Use a proof tree visitor to refine the Obligation for error reporting in new solver

compiler/rustc_middle/src/traits/solve.rs

@@ -273,6 +273,8 @@ pub enum GoalSource {
     /// they are from an impl where-clause. This is necessary due to
     /// backwards compatability, cc trait-system-refactor-initiatitive#70.
     ImplWhereBound,
+    /// Instantiating a higher-ranked goal and re-proving it.
+    InstantiateHigherRanked,
 }
 
 /// Possible ways the given goal can be proven.

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -127,6 +127,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
                         let source = match source {
                             GoalSource::Misc => "misc",
                             GoalSource::ImplWhereBound => "impl where-bound",
+                            GoalSource::InstantiateHigherRanked => "higher-ranked goal",
                         };
                         writeln!(this.f, "ADDED GOAL ({source}): {goal:?}")?
                     }

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -58,15 +58,15 @@ pub(super) trait GoalKind<'tcx>:
     /// goal by equating it with the assumption.
     fn probe_and_consider_implied_clause(
         ecx: &mut EvalCtxt<'_, 'tcx>,
-        source: CandidateSource,
+        parent_source: CandidateSource,
         goal: Goal<'tcx, Self>,
         assumption: ty::Clause<'tcx>,
-        requirements: impl IntoIterator<Item = Goal<'tcx, ty::Predicate<'tcx>>>,
+        requirements: impl IntoIterator<Item = (GoalSource, Goal<'tcx, ty::Predicate<'tcx>>)>,
     ) -> Result<Candidate<'tcx>, NoSolution> {
-        Self::probe_and_match_goal_against_assumption(ecx, source, goal, assumption, |ecx| {
-            // FIXME(-Znext-solver=coinductive): check whether this should be
-            // `GoalSource::ImplWhereBound` for any caller.
-            ecx.add_goals(GoalSource::Misc, requirements);
+        Self::probe_and_match_goal_against_assumption(ecx, parent_source, goal, assumption, |ecx| {
+            for (nested_source, goal) in requirements {
+                ecx.add_goal(nested_source, goal);
+            }
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }
@@ -85,9 +85,8 @@ pub(super) trait GoalKind<'tcx>:
             let ty::Dynamic(bounds, _, _) = *goal.predicate.self_ty().kind() else {
                 bug!("expected object type in `probe_and_consider_object_bound_candidate`");
             };
-            // FIXME(-Znext-solver=coinductive): Should this be `GoalSource::ImplWhereBound`?
             ecx.add_goals(
-                GoalSource::Misc,
+                GoalSource::ImplWhereBound,
                 structural_traits::predicates_for_object_candidate(
                     ecx,
                     goal.param_env,

compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs

@@ -90,6 +90,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         &mut self,
         certainty: Certainty,
     ) -> QueryResult<'tcx> {
+        self.inspect.make_canonical_response(certainty);
+
         let goals_certainty = self.try_evaluate_added_goals()?;
         assert_eq!(
             self.tainted,
@@ -98,8 +100,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             previous call to `try_evaluate_added_goals!`"
         );
 
-        self.inspect.make_canonical_response(certainty);
-
         // When normalizing, we've replaced the expected term with an unconstrained
         // inference variable. This means that we dropped information which could
         // have been important. We handle this by instead returning the nested goals

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -454,7 +454,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         } else {
             self.infcx.enter_forall(kind, |kind| {
                 let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
-                self.add_goal(GoalSource::Misc, goal);
+                self.add_goal(GoalSource::InstantiateHigherRanked, goal);
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             })
         }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -1,15 +1,19 @@
 use std::mem;
+use std::ops::ControlFlow;
 
 use rustc_infer::infer::InferCtxt;
-use rustc_infer::traits::solve::MaybeCause;
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::solve::inspect::ProbeKind;
+use rustc_infer::traits::solve::{CandidateSource, GoalSource, MaybeCause};
 use rustc_infer::traits::{
-    query::NoSolution, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
-    PredicateObligation, SelectionError, TraitEngine,
+    self, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation,
+    ObligationCause, PredicateObligation, SelectionError, TraitEngine,
 };
-use rustc_middle::ty;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::{self, TyCtxt};
 
 use super::eval_ctxt::GenerateProofTree;
+use super::inspect::{ProofTreeInferCtxtExt, ProofTreeVisitor};
 use super::{Certainty, InferCtxtEvalExt};
 
 /// A trait engine using the new trait solver.
@@ -133,9 +137,9 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
             .collect();
 
         errors.extend(self.obligations.overflowed.drain(..).map(|obligation| FulfillmentError {
-            root_obligation: obligation.clone(),
+            obligation: find_best_leaf_obligation(infcx, &obligation),
             code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
-            obligation,
+            root_obligation: obligation,
         }));
 
         errors
@@ -192,8 +196,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
 
 fn fulfillment_error_for_no_solution<'tcx>(
     infcx: &InferCtxt<'tcx>,
-    obligation: PredicateObligation<'tcx>,
+    root_obligation: PredicateObligation<'tcx>,
 ) -> FulfillmentError<'tcx> {
+    let obligation = find_best_leaf_obligation(infcx, &root_obligation);
+
     let code = match obligation.predicate.kind().skip_binder() {
         ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
             FulfillmentErrorCode::ProjectionError(
@@ -234,7 +240,8 @@ fn fulfillment_error_for_no_solution<'tcx>(
             bug!("unexpected goal: {obligation:?}")
         }
     };
-    FulfillmentError { root_obligation: obligation.clone(), code, obligation }
+
+    FulfillmentError { obligation, code, root_obligation }
 }
 
 fn fulfillment_error_for_stalled<'tcx>(
@@ -258,5 +265,136 @@ fn fulfillment_error_for_stalled<'tcx>(
         }
     });
 
