update new solver candidate assembly

6 files changed, 180 insertions, 89 deletions

diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
index 198ccb000f3..02f6439b77f 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -11,6 +11,7 @@ use rustc_type_ir::visit::TypeVisitableExt as _;
 use rustc_type_ir::{self as ty, Interner, TypingMode, Upcast as _, elaborate};
 use tracing::{debug, instrument};
 
+use super::trait_goals::TraitGoalProvenVia;
 use crate::delegate::SolverDelegate;
 use crate::solve::inspect::ProbeKind;
 use crate::solve::{
@@ -337,15 +338,6 @@ where
 
         self.assemble_param_env_candidates(goal, &mut candidates);
 
-        match self.typing_mode() {
-            TypingMode::Coherence => {}
-            TypingMode::Analysis { .. }
-            | TypingMode::PostBorrowckAnalysis { .. }
-            | TypingMode::PostAnalysis => {
-                self.discard_impls_shadowed_by_env(goal, &mut candidates);
-            }
-        }
-
         candidates
     }
 
@@ -500,7 +492,7 @@ where
         goal: Goal<I, G>,
         candidates: &mut Vec<Candidate<I>>,
     ) {
-        for (i, assumption) in goal.param_env.caller_bounds().into_iter().enumerate() {
+        for (i, assumption) in goal.param_env.caller_bounds().iter().enumerate() {
             candidates.extend(G::probe_and_consider_implied_clause(
                 self,
                 CandidateSource::ParamEnv(i),
@@ -733,72 +725,64 @@ where
         })
     }
 
-    /// If there's a where-bound for the current goal, do not use any impl candidates
-    /// to prove the current goal. Most importantly, if there is a where-bound which does
-    /// not specify any associated types, we do not allow normalizing the associated type
-    /// by using an impl, even if it would apply.
+    /// We sadly can't simply take all possible candidates for normalization goals
+    /// and check whether they result in the same constraints. We want to make sure
+    /// that trying to normalize an alias doesn't result in constraints which aren't
+    /// otherwise required.
+    ///
+    /// Most notably, when proving a trait goal by via a where-bound, we should not
+    /// normalize via impls which have stricter region constraints than the where-bound:
+    ///
+    /// ```rust
+    /// trait Trait<'a> {
+    ///     type Assoc;
+    /// }
+    ///
+    /// impl<'a, T: 'a> Trait<'a> for T {
+    ///     type Assoc = u32;
+    /// }
+    ///
+    /// fn with_bound<'a, T: Trait<'a>>(_value: T::Assoc) {}
+    /// ```
     ///
-    ///  <https://github.com/rust-lang/trait-system-refactor-initiative/issues/76>
-    // FIXME(@lcnr): The current structure here makes me unhappy and feels ugly. idk how
-    // to improve this however. However, this should make it fairly straightforward to refine
-    // the filtering going forward, so it seems alright-ish for now.
-    #[instrument(level = "debug", skip(self, goal))]
-    fn discard_impls_shadowed_by_env<G: GoalKind<D>>(
+    /// The where-bound of `with_bound` doesn't specify the associated type, so we would
+    /// only be able to normalize `<T as Trait<'a>>::Assoc` by using the impl. This impl
+    /// adds a `T: 'a` bound however, which would result in a region error. Given that the
+    /// user explicitly wrote that `T: Trait<'a>` holds, this is undesirable and we instead
+    /// treat the alias as rigid.
+    ///
+    /// See trait-system-refactor-initiative#124 for more details.
+    #[instrument(level = "debug", skip(self), ret)]
+    pub(super) fn merge_candidates(
         &mut self,
-        goal: Goal<I, G>,
-        candidates: &mut Vec<Candidate<I>>,
-    ) {
-        let cx = self.cx();
-        let trait_goal: Goal<I, ty::TraitPredicate<I>> =
-            goal.with(cx, goal.predicate.trait_ref(cx));
-
-        let mut trait_candidates_from_env = vec![];
-        self.probe(|_| ProbeKind::ShadowedEnvProbing).enter(|ecx| {
-            ecx.assemble_param_env_candidates(trait_goal, &mut trait_candidates_from_env);
-            ecx.assemble_alias_bound_candidates(trait_goal, &mut trait_candidates_from_env);
-        });
+        proven_via: Option<TraitGoalProvenVia>,
+        candidates: Vec<Candidate<I>>,
+    ) -> QueryResult<I> {
+        let Some(proven_via) = proven_via else {
+            // We don't care about overflow. If proving the trait goal overflowed, then
+            // it's enough to report an overflow error for that, we don't also have to
+            // overflow during normalization.
+            return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Ambiguity));
+        };
 
