implement the skeleton of the updated trait solver
This commit is contained in:
lcnr
324
compiler/rustc_trait_selection/src/solve/project_goals.rs
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324
compiler/rustc_trait_selection/src/solve/project_goals.rs
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@@ -0,0 +1,324 @@
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use crate::traits::{specialization_graph, translate_substs};
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use super::infcx_ext::InferCtxtExt;
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use super::{
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fixme_instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty,
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EvalCtxt, Goal, QueryResult,
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};
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use rustc_errors::ErrorGuaranteed;
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use rustc_hir::def::DefKind;
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use rustc_hir::def_id::DefId;
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use rustc_infer::infer::canonical::{CanonicalVarValues, OriginalQueryValues};
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use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
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use rustc_infer::traits::query::NoSolution;
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use rustc_infer::traits::specialization_graph::LeafDef;
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use rustc_infer::traits::{ObligationCause, Reveal};
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use rustc_middle::ty;
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use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
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use rustc_middle::ty::ProjectionPredicate;
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use rustc_middle::ty::TypeVisitable;
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use rustc_span::DUMMY_SP;
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use std::iter;
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// FIXME: Deduplicate the candidate code between projection and trait goal.
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/// Similar to [super::trait_goals::Candidate] but for `Projection` goals.
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#[derive(Debug, Clone)]
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struct Candidate<'tcx> {
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source: CandidateSource,
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result: CanonicalResponse<'tcx>,
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}
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#[allow(dead_code)] // FIXME: implement and use all variants.
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#[derive(Debug, Clone, Copy)]
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enum CandidateSource {
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Impl(DefId),
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ParamEnv(usize),
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Builtin,
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}
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impl<'tcx> EvalCtxt<'tcx> {
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pub(super) fn compute_projection_goal(
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&mut self,
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goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
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) -> QueryResult<'tcx> {
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let candidates = self.assemble_and_evaluate_project_candidates(goal);
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self.merge_project_candidates(candidates)
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}
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fn assemble_and_evaluate_project_candidates(
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&mut self,
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goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
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) -> Vec<Candidate<'tcx>> {
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let (ref infcx, goal, var_values) =
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self.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
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let mut acx = AssemblyCtxt { cx: self, infcx, var_values, candidates: Vec::new() };
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acx.assemble_candidates_after_normalizing_self_ty(goal);
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acx.assemble_impl_candidates(goal);
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acx.candidates
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}
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fn merge_project_candidates(
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&mut self,
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mut candidates: Vec<Candidate<'tcx>>,
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) -> QueryResult<'tcx> {
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match candidates.len() {
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0 => return Err(NoSolution),
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1 => return Ok(candidates.pop().unwrap().result),
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_ => {}
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}
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if candidates.len() > 1 {
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let mut i = 0;
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'outer: while i < candidates.len() {
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for j in (0..candidates.len()).filter(|&j| i != j) {
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if self.project_candidate_should_be_dropped_in_favor_of(
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&candidates[i],
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&candidates[j],
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) {
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debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
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candidates.swap_remove(i);
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continue 'outer;
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}
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}
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debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
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// If there are *STILL* multiple candidates, give up
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// and report ambiguity.
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i += 1;
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if i > 1 {
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debug!("multiple matches, ambig");
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// FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
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unimplemented!();
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}
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}
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}
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Ok(candidates.pop().unwrap().result)
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}
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fn project_candidate_should_be_dropped_in_favor_of(
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&self,
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candidate: &Candidate<'tcx>,
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other: &Candidate<'tcx>,
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) -> bool {
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// FIXME: implement this
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match (candidate.source, other.source) {
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(CandidateSource::Impl(_), _)
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| (CandidateSource::ParamEnv(_), _)
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| (CandidateSource::Builtin, _) => unimplemented!(),
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}
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}
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}
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/// Similar to [super::trait_goals::AssemblyCtxt] but for `Projection` goals.
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struct AssemblyCtxt<'a, 'tcx> {
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cx: &'a mut EvalCtxt<'tcx>,
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infcx: &'a InferCtxt<'tcx>,
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var_values: CanonicalVarValues<'tcx>,
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candidates: Vec<Candidate<'tcx>>,
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}
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impl<'tcx> AssemblyCtxt<'_, 'tcx> {
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fn try_insert_candidate(&mut self, source: CandidateSource, certainty: Certainty) {
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match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
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Ok(result) => self.candidates.push(Candidate { source, result }),
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Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
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}
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}
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fn assemble_candidates_after_normalizing_self_ty(
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&mut self,
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goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
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) {
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let tcx = self.cx.tcx;
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let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.projection_ty.self_ty().kind() else {
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return
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};
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self.infcx.probe(|_| {
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let normalized_ty = self.infcx.next_ty_infer();
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let normalizes_to_goal = goal.with(
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tcx,
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ty::Binder::dummy(ty::ProjectionPredicate {
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projection_ty,
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term: normalized_ty.into(),
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}),
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);
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let normalization_certainty =
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match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
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Ok((_, certainty)) => certainty,
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Err(NoSolution) => return,
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};
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// NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
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// This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
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let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
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let mut orig_values = OriginalQueryValues::default();
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let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
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let normalized_candidates = self.cx.assemble_and_evaluate_project_candidates(goal);
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// Map each candidate from being canonical wrt the current inference context to being
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// canonical wrt the caller.
