Only prefer param-env candidates if they remain non-global after norm

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

@@ -2,21 +2,24 @@
 
 pub(super) mod structural_traits;
 
+use std::ops::ControlFlow;
+
 use derive_where::derive_where;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
 use rustc_type_ir::{
-    self as ty, Interner, TypeFoldable, TypeVisitableExt as _, TypingMode, Upcast as _, elaborate,
+    self as ty, Interner, TypeFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitableExt as _,
+    TypeVisitor, TypingMode, Upcast as _, elaborate,
 };
 use tracing::{debug, instrument};
 
-use super::has_only_region_constraints;
 use super::trait_goals::TraitGoalProvenVia;
+use super::{has_only_region_constraints, inspect};
 use crate::delegate::SolverDelegate;
 use crate::solve::inspect::ProbeKind;
 use crate::solve::{
     BuiltinImplSource, CandidateSource, CanonicalResponse, Certainty, EvalCtxt, Goal, GoalSource,
-    MaybeCause, NoSolution, QueryResult,
+    MaybeCause, NoSolution, ParamEnvSource, QueryResult,
 };
 
 enum AliasBoundKind {
@@ -49,18 +52,6 @@ where
 
     fn trait_def_id(self, cx: I) -> I::DefId;
 
-    /// Try equating an assumption predicate against a goal's predicate. If it
-    /// holds, then execute the `then` callback, which should do any additional
-    /// work, then produce a response (typically by executing
-    /// [`EvalCtxt::evaluate_added_goals_and_make_canonical_response`]).
-    fn probe_and_match_goal_against_assumption(
-        ecx: &mut EvalCtxt<'_, D>,
-        source: CandidateSource<I>,
-        goal: Goal<I, Self>,
-        assumption: I::Clause,
-        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
-    ) -> Result<Candidate<I>, NoSolution>;
-
     /// Consider a clause, which consists of a "assumption" and some "requirements",
     /// to satisfy a goal. If the requirements hold, then attempt to satisfy our
     /// goal by equating it with the assumption.
@@ -119,6 +110,67 @@ where
         alias_ty: ty::AliasTy<I>,
     ) -> Vec<Candidate<I>>;
 
+    fn probe_and_consider_param_env_candidate(
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
+        assumption: I::Clause,
+    ) -> Result<Candidate<I>, NoSolution> {
+        Self::fast_reject_assumption(ecx, goal, assumption)?;
+
+        ecx.probe(|candidate: &Result<Candidate<I>, NoSolution>| match candidate {
+            Ok(candidate) => inspect::ProbeKind::TraitCandidate {
+                source: candidate.source,
+                result: Ok(candidate.result),
+            },
+            Err(NoSolution) => inspect::ProbeKind::TraitCandidate {
+                source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
+                result: Err(NoSolution),
+            },
+        })
+        .enter(|ecx| {
+            Self::match_assumption(ecx, goal, assumption)?;
+            let source = ecx.characterize_param_env_assumption(goal.param_env, assumption)?;
+            Ok(Candidate {
+                source,
+                result: ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)?,
+            })
+        })
+    }
+
+    /// Try equating an assumption predicate against a goal's predicate. If it
+    /// holds, then execute the `then` callback, which should do any additional
+    /// work, then produce a response (typically by executing
+    /// [`EvalCtxt::evaluate_added_goals_and_make_canonical_response`]).
+    fn probe_and_match_goal_against_assumption(
+        ecx: &mut EvalCtxt<'_, D>,
+        source: CandidateSource<I>,
+        goal: Goal<I, Self>,
+        assumption: I::Clause,
+        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
+    ) -> Result<Candidate<I>, NoSolution> {
+        Self::fast_reject_assumption(ecx, goal, assumption)?;
+
+        ecx.probe_trait_candidate(source).enter(|ecx| {
+            Self::match_assumption(ecx, goal, assumption)?;
+            then(ecx)
+        })
+    }
+
+    /// Try to reject the assumption based off of simple heuristics, such as [`ty::ClauseKind`]
+    /// and [`I::DefId`].
+    fn fast_reject_assumption(
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
+        assumption: I::Clause,
+    ) -> Result<(), NoSolution>;
+
+    /// Relate the goal and assumption.
+    fn match_assumption(
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
+        assumption: I::Clause,
+    ) -> Result<(), NoSolution>;
+
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
@@ -500,14 +552,8 @@ where
         goal: Goal<I, G>,
         candidates: &mut Vec<Candidate<I>>,
     ) {
-        for (i, assumption) in goal.param_env.caller_bounds().iter().enumerate() {
-            candidates.extend(G::probe_and_consider_implied_clause(
-                self,
-                CandidateSource::ParamEnv(i),
-                goal,
-                assumption,
-                [],
-            ));
+        for assumption in goal.param_env.caller_bounds().iter() {
+            candidates.extend(G::probe_and_consider_param_env_candidate(self, goal, assumption));
         }
     }
 
