cx.assemble_and_evaluate_project_candidates(goal);
 160             // Map each candidate from being canonical wrt the current inference context to being
 161             // canonical wrt the caller.
 162             for Candidate { source, result } in normalized_candidates {
 163                 self.infcx.probe(|_| {
 164                     let candidate_certainty = fixme_instantiate_canonical_query_response(
 165                         self.infcx,
 166                         &orig_values,
 167                         result,
 168                     );
 169                     self.try_insert_candidate(
 170                         source,
 171                         normalization_certainty.unify_and(candidate_certainty),
 172                     )
 173                 })
 174             }
 175         })
 176     }
 177
 178     fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, ProjectionPredicate<'tcx>>) {
 179         self.cx.tcx.for_each_relevant_impl(
 180             goal.predicate.trait_def_id(self.cx.tcx),
 181             goal.predicate.self_ty(),
 182             |impl_def_id| self.consider_impl_candidate(goal, impl_def_id),
 183         );
 184     }
 185
 186     fn consider_impl_candidate(
 187         &mut self,
 188         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
 189         impl_def_id: DefId,
 190     ) {
 191         let tcx = self.cx.tcx;
 192         let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
 193         let impl_trait_ref = tcx.bound_impl_trait_ref(impl_def_id).unwrap();
 194         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
 195         if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
 196             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
 197         {
 198             return;
 199         }
 200
 201         self.infcx.probe(|_| {
 202             let impl_substs = self.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
 203             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
 204
 205             let Ok(InferOk { obligations, .. }) = self
 206                 .infcx
 207                 .at(&ObligationCause::dummy(), goal.param_env)
 208                 .define_opaque_types(false)
 209                 .eq(goal_trait_ref, impl_trait_ref)
 210                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
 211             else {
 212                 return
 213             };
 214
 215             let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
 216             let Ok(trait_ref_certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
 217
 218             let Some(assoc_def) = self.fetch_eligible_assoc_item_def(
 219                 goal.param_env,
 220                 goal_trait_ref,
 221                 goal.predicate.def_id(),
 222                 impl_def_id
 223             ) else {
 224                 return
 225             };
 226
 227             if !assoc_def.item.defaultness(tcx).has_value() {
 228                 tcx.sess.delay_span_bug(
 229                     tcx.def_span(assoc_def.item.def_id),
 230                     "missing value for assoc item in impl",
 231                 );
 232             }
 233
 234             // Getting the right substitutions here is complex, e.g. given:
 235             // - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
 236             // - the applicable impl `impl<T> Trait<i32> for Vec<T>`
 237             // - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
 238             //
 239             // We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
 240             // to `[u32, u64]`.
 241             //
 242             // And then map these substs to the substs of the defining impl of `Assoc`, going
 243             // from `[u32, u64]` to `[u32, i32, u64]`.
 244             let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
 245                 tcx,
 246                 goal_trait_ref.def_id,
 247                 impl_trait_ref.substs,
 248             );
 249             let substs = translate_substs(
 250                 self.infcx,
 251                 goal.param_env,
 252                 impl_def_id,
 253                 impl_substs_with_gat,
 254                 assoc_def.defining_node,
 255             );
 256
 257             // Finally we construct the actual value of the associated type.
 258             let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
 259             let ty = tcx.bound_type_of(assoc_def.item.def_id);
 260             let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
 261                 let identity_substs = ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
 262                 let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
 263                 let kind =
 264                     ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
 265                 ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
 266             } else {
 267                 ty.map_bound(|ty| ty.into())
 268             };
 269
 270             let Ok(InferOk { obligations, .. }) = self
 271                 .infcx
 272                 .at(&ObligationCause::dummy(), goal.param_env)
 273                 .define_opaque_types(false)
 274                 .eq(goal.predicate.term,  term.subst(tcx, substs))
 275                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
 276             else {
 277                 return
 278             };
 279
 280             let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
 281             let Ok(rhs_certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
 282
 283             let certainty = trait_ref_certainty.unify_and(rhs_certainty);
 284             self.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
 285         })
 286     }
 287
 288     /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
 289     ///
 290     /// FIXME: We should merge these 3 implementations as it's likely that they otherwise
 291     /// diverge.
 292     #[instrument(level = "debug", skip(self, param_env), ret)]
 293     fn fetch_eligible_assoc_item_def(
 294         &self,
 295         param_env: ty::ParamEnv<'tcx>,
 296         goal_trait_ref: ty::TraitRef<'tcx>,
 297         trait_assoc_def_id: DefId,
 298         impl_def_id: DefId,
 299     ) -> Option<LeafDef> {
 300         let node_item =
 301             specialization_graph::assoc_def(self.cx.tcx, impl_def_id, trait_assoc_def_id)
 302                 .map_err(|ErrorGuaranteed { .. }| ())
 303                 .ok()?;
 304
 305         let eligible = if node_item.is_final() {
 306             // Non-specializable items are always projectable.
 307             true
 308         } else {
 309             // Only reveal a specializable default if we're past type-checking
 310             // and the obligation is monomorphic, otherwise passes such as
 311             // transmute checking and polymorphic MIR optimizations could
 312             // get a result which isn't correct for all monomorphizations.
 313             if param_env.reveal() == Reveal::All {
 314                 let poly_trait_ref = self.infcx.resolve_vars_if_possible(goal_trait_ref);
 315                 !poly_trait_ref.still_further_specializable()
 316             } else {
 317                 debug!(?node_item.item.def_id, "not eligible due to default");
 318                 false
 319             }
 320         };
 321
 322         if eligible { Some(node_item) } else { None }
 323     }
 324 }