do not attempt to prove unknowable goals

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

@@ -304,6 +304,11 @@ where
 
         let mut candidates = vec![];
 
+        if self.solver_mode() == SolverMode::Coherence {
+            if let Ok(candidate) = self.consider_coherence_unknowable_candidate(goal) {
+                return vec![candidate];
+            }
+        }
         self.assemble_impl_candidates(goal, &mut candidates);
 
         self.assemble_builtin_impl_candidates(goal, &mut candidates);
@@ -314,11 +319,8 @@ where
 
         self.assemble_param_env_candidates(goal, &mut candidates);
 
-        match self.solver_mode() {
-            SolverMode::Normal => self.discard_impls_shadowed_by_env(goal, &mut candidates),
-            SolverMode::Coherence => {
-                self.assemble_coherence_unknowable_candidates(goal, &mut candidates)
-            }
+        if self.solver_mode() == SolverMode::Normal {
+            self.discard_impls_shadowed_by_env(goal, &mut candidates);
         }
 
         candidates
@@ -682,38 +684,34 @@ where
     /// also consider impls which may get added in a downstream or sibling crate
     /// or which an upstream impl may add in a minor release.
     ///
-    /// To do so we add an ambiguous candidate in case such an unknown impl could
-    /// apply to the current goal.
+    /// To do so we return a single ambiguous candidate in case such an unknown
+    /// impl could apply to the current goal.
     #[instrument(level = "trace", skip_all)]
-    fn assemble_coherence_unknowable_candidates<G: GoalKind<D>>(
+    fn consider_coherence_unknowable_candidate<G: GoalKind<D>>(
         &mut self,
         goal: Goal<I, G>,
-        candidates: &mut Vec<Candidate<I>>,
-    ) {
-        let cx = self.cx();
-
-        candidates.extend(self.probe_trait_candidate(CandidateSource::CoherenceUnknowable).enter(
-            |ecx| {
-                let trait_ref = goal.predicate.trait_ref(cx);
-                if ecx.trait_ref_is_knowable(goal.param_env, trait_ref)? {
-                    Err(NoSolution)
-                } else {
-                    // While the trait bound itself may be unknowable, we may be able to
-                    // prove that a super trait is not implemented. For this, we recursively
-                    // prove the super trait bounds of the current goal.
-                    //
-                    // We skip the goal itself as that one would cycle.
-                    let predicate: I::Predicate = trait_ref.upcast(cx);
-                    ecx.add_goals(
-                        GoalSource::Misc,
-                        elaborate::elaborate(cx, [predicate])
-                            .skip(1)
-                            .map(|predicate| goal.with(cx, predicate)),
-                    );
-                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                }
-            },
-        ))
+    ) -> Result<Candidate<I>, NoSolution> {
+        self.probe_trait_candidate(CandidateSource::CoherenceUnknowable).enter(|ecx| {
+            let cx = ecx.cx();
+            let trait_ref = goal.predicate.trait_ref(cx);
+            if ecx.trait_ref_is_knowable(goal.param_env, trait_ref)? {
+                Err(NoSolution)
+            } else {
+                // While the trait bound itself may be unknowable, we may be able to
+                // prove that a super trait is not implemented. For this, we recursively
+                // prove the super trait bounds of the current goal.
+                //
+                // We skip the goal itself as that one would cycle.
+                let predicate: I::Predicate = trait_ref.upcast(cx);
+                ecx.add_goals(
+                    GoalSource::Misc,
+                    elaborate::elaborate(cx, [predicate])
+                        .skip(1)
+                        .map(|predicate| goal.with(cx, predicate)),
+                );
+                ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+            }
+        })
     }
 
     /// If there's a where-bound for the current goal, do not use any impl candidates