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for Candidate { source, result } in normalized_candidates {
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self.infcx.probe(|_| {
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let candidate_certainty = fixme_instantiate_canonical_query_response(
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self.infcx,
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&orig_values,
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result,
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);
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self.try_insert_candidate(
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source,
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normalization_certainty.unify_and(candidate_certainty),
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)
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})
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}
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})
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}
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fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, ProjectionPredicate<'tcx>>) {
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self.cx.tcx.for_each_relevant_impl(
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goal.predicate.trait_def_id(self.cx.tcx),
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goal.predicate.self_ty(),
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|impl_def_id| self.consider_impl_candidate(goal, impl_def_id),
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);
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}
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fn consider_impl_candidate(
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&mut self,
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goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
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impl_def_id: DefId,
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) {
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let tcx = self.cx.tcx;
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let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
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let impl_trait_ref = tcx.bound_impl_trait_ref(impl_def_id).unwrap();
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let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
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if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
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.any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
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{
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return;
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}
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self.infcx.probe(|_| {
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let impl_substs = self.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
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let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
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let Ok(InferOk { obligations, .. }) = self
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.infcx
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.at(&ObligationCause::dummy(), goal.param_env)
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.define_opaque_types(false)
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.eq(goal_trait_ref, impl_trait_ref)
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.map_err(|e| debug!("failed to equate trait refs: {e:?}"))
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else {
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return
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};
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let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
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let Ok(trait_ref_certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
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let Some(assoc_def) = self.fetch_eligible_assoc_item_def(
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goal.param_env,
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goal_trait_ref,
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goal.predicate.def_id(),
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impl_def_id
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) else {
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return
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};
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if !assoc_def.item.defaultness(tcx).has_value() {
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tcx.sess.delay_span_bug(
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tcx.def_span(assoc_def.item.def_id),
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"missing value for assoc item in impl",
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);
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}
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// Getting the right substitutions here is complex, e.g. given:
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// - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
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// - the applicable impl `impl<T> Trait<i32> for Vec<T>`
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// - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
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//
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// We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
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// to `[u32, u64]`.
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//
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// And then map these substs to the substs of the defining impl of `Assoc`, going
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// from `[u32, u64]` to `[u32, i32, u64]`.
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let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
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tcx,
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goal_trait_ref.def_id,
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impl_trait_ref.substs,
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);
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let substs = translate_substs(
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self.infcx,
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goal.param_env,
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impl_def_id,
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impl_substs_with_gat,
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assoc_def.defining_node,
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);
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// Finally we construct the actual value of the associated type.
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let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
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let ty = tcx.bound_type_of(assoc_def.item.def_id);
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let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
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let identity_substs = ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
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let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
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let kind =
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ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
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ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
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} else {
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ty.map_bound(|ty| ty.into())
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};
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let Ok(InferOk { obligations, .. }) = self
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.infcx
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.at(&ObligationCause::dummy(), goal.param_env)
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.define_opaque_types(false)
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.eq(goal.predicate.term, term.subst(tcx, substs))
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.map_err(|e| debug!("failed to equate trait refs: {e:?}"))
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else {
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return
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};
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let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
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let Ok(rhs_certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
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let certainty = trait_ref_certainty.unify_and(rhs_certainty);
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self.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
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})
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}
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/// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
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///
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/// FIXME: We should merge these 3 implementations as it's likely that they otherwise
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/// diverge.
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#[instrument(level = "debug", skip(self, param_env), ret)]
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fn fetch_eligible_assoc_item_def(
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&self,
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param_env: ty::ParamEnv<'tcx>,
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goal_trait_ref: ty::TraitRef<'tcx>,
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trait_assoc_def_id: DefId,
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impl_def_id: DefId,
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) -> Option<LeafDef> {
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let node_item =
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specialization_graph::assoc_def(self.cx.tcx, impl_def_id, trait_assoc_def_id)
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.map_err(|ErrorGuaranteed { .. }| ())
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.ok()?;
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let eligible = if node_item.is_final() {
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// Non-specializable items are always projectable.
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true
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} else {
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// Only reveal a specializable default if we're past type-checking
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// and the obligation is monomorphic, otherwise passes such as
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// transmute checking and polymorphic MIR optimizations could
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// get a result which isn't correct for all monomorphizations.
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if param_env.reveal() == Reveal::All {
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let poly_trait_ref = self.infcx.resolve_vars_if_possible(goal_trait_ref);
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!poly_trait_ref.still_further_specializable()
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} else {
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debug!(?node_item.item.def_id, "not eligible due to default");
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false
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}
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};
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if eligible { Some(node_item) } else { None }
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}
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}
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