-        if !trait_candidates_from_env.is_empty() {
-            let trait_env_result = self.merge_candidates(trait_candidates_from_env);
-            match trait_env_result.unwrap().value.certainty {
-                // If proving the trait goal succeeds by using the env,
-                // we freely drop all impl candidates.
-                //
-                // FIXME(@lcnr): It feels like this could easily hide
-                // a forced ambiguity candidate added earlier.
-                // This feels dangerous.
-                Certainty::Yes => {
-                    candidates.retain(|c| match c.source {
-                        CandidateSource::Impl(_) | CandidateSource::BuiltinImpl(_) => {
-                            debug!(?c, "discard impl candidate");
-                            false
-                        }
-                        CandidateSource::ParamEnv(_) | CandidateSource::AliasBound => true,
-                        CandidateSource::CoherenceUnknowable => panic!("uh oh"),
-                    });
-                }
-                // If it is still ambiguous we instead just force the whole goal
-                // to be ambig and wait for inference constraints. See
-                // tests/ui/traits/next-solver/env-shadows-impls/ambig-env-no-shadow.rs
-                Certainty::Maybe(cause) => {
-                    debug!(?cause, "force ambiguity");
-                    *candidates = self.forced_ambiguity(cause).into_iter().collect();
-                }
-            }
-        }
-    }
+        let responses: Vec<_> = match proven_via {
+            // Even when a trait bound has been proven using a where-bound, we
+            // still need to consider alias-bounds for normalization, see
+            // tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
+            //
+            // FIXME(const_trait_impl): should this behavior also be used by
+            // constness checking. Doing so is *at least theoretically* breaking,
+            // see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
+            TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => candidates
+                .iter()
+                .filter(|c| {
+                    matches!(c.source, CandidateSource::AliasBound | CandidateSource::ParamEnv(_))
+                })
+                .map(|c| c.result)
+                .collect(),
+            TraitGoalProvenVia::Misc => candidates.iter().map(|c| c.result).collect(),
+        };
 
-    /// If there are multiple ways to prove a trait or projection goal, we have
-    /// to somehow try to merge the candidates into one. If that fails, we return
-    /// ambiguity.
-    #[instrument(level = "debug", skip(self), ret)]
-    pub(super) fn merge_candidates(&mut self, candidates: Vec<Candidate<I>>) -> QueryResult<I> {
-        // First try merging all candidates. This is complete and fully sound.
-        let responses = candidates.iter().map(|c| c.result).collect::<Vec<_>>();
-        if let Some(result) = self.try_merge_responses(&responses) {
-            return Ok(result);
-        } else {
-            self.flounder(&responses)
-        }
+        self.try_merge_responses(&responses).map_or_else(|| self.flounder(&responses), Ok)
     }
 }
diff --git a/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs b/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
index 81b5199002b..ce7552e30f0 100644
--- a/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
@@ -4,6 +4,7 @@
 use rustc_type_ir::fast_reject::DeepRejectCtxt;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
+use rustc_type_ir::solve::inspect::ProbeKind;
 use rustc_type_ir::{self as ty, Interner, elaborate};
 use tracing::instrument;
 