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -29,6 +29,7 @@ use crate::infer::outlives::env::OutlivesEnvironment;
 use crate::infer::InferOk;
 use crate::solve::inspect::{InspectGoal, ProofTreeInferCtxtExt, ProofTreeVisitor};
 use crate::solve::{deeply_normalize_for_diagnostics, inspect};
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::{
     util, FulfillmentErrorCode, NormalizeExt, Obligation, ObligationCause, PredicateObligation,
@@ -624,14 +625,13 @@ impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a, 'tcx> {
         // at ambiguous goals, as for others the coherence unknowable candidate
         // was irrelevant.
         match goal.result() {
-            Ok(Certainty::Maybe(_)) => {}
             Ok(Certainty::Yes) | Err(NoSolution) => return,
+            Ok(Certainty::Maybe(_)) => {}
         }
 
-        let Goal { param_env, predicate } = goal.goal();
-
         // For bound predicates we simply call `infcx.enter_forall`
         // and then prove the resulting predicate as a nested goal.
+        let Goal { param_env, predicate } = goal.goal();
         let trait_ref = match predicate.kind().no_bound_vars() {
             Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(tr))) => tr.trait_ref,
             Some(ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)))
@@ -645,7 +645,11 @@ impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a, 'tcx> {
             _ => return,
         };
 
-        // Add ambiguity causes for reservation impls.
+        if trait_ref.references_error() {
+            return;
+        }
+
+        let mut candidates = goal.candidates();
         for cand in goal.candidates() {
             if let inspect::ProbeKind::TraitCandidate {
                 source: CandidateSource::Impl(def_id),
@@ -664,78 +668,68 @@ impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a, 'tcx> {
             }
         }
 
-        // Add ambiguity causes for unknowable goals.
-        let mut ambiguity_cause = None;
-        for cand in goal.candidates() {
-            if let inspect::ProbeKind::TraitCandidate {
-                source: CandidateSource::CoherenceUnknowable,
-                result: Ok(_),
-            } = cand.kind()
-            {
-                let lazily_normalize_ty = |mut ty: Ty<'tcx>| {
-                    if matches!(ty.kind(), ty::Alias(..)) {
-                        let ocx = ObligationCtxt::new(infcx);
-                        ty = ocx
-                            .structurally_normalize(&ObligationCause::dummy(), param_env, ty)
-                            .map_err(|_| ())?;
-                        if !ocx.select_where_possible().is_empty() {
-                            return Err(());
-                        }
-                    }
-                    Ok(ty)
-                };
+        // We also look for unknowable candidates. In case a goal is unknowable, there's
+        // always exactly 1 candidate.
+        let Some(cand) = candidates.pop() else {
+            return;
+        };
 
-                infcx.probe(|_| {
-                    match trait_ref_is_knowable(infcx, trait_ref, lazily_normalize_ty) {
-                        Err(()) => {}
-                        Ok(Ok(())) => warn!("expected an unknowable trait ref: {trait_ref:?}"),
-                        Ok(Err(conflict)) => {
-                            if !trait_ref.references_error() {
-                                // Normalize the trait ref for diagnostics, ignoring any errors if this fails.
-                                let trait_ref =
-                                    deeply_normalize_for_diagnostics(infcx, param_env, trait_ref);
-
-                                let self_ty = trait_ref.self_ty();
-                                let self_ty = self_ty.has_concrete_skeleton().then(|| self_ty);
-                                ambiguity_cause = Some(match conflict {
-                                    Conflict::Upstream => {
-                                        IntercrateAmbiguityCause::UpstreamCrateUpdate {
-                                            trait_ref,
-                                            self_ty,
-                                        }
-                                    }
-                                    Conflict::Downstream => {
-                                        IntercrateAmbiguityCause::DownstreamCrate {
-                                            trait_ref,
-                                            self_ty,
-                                        }
-                                    }
-                                });
-                            }
-                        }
-                    }
-                })
-            } else {
-                match cand.result() {
-                    // We only add an ambiguity cause if the goal would otherwise
-                    // result in an error.
-                    //
-                    // FIXME: While this matches the behavior of the
-                    // old solver, it is not the only way in which the unknowable
-                    // candidates *weaken* coherence, they can also force otherwise
-                    // successful normalization to be ambiguous.
-                    Ok(Certainty::Maybe(_) | Certainty::Yes) => {
-                        ambiguity_cause = None;
-                        break;
-                    }
-                    Err(NoSolution) => continue,
+        let inspect::ProbeKind::TraitCandidate {
+            source: CandidateSource::CoherenceUnknowable,
+            result: Ok(_),
+        } = cand.kind()
+        else {
+            return;
+        };
+
+        let lazily_normalize_ty = |mut ty: Ty<'tcx>| {
+            if matches!(ty.kind(), ty::Alias(..)) {
+                let ocx = ObligationCtxt::new(infcx);
+                ty = ocx
+                    .structurally_normalize(&ObligationCause::dummy(), param_env, ty)
+                    .map_err(|_| ())?;
+                if !ocx.select_where_possible().is_empty() {
+                    return Err(());
                 }
             }
-        }
+            Ok(ty)
+        };
 
