compiler/rustc_trait_selection/src/solve/assembly.rs

   1 //! Code shared by trait and projection goals for candidate assembly.
   2
   3 use super::infcx_ext::InferCtxtExt;
   4 use super::{
   5     instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty, EvalCtxt,
   6     Goal,
   7 };
   8 use rustc_hir::def_id::DefId;
   9 use rustc_infer::infer::TyCtxtInferExt;
  10 use rustc_infer::infer::{
  11     canonical::{CanonicalVarValues, OriginalQueryValues},
  12     InferCtxt,
  13 };
  14 use rustc_infer::traits::query::NoSolution;
  15 use rustc_middle::ty::TypeFoldable;
  16 use rustc_middle::ty::{self, Ty, TyCtxt};
  17 use rustc_span::DUMMY_SP;
  18 use std::fmt::Debug;
  19
  20 /// A candidate is a possible way to prove a goal.
  21 ///
  22 /// It consists of both the `source`, which describes how that goal would be proven,
  23 /// and the `result` when using the given `source`.
  24 ///
  25 /// For the list of possible candidates, please look at the documentation of
  26 /// [super::trait_goals::CandidateSource] and [super::project_goals::CandidateSource].
  27 #[derive(Debug, Clone)]
  28 pub(super) struct Candidate<'tcx, G: GoalKind<'tcx>> {
  29     pub(super) source: G::CandidateSource,
  30     pub(super) result: CanonicalResponse<'tcx>,
  31 }
  32
  33 pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
  34     type CandidateSource: Debug + Copy;
  35
  36     fn self_ty(self) -> Ty<'tcx>;
  37
  38     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
  39
  40     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;
  41
  42     fn consider_impl_candidate(
  43         acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
  44         goal: Goal<'tcx, Self>,
  45         impl_def_id: DefId,
  46     );
  47 }
  48
  49 /// An abstraction which correctly deals with the canonical results for candidates.
  50 ///
  51 /// It also deduplicates the behavior between trait and projection predicates.
  52 pub(super) struct AssemblyCtxt<'a, 'tcx, G: GoalKind<'tcx>> {
  53     pub(super) cx: &'a mut EvalCtxt<'tcx>,
  54     pub(super) infcx: &'a InferCtxt<'tcx>,
  55     var_values: CanonicalVarValues<'tcx>,
  56     candidates: Vec<Candidate<'tcx, G>>,
  57 }
  58
  59 impl<'a, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'tcx, G> {
  60     pub(super) fn assemble_and_evaluate_candidates(
  61         cx: &'a mut EvalCtxt<'tcx>,
  62         goal: CanonicalGoal<'tcx, G>,
  63     ) -> Vec<Candidate<'tcx, G>> {
  64         let (ref infcx, goal, var_values) =
  65             cx.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
  66         let mut acx = AssemblyCtxt { cx, infcx, var_values, candidates: Vec::new() };
  67
  68         acx.assemble_candidates_after_normalizing_self_ty(goal);
  69
  70         acx.assemble_impl_candidates(goal);
  71
  72         acx.candidates
  73     }
  74
  75     pub(super) fn try_insert_candidate(
  76         &mut self,
  77         source: G::CandidateSource,
  78         certainty: Certainty,
  79     ) {
  80         match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
  81             Ok(result) => self.candidates.push(Candidate { source, result }),
  82             Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
  83         }
  84     }
  85
  86     /// If the self type of a goal is a projection, computing the relevant candidates is difficult.
  87     ///
  88     /// To deal with this, we first try to normalize the self type and add the candidates for the normalized
  89     /// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in
  90     /// this case as projections as self types add `
  91     fn assemble_candidates_after_normalizing_self_ty(&mut self, goal: Goal<'tcx, G>) {
  92         let tcx = self.cx.tcx;
  93         // FIXME: We also have to normalize opaque types, not sure where to best fit that in.
  94         let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.self_ty().kind() else {
  95             return
  96         };
  97         self.infcx.probe(|_| {
  98             let normalized_ty = self.infcx.next_ty_infer();
  99             let normalizes_to_goal = goal.with(
 100                 tcx,
 101                 ty::Binder::dummy(ty::ProjectionPredicate {
 102                     projection_ty,
 103                     term: normalized_ty.into(),
 104                 }),
 105             );
 106             let normalization_certainty =
 107                 match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
 108                     Ok((_, certainty)) => certainty,
 109                     Err(NoSolution) => return,
 110                 };
 111
 112             // NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
 113             // This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
 114             let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
 115             let mut orig_values = OriginalQueryValues::default();
 116             let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 117             let normalized_candidates =
 118                 AssemblyCtxt::assemble_and_evaluate_candidates(self.cx, goal);
 119
 120             // Map each candidate from being canonical wrt the current inference context to being
 121             // canonical wrt the caller.
 122             for Candidate { source, result } in normalized_candidates {
 123                 self.infcx.probe(|_| {
 124                     let candidate_certainty =
 125                         instantiate_canonical_query_response(&self.infcx, &orig_values, result);
 126
 127                     // FIXME: This is a bit scary if the `normalizes_to_goal` overflows.
 128                     //
 129                     // If we have an ambiguous candidate it hides that normalization
 130                     // caused an overflow which may cause issues.
 131                     self.try_insert_candidate(
 132                         source,
 133                         normalization_certainty.unify_and(candidate_certainty),
 134                     )
 135                 })
 136             }
 137         })
 138     }
 139
 140     fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, G>) {
 141         self.cx.tcx.for_each_relevant_impl(
 142             goal.predicate.trait_def_id(self.cx.tcx),
 143             goal.predicate.self_ty(),
 144             |impl_def_id| G::consider_impl_candidate(self, goal, impl_def_id),
 145         );
 146     }
 147 }