@@ -943,4 +989,76 @@ where
             }
         }
     }
+
+    fn characterize_param_env_assumption(
+        &mut self,
+        param_env: I::ParamEnv,
+        assumption: I::Clause,
+    ) -> Result<CandidateSource<I>, NoSolution> {
+        // FIXME:
+        if assumption.has_bound_vars() {
+            return Ok(CandidateSource::ParamEnv(ParamEnvSource::NonGlobal));
+        }
+
+        match assumption.visit_with(&mut FindParamInClause { ecx: self, param_env }) {
+            ControlFlow::Break(Err(NoSolution)) => Err(NoSolution),
+            ControlFlow::Break(Ok(())) => Ok(CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)),
+            ControlFlow::Continue(()) => Ok(CandidateSource::ParamEnv(ParamEnvSource::Global)),
+        }
+    }
+}
+
+struct FindParamInClause<'a, 'b, D: SolverDelegate<Interner = I>, I: Interner> {
+    ecx: &'a mut EvalCtxt<'b, D>,
+    param_env: I::ParamEnv,
+}
+
+impl<D, I> TypeVisitor<I> for FindParamInClause<'_, '_, D, I>
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
+{
+    type Result = ControlFlow<Result<(), NoSolution>>;
+
+    fn visit_binder<T: TypeFoldable<I>>(&mut self, t: &ty::Binder<I, T>) -> Self::Result {
+        self.ecx.enter_forall(t.clone(), |ecx, v| {
+            v.visit_with(&mut FindParamInClause { ecx, param_env: self.param_env })
+        })
+    }
+
+    fn visit_ty(&mut self, ty: I::Ty) -> Self::Result {
+        let Ok(ty) = self.ecx.structurally_normalize_ty(self.param_env, ty) else {
+            return ControlFlow::Break(Err(NoSolution));
+        };
+        let ty = self.ecx.eager_resolve(ty);
+
+        if let ty::Placeholder(_) = ty.kind() {
+            ControlFlow::Break(Ok(()))
+        } else {
+            ty.super_visit_with(self)
+        }
+    }
+
+    fn visit_const(&mut self, ct: I::Const) -> Self::Result {
+        let Ok(ct) = self.ecx.structurally_normalize_const(self.param_env, ct) else {
+            return ControlFlow::Break(Err(NoSolution));
+        };
+        let ct = self.ecx.eager_resolve(ct);
+
+        if let ty::ConstKind::Placeholder(_) = ct.kind() {
+            ControlFlow::Break(Ok(()))
+        } else {
+            ct.super_visit_with(self)
+        }
+    }
+
+    fn visit_region(&mut self, r: I::Region) -> Self::Result {
+        match r.kind() {
+            ty::ReStatic | ty::ReError(_) => ControlFlow::Continue(()),
+            ty::ReVar(_) | ty::RePlaceholder(_) => ControlFlow::Break(Ok(())),
+            ty::ReErased | ty::ReEarlyParam(_) | ty::ReLateParam(_) | ty::ReBound(..) => {
+                unreachable!()
+            }
+        }
+    }
 }