@@ -391,6 +392,11 @@ where
         goal: Goal<I, ty::HostEffectPredicate<I>>,
     ) -> QueryResult<I> {
         let candidates = self.assemble_and_evaluate_candidates(goal);
-        self.merge_candidates(candidates)
+        let (_, proven_via) = self.probe(|_| ProbeKind::ShadowedEnvProbing).enter(|ecx| {
+            let trait_goal: Goal<I, ty::TraitPredicate<I>> =
+                goal.with(ecx.cx(), goal.predicate.trait_ref);
+            ecx.compute_trait_goal(trait_goal)
+        })?;
+        self.merge_candidates(proven_via, candidates)
     }
 }
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index 70ceb22bfea..8c74490e0e0 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -440,7 +440,7 @@ where
         if let Some(kind) = kind.no_bound_vars() {
             match kind {
                 ty::PredicateKind::Clause(ty::ClauseKind::Trait(predicate)) => {
-                    self.compute_trait_goal(Goal { param_env, predicate })
+                    self.compute_trait_goal(Goal { param_env, predicate }).map(|(r, _via)| r)
                 }
                 ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(predicate)) => {
                     self.compute_host_effect_goal(Goal { param_env, predicate })
diff --git a/compiler/rustc_next_trait_solver/src/solve/mod.rs b/compiler/rustc_next_trait_solver/src/solve/mod.rs
index ebf1013db1e..37678bfd880 100644
--- a/compiler/rustc_next_trait_solver/src/solve/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/mod.rs
@@ -243,22 +243,27 @@ where
             .copied()
     }
 
+    fn bail_with_ambiguity(&mut self, responses: &[CanonicalResponse<I>]) -> CanonicalResponse<I> {
+        debug_assert!(!responses.is_empty());
+        if let Certainty::Maybe(maybe_cause) =
+            responses.iter().fold(Certainty::AMBIGUOUS, |certainty, response| {
+                certainty.unify_with(response.value.certainty)
+            })
+        {
+            self.make_ambiguous_response_no_constraints(maybe_cause)
+        } else {
+            panic!("expected flounder response to be ambiguous")
+        }
+    }
+
     /// If we fail to merge responses we flounder and return overflow or ambiguity.
     #[instrument(level = "trace", skip(self), ret)]
     fn flounder(&mut self, responses: &[CanonicalResponse<I>]) -> QueryResult<I> {
         if responses.is_empty() {
             return Err(NoSolution);
+        } else {
+            Ok(self.bail_with_ambiguity(responses))
         }
-
-        let Certainty::Maybe(maybe_cause) =
-            responses.iter().fold(Certainty::AMBIGUOUS, |certainty, response| {
-                certainty.unify_with(response.value.certainty)
-            })
-        else {
-            panic!("expected flounder response to be ambiguous")
-        };
-
-        Ok(self.make_ambiguous_response_no_constraints(maybe_cause))
     }
 
