dedup assembly

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -0,0 +1,150 @@
+//! Code shared by trait and projection goals for candidate assembly.
+
+use super::infcx_ext::InferCtxtExt;
+use super::{
+    fixme_instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty,
+    EvalCtxt, Goal,
+};
+use rustc_hir::def_id::DefId;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::{
+    canonical::{CanonicalVarValues, OriginalQueryValues},
+    InferCtxt,
+};
+use rustc_infer::traits::query::NoSolution;
+use rustc_middle::ty::TypeFoldable;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::DUMMY_SP;
+use std::fmt::Debug;
+
+/// A candidate is a possible way to prove a goal.
+///
+/// It consists of both the `source`, which describes how that goal would be proven,
+/// and the `result` when using the given `source`.
+///
+/// For the list of possible candidates, please look at the documentation of
+/// [super::trait_goals::CandidateSource] and [super::project_goals::CandidateSource].
+#[derive(Debug, Clone)]
+pub(super) struct Candidate<'tcx, G: GoalKind<'tcx>> {
+    pub(super) source: G::CandidateSource,
+    pub(super) result: CanonicalResponse<'tcx>,
+}
+
+pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
+    type CandidateSource: Debug + Copy;
+
+    fn self_ty(self) -> Ty<'tcx>;
+
+    fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
+
+    fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;
+
+    fn consider_impl_candidate(
+        acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
+        goal: Goal<'tcx, Self>,
+        impl_def_id: DefId,
+    );
+}
+
+/// An abstraction which correctly deals with the canonical results for candidates.
+///
+/// It also deduplicates the behavior between trait and projection predicates.
+pub(super) struct AssemblyCtxt<'a, 'tcx, G: GoalKind<'tcx>> {
+    pub(super) cx: &'a mut EvalCtxt<'tcx>,
+    pub(super) infcx: &'a InferCtxt<'tcx>,
+    var_values: CanonicalVarValues<'tcx>,
+    candidates: Vec<Candidate<'tcx, G>>,
+}
+
+impl<'a, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'tcx, G> {
+    pub(super) fn assemble_and_evaluate_candidates(
+        cx: &'a mut EvalCtxt<'tcx>,
+        goal: CanonicalGoal<'tcx, G>,
+    ) -> Vec<Candidate<'tcx, G>> {
+        let (ref infcx, goal, var_values) =
+            cx.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
+        let mut acx = AssemblyCtxt { cx, infcx, var_values, candidates: Vec::new() };
+
+        acx.assemble_candidates_after_normalizing_self_ty(goal);
+
+        acx.assemble_impl_candidates(goal);
+
+        acx.candidates
+    }
+
+    pub(super) fn try_insert_candidate(
+        &mut self,
+        source: G::CandidateSource,
+        certainty: Certainty,
+    ) {
+        match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
+            Ok(result) => self.candidates.push(Candidate { source, result }),
+            Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
+        }
+    }
+
+    /// If the self type of a goal is a projection, computing the relevant candidates is difficult.
+    ///
+    /// To deal with this, we first try to normalize the self type and add the candidates for the normalized
+    /// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in
+    /// this case as projections as self types add `
+    fn assemble_candidates_after_normalizing_self_ty(&mut self, goal: Goal<'tcx, G>) {
+        let tcx = self.cx.tcx;
+        // FIXME: We also have to normalize opaque types, not sure where to best fit that in.
+        let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.self_ty().kind() else {
+            return
+        };
+        self.infcx.probe(|_| {
+            let normalized_ty = self.infcx.next_ty_infer();
+            let normalizes_to_goal = goal.with(
+                tcx,
+                ty::Binder::dummy(ty::ProjectionPredicate {
+                    projection_ty,
+                    term: normalized_ty.into(),
+                }),
+            );
+            let normalization_certainty =
+                match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
+                    Ok((_, certainty)) => certainty,
+                    Err(NoSolution) => return,
+                };
+
+            // NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
+            // This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
+            let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
+            let mut orig_values = OriginalQueryValues::default();
+            let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
+            let normalized_candidates =
+                AssemblyCtxt::assemble_and_evaluate_candidates(self.cx, goal);
+
+            // Map each candidate from being canonical wrt the current inference context to being
+            // canonical wrt the caller.
+            for Candidate { source, result } in normalized_candidates {
+                self.infcx.probe(|_| {
+                    let candidate_certainty = fixme_instantiate_canonical_query_response(
+                        &self.infcx,
+                        &orig_values,
+                        result,
+                    );
+
+                    // FIXME: This is a bit scary if the `normalizes_to_goal` overflows.
+                    //
+                    // If we have an ambiguous candidate it hides that normalization
+                    // caused an overflow which may cause issues.
+                    self.try_insert_candidate(
+                        source,
+                        normalization_certainty.unify_and(candidate_certainty),
+                    )
+                })
+            }
+        })
+    }
+
+    fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, G>) {
+        self.cx.tcx.for_each_relevant_impl(
+            goal.predicate.trait_def_id(self.cx.tcx),
+            goal.predicate.self_ty(),
+            |impl_def_id| G::consider_impl_candidate(self, goal, impl_def_id),
+        );
+    }
+}