-        if let Some(ambiguity_cause) = ambiguity_cause {
-            self.causes.insert(ambiguity_cause);
-        }
+        infcx.probe(|_| {
+            let conflict = match trait_ref_is_knowable(infcx, trait_ref, lazily_normalize_ty) {
+                Err(()) => return,
+                Ok(Ok(())) => {
+                    warn!("expected an unknowable trait ref: {trait_ref:?}");
+                    return;
+                }
+                Ok(Err(conflict)) => conflict,
+            };
+
+            // It is only relevant that a goal is unknowable if it would have otherwise
+            // failed.
+            let non_intercrate_infcx = infcx.fork_with_intercrate(false);
+            if non_intercrate_infcx.predicate_may_hold(&Obligation::new(
+                infcx.tcx,
+                ObligationCause::dummy(),
+                param_env,
+                predicate,
+            )) {
+                return;
+            }
+
+            // Normalize the trait ref for diagnostics, ignoring any errors if this fails.
+            let trait_ref = deeply_normalize_for_diagnostics(infcx, param_env, trait_ref);
+            let self_ty = trait_ref.self_ty();
+            let self_ty = self_ty.has_concrete_skeleton().then(|| self_ty);
+            self.causes.insert(match conflict {
+                Conflict::Upstream => {
+                    IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_ref, self_ty }
+                }
+                Conflict::Downstream => {
+                    IntercrateAmbiguityCause::DownstreamCrate { trait_ref, self_ty }
+                }
+            });
+        });
     }
 }
 

compiler/rustc_type_ir/src/solve/mod.rs

@@ -58,7 +58,7 @@ pub enum Reveal {
     All,
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum SolverMode {
     /// Ordinary trait solving, using everywhere except for coherence.
     Normal,

tests/ui/coherence/normalize-for-errors.next.stderr

@@ -7,7 +7,7 @@ LL |
 LL | impl<S: Iterator> MyTrait<S> for (Box<<(MyType,) as Mirror>::Assoc>, S::Item) {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(Box<(MyType,)>, <_ as Iterator>::Item)`
    |
-   = note: upstream crates may add a new impl of trait `std::clone::Clone` for type `(MyType,)` in future versions
+   = note: upstream crates may add a new impl of trait `std::clone::Clone` for type `std::boxed::Box<(MyType,)>` in future versions
    = note: upstream crates may add a new impl of trait `std::marker::Copy` for type `std::boxed::Box<(MyType,)>` in future versions
 
 error: aborting due to 1 previous error

tests/ui/coherence/normalize-for-errors.rs

@@ -18,6 +18,6 @@ impl<S: Iterator> MyTrait<S> for (Box<<(MyType,) as Mirror>::Assoc>, S::Item) {}
 //~^ ERROR conflicting implementations of trait `MyTrait<_>` for type `(Box<(MyType,)>,
 //~| NOTE conflicting implementation for `(Box<(MyType,)>,
 //~| NOTE upstream crates may add a new impl of trait `std::marker::Copy` for type `std::boxed::Box<(MyType,)>` in future versions
-//[next]~| NOTE upstream crates may add a new impl of trait `std::clone::Clone` for type `(MyType,)` in future versions
+//[next]~| NOTE upstream crates may add a new impl of trait `std::clone::Clone` for type `std::boxed::Box<(MyType,)>` in future versions
 
 fn main() {}