we only merge candidates for trait and normalizes-to goals

3 files changed, 48 insertions, 69 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 231fb3e2964..5e08c3a03d8 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -959,36 +959,23 @@ where
                 // 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`.
-                let candidates_from_env_and_bounds: Vec<_> = self
+                let mut candidates: Vec<_> = self
                     .assemble_and_evaluate_candidates(goal, AssembleCandidatesFrom::EnvAndBounds);
 
                 // We still need to prefer where-bounds over alias-bounds however.
                 // See `tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs`.
-                let mut considered_candidates: Vec<_> = if candidates_from_env_and_bounds
-                    .iter()
-                    .any(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
-                {
-                    candidates_from_env_and_bounds
-                        .into_iter()
-                        .filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
-                        .map(|c| c.result)
-                        .collect()
-                } else {
-                    candidates_from_env_and_bounds.into_iter().map(|c| c.result).collect()
-                };
-
-                // If the trait goal has been proven by using the environment, we want to treat
-                // aliases as rigid if there are no applicable projection bounds in the environment.
-                if considered_candidates.is_empty() {
-                    if let Ok(response) = inject_normalize_to_rigid_candidate(self) {
-                        considered_candidates.push(response);
-                    }
+                if candidates.iter().any(|c| matches!(c.source, CandidateSource::ParamEnv(_))) {
+                    candidates.retain(|c| matches!(c.source, CandidateSource::ParamEnv(_)));
+                } else if candidates.is_empty() {
+                    // If the trait goal has been proven by using the environment, we want to treat
+                    // aliases as rigid if there are no applicable projection bounds in the environment.
+                    return inject_normalize_to_rigid_candidate(self);
                 }
 
-                if let Some(response) = self.try_merge_responses(&considered_candidates) {
+                if let Some(response) = self.try_merge_candidates(&candidates) {
                     Ok(response)
                 } else {
-                    self.flounder(&considered_candidates)
+                    self.flounder(&candidates)
                 }
             }
             TraitGoalProvenVia::Misc => {
@@ -998,11 +985,9 @@ where
                 // Prefer "orphaned" param-env normalization predicates, which are used
                 // (for example, and ideally only) when proving item bounds for an impl.
                 let candidates_from_env: Vec<_> = candidates
-                    .iter()
-                    .filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
-                    .map(|c| c.result)
+                    .extract_if(.., |c| matches!(c.source, CandidateSource::ParamEnv(_)))
                     .collect();
-                if let Some(response) = self.try_merge_responses(&candidates_from_env) {
+                if let Some(response) = self.try_merge_candidates(&candidates_from_env) {
                     return Ok(response);
                 }
 
@@ -1012,12 +997,10 @@ where
                 // means we can just ignore inference constraints and don't have to special-case
                 // constraining the normalized-to `term`.
                 self.filter_specialized_impls(AllowInferenceConstraints::Yes, &mut candidates);
-
-                let responses: Vec<_> = candidates.iter().map(|c| c.result).collect();
-                if let Some(response) = self.try_merge_responses(&responses) {
+                if let Some(response) = self.try_merge_candidates(&candidates) {
                     Ok(response)
                 } else {
-                    self.flounder(&responses)
+                    self.flounder(&candidates)
                 }
             }
         }
diff --git a/compiler/rustc_next_trait_solver/src/solve/mod.rs b/compiler/rustc_next_trait_solver/src/solve/mod.rs
index aec9594b834..2feebe270a6 100644
--- a/compiler/rustc_next_trait_solver/src/solve/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/mod.rs
@@ -29,6 +29,7 @@ use tracing::instrument;
 
 pub use self::eval_ctxt::{EvalCtxt, GenerateProofTree, SolverDelegateEvalExt};
 use crate::delegate::SolverDelegate;
+use crate::solve::assembly::Candidate;
 
 /// How many fixpoint iterations we should attempt inside of the solver before bailing
 /// with overflow.
@@ -244,50 +245,51 @@ where
     ///
     /// In this case we tend to flounder and return ambiguity by calling `[EvalCtxt::flounder]`.
     #[instrument(level = "trace", skip(self), ret)]
-    fn try_merge_responses(
+    fn try_merge_candidates(
         &mut self,
-        responses: &[CanonicalResponse<I>],
+        candidates: &[Candidate<I>],
     ) -> Option<CanonicalResponse<I>> {
-        if responses.is_empty() {
+        if candidates.is_empty() {
             return None;
         }
 