-    FulfillmentError { obligation: obligation.clone(), code, root_obligation: obligation }
+    FulfillmentError {
+        obligation: find_best_leaf_obligation(infcx, &obligation),
+        code,
+        root_obligation: obligation,
+    }
+}
+
+fn find_best_leaf_obligation<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligation: &PredicateObligation<'tcx>,
+) -> PredicateObligation<'tcx> {
+    let obligation = infcx.resolve_vars_if_possible(obligation.clone());
+    infcx
+        .visit_proof_tree(
+            obligation.clone().into(),
+            &mut BestObligation { obligation: obligation.clone() },
+        )
+        .break_value()
+        .unwrap_or(obligation)
+}
+
+struct BestObligation<'tcx> {
+    obligation: PredicateObligation<'tcx>,
+}
+
+impl<'tcx> BestObligation<'tcx> {
+    fn with_derived_obligation(
+        &mut self,
+        derived_obligation: PredicateObligation<'tcx>,
+        and_then: impl FnOnce(&mut Self) -> <Self as ProofTreeVisitor<'tcx>>::Result,
+    ) -> <Self as ProofTreeVisitor<'tcx>>::Result {
+        let old_obligation = std::mem::replace(&mut self.obligation, derived_obligation);
+        let res = and_then(self);
+        self.obligation = old_obligation;
+        res
+    }
+}
+
+impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
+    type Result = ControlFlow<PredicateObligation<'tcx>>;
+
+    fn span(&self) -> rustc_span::Span {
+        self.obligation.cause.span
+    }
+
+    fn visit_goal(&mut self, goal: &super::inspect::InspectGoal<'_, 'tcx>) -> Self::Result {
+        // FIXME: Throw out candidates that have no failing WC and >0 failing misc goal.
+        // This most likely means that the goal just didn't unify at all, e.g. a param
+        // candidate with an alias in it.
+        let candidates = goal.candidates();
+
+        let [candidate] = candidates.as_slice() else {
+            return ControlFlow::Break(self.obligation.clone());
+        };
+
+        // FIXME: Could we extract a trait ref from a projection here too?
+        // FIXME: Also, what about considering >1 layer up the stack? May be necessary
+        // for normalizes-to.
+        let Some(parent_trait_pred) = goal.goal().predicate.to_opt_poly_trait_pred() else {
+            return ControlFlow::Break(self.obligation.clone());
+        };
+
+        let tcx = goal.infcx().tcx;
+        let mut impl_where_bound_count = 0;
+        for nested_goal in candidate.instantiate_nested_goals(self.span()) {
+            let obligation;
+            match nested_goal.source() {
+                GoalSource::Misc => {
+                    continue;
+                }
+                GoalSource::ImplWhereBound => {
+                    obligation = Obligation {
+                        cause: derive_cause(
+                            tcx,
+                            candidate.kind(),
+                            self.obligation.cause.clone(),
+                            impl_where_bound_count,
+                            parent_trait_pred,
+                        ),
+                        param_env: nested_goal.goal().param_env,
+                        predicate: nested_goal.goal().predicate,
+                        recursion_depth: self.obligation.recursion_depth + 1,
+                    };
+                    impl_where_bound_count += 1;
+                }
+                GoalSource::InstantiateHigherRanked => {
+                    obligation = self.obligation.clone();
+                }
+            }
+
+            // Skip nested goals that hold.
+            //FIXME: We should change the max allowed certainty based on if we're
+            // visiting an ambiguity or error obligation.
+            if matches!(nested_goal.result(), Ok(Certainty::Yes)) {
+                continue;
+            }
+
+            self.with_derived_obligation(obligation, |this| nested_goal.visit_with(this))?;
+        }
+
+        ControlFlow::Break(self.obligation.clone())
+    }
+}
+
+fn derive_cause<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    candidate_kind: ProbeKind<'tcx>,
+    mut cause: ObligationCause<'tcx>,
+    idx: usize,
+    parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
+) -> ObligationCause<'tcx> {
+    match candidate_kind {
+        ProbeKind::TraitCandidate { source: CandidateSource::Impl(impl_def_id), result: _ } => {
+            if let Some((_, span)) =
+                tcx.predicates_of(impl_def_id).instantiate_identity(tcx).iter().nth(idx)
+            {
+                cause = cause.derived_cause(parent_trait_pred, |derived| {
+                    traits::ImplDerivedObligation(Box::new(traits::ImplDerivedObligationCause {
+                        derived,
+                        impl_or_alias_def_id: impl_def_id,
+                        impl_def_predicate_index: Some(idx),
+                        span,
+                    }))
+                })
+            }
+        }
+        ProbeKind::TraitCandidate { source: CandidateSource::BuiltinImpl(..), result: _ } => {
+            cause = cause.derived_cause(parent_trait_pred, traits::BuiltinDerivedObligation);
+        }
+        _ => {}
+    };
+    cause
 }