     /// Normalize a type for when it is structurally matched on.
diff --git a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
index 63dbee2640b..b8867192225 100644
--- a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
@@ -88,10 +88,17 @@ where
     /// returns `NoSolution`.
     #[instrument(level = "trace", skip(self), ret)]
     fn normalize_at_least_one_step(&mut self, goal: Goal<I, NormalizesTo<I>>) -> QueryResult<I> {
-        match goal.predicate.alias.kind(self.cx()) {
+        let cx = self.cx();
+        match goal.predicate.alias.kind(cx) {
             ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst => {
                 let candidates = self.assemble_and_evaluate_candidates(goal);
-                self.merge_candidates(candidates)
+                let (_, proven_via) =
+                    self.probe(|_| ProbeKind::ShadowedEnvProbing).enter(|ecx| {
+                        let trait_goal: Goal<I, ty::TraitPredicate<I>> =
+                            goal.with(cx, goal.predicate.alias.trait_ref(cx));
+                        ecx.compute_trait_goal(trait_goal)
+                    })?;
+                self.merge_candidates(proven_via, candidates)
             }
             ty::AliasTermKind::InherentTy => self.normalize_inherent_associated_type(goal),
             ty::AliasTermKind::OpaqueTy => self.normalize_opaque_type(goal),
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index 12df35d30b8..886cdec0345 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -5,6 +5,7 @@ use rustc_type_ir::data_structures::IndexSet;
 use rustc_type_ir::fast_reject::DeepRejectCtxt;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
+use rustc_type_ir::solve::CanonicalResponse;
 use rustc_type_ir::visit::TypeVisitableExt as _;
 use rustc_type_ir::{self as ty, Interner, TraitPredicate, TypingMode, Upcast as _, elaborate};
 use tracing::{instrument, trace};
@@ -1147,13 +1148,101 @@ where
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }
+}
+
+/// How we've proven this trait goal.
+///
+/// This is used by `NormalizesTo` goals to only normalize
+/// by using the same 'kind of candidate' we've used to prove
+/// its corresponding trait goal. Most notably, we do not
+/// normalize by using an impl if the trait goal has been
+/// proven via a `ParamEnv` candidate.
+///
+/// This is necessary to avoid unnecessary region constraints,
+/// see trait-system-refactor-initiative#125 for more details.
+#[derive(Debug, Clone, Copy)]
+pub(super) enum TraitGoalProvenVia {
+    /// We've proven the trait goal by something which is
+    /// is not a non-global where-bound or an alias-bound.
+    ///
+    /// This means we don't disable any candidates during
+    /// normalization.
+    Misc,
+    ParamEnv,
+    AliasBound,
+}
+
+impl<D, I> EvalCtxt<'_, D>
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
+{
+    pub(super) fn merge_trait_candidates(
+        &mut self,
+        goal: Goal<I, TraitPredicate<I>>,
+        candidates: Vec<Candidate<I>>,
+    ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
+        if let TypingMode::Coherence = self.typing_mode() {
+            let all_candidates: Vec<_> = candidates.into_iter().map(|c| c.result).collect();
+            return if let Some(response) = self.try_merge_responses(&all_candidates) {
+                Ok((response, Some(TraitGoalProvenVia::Misc)))
+            } else {
+                self.flounder(&all_candidates).map(|r| (r, None))
+            };
+        }
+
+        // FIXME: prefer trivial builtin impls
+
+        // If there are non-global where-bounds, prefer where-bounds
+        // (including global ones) over everything else.
+        let has_non_global_where_bounds = candidates.iter().any(|c| match c.source {
+            CandidateSource::ParamEnv(idx) => {
+                let where_bound = goal.param_env.caller_bounds().get(idx);
+                where_bound.has_bound_vars() || !where_bound.is_global()
+            }
+            _ => false,
+        });
+        if has_non_global_where_bounds {
+            let where_bounds: Vec<_> = candidates
+                .iter()
+                .filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
+                .map(|c| c.result)
+                .collect();
+
+            return if let Some(response) = self.try_merge_responses(&where_bounds) {
+                Ok((response, Some(TraitGoalProvenVia::ParamEnv)))
+            } else {
+                Ok((self.bail_with_ambiguity(&where_bounds), None))
+            };
+        }
+
+        if candidates.iter().any(|c| matches!(c.source, CandidateSource::AliasBound)) {
+            let alias_bounds: Vec<_> = candidates
+                .iter()
+                .filter(|c| matches!(c.source, CandidateSource::AliasBound))
+                .map(|c| c.result)
+                .collect();
+            return if let Some(response) = self.try_merge_responses(&alias_bounds) {
+                Ok((response, Some(TraitGoalProvenVia::AliasBound)))
+            } else {
+                Ok((self.bail_with_ambiguity(&alias_bounds), None))
+            };
+        }
+
+        let all_candidates: Vec<_> = candidates.into_iter().map(|c| c.result).collect();
+        if let Some(response) = self.try_merge_responses(&all_candidates) {
+            Ok((response, Some(TraitGoalProvenVia::Misc)))
+        } else {
+            self.flounder(&all_candidates).map(|r| (r, None))
+        }
+    }
 
     #[instrument(level = "trace", skip(self))]
     pub(super) fn compute_trait_goal(
         &mut self,
         goal: Goal<I, TraitPredicate<I>>,
-    ) -> QueryResult<I> {
+    ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
         let candidates = self.assemble_and_evaluate_candidates(goal);
-        self.merge_candidates(candidates)
+        self.merge_trait_candidates(goal, candidates)
     }
 }