-        let one = responses[0];
-        if responses[1..].iter().all(|&resp| resp == one) {
+        let one: CanonicalResponse<I> = candidates[0].result;
+        if candidates[1..].iter().all(|candidate| candidate.result == one) {
             return Some(one);
         }
 
-        responses
+        candidates
             .iter()
-            .find(|response| {
-                response.value.certainty == Certainty::Yes
-                    && has_no_inference_or_external_constraints(**response)
+            .find(|candidate| {
+                candidate.result.value.certainty == Certainty::Yes
+                    && has_no_inference_or_external_constraints(candidate.result)
             })
-            .copied()
+            .map(|candidate| candidate.result)
     }
 
-    fn bail_with_ambiguity(&mut self, responses: &[CanonicalResponse<I>]) -> CanonicalResponse<I> {
-        debug_assert!(responses.len() > 1);
-        let maybe_cause = responses.iter().fold(MaybeCause::Ambiguity, |maybe_cause, response| {
-            // Pull down the certainty of `Certainty::Yes` to ambiguity when combining
-            // these responses, b/c we're combining more than one response and this we
-            // don't know which one applies.
-            let candidate = match response.value.certainty {
-                Certainty::Yes => MaybeCause::Ambiguity,
-                Certainty::Maybe(candidate) => candidate,
-            };
-            maybe_cause.or(candidate)
-        });
+    fn bail_with_ambiguity(&mut self, candidates: &[Candidate<I>]) -> CanonicalResponse<I> {
+        debug_assert!(candidates.len() > 1);
+        let maybe_cause =
+            candidates.iter().fold(MaybeCause::Ambiguity, |maybe_cause, candidates| {
+                // Pull down the certainty of `Certainty::Yes` to ambiguity when combining
+                // these responses, b/c we're combining more than one response and this we
+                // don't know which one applies.
+                let candidate = match candidates.result.value.certainty {
+                    Certainty::Yes => MaybeCause::Ambiguity,
+                    Certainty::Maybe(candidate) => candidate,
+                };
+                maybe_cause.or(candidate)
+            });
         self.make_ambiguous_response_no_constraints(maybe_cause)
     }
 
     /// 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() {
+    fn flounder(&mut self, candidates: &[Candidate<I>]) -> QueryResult<I> {
+        if candidates.is_empty() {
             return Err(NoSolution);
         } else {
-            Ok(self.bail_with_ambiguity(responses))
+            Ok(self.bail_with_ambiguity(candidates))
         }
     }
 
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 f760c2c07ff..60bae738e61 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -1346,11 +1346,10 @@ where
         mut 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) {
+            return if let Some(response) = self.try_merge_candidates(&candidates) {
                 Ok((response, Some(TraitGoalProvenVia::Misc)))
             } else {
-                self.flounder(&all_candidates).map(|r| (r, None))
+                self.flounder(&candidates).map(|r| (r, None))
             };
         }
 
@@ -1375,11 +1374,9 @@ where
             .any(|c| matches!(c.source, CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)));
         if has_non_global_where_bounds {
             let where_bounds: Vec<_> = candidates
-                .iter()
-                .filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
-                .map(|c| c.result)
+                .extract_if(.., |c| matches!(c.source, CandidateSource::ParamEnv(_)))
                 .collect();
-            return if let Some(response) = self.try_merge_responses(&where_bounds) {
+            return if let Some(response) = self.try_merge_candidates(&where_bounds) {
                 Ok((response, Some(TraitGoalProvenVia::ParamEnv)))
             } else {
                 Ok((self.bail_with_ambiguity(&where_bounds), None))
@@ -1388,11 +1385,9 @@ where
 
         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)
+                .extract_if(.., |c| matches!(c.source, CandidateSource::AliasBound))
                 .collect();
-            return if let Some(response) = self.try_merge_responses(&alias_bounds) {
+            return if let Some(response) = self.try_merge_candidates(&alias_bounds) {
                 Ok((response, Some(TraitGoalProvenVia::AliasBound)))
             } else {
                 Ok((self.bail_with_ambiguity(&alias_bounds), None))
@@ -1417,11 +1412,10 @@ where
             TraitGoalProvenVia::Misc
         };
 
-        let all_candidates: Vec<_> = candidates.into_iter().map(|c| c.result).collect();
-        if let Some(response) = self.try_merge_responses(&all_candidates) {
+        if let Some(response) = self.try_merge_candidates(&candidates) {
             Ok((response, Some(proven_via)))
         } else {
-            self.flounder(&all_candidates).map(|r| (r, None))
+            self.flounder(&candidates).map(|r| (r, None))
         }
     }