Allocate one vec, instead of one per invocation

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -14,10 +14,10 @@ use crate::infer::canonical::{
 };
 use crate::infer::nll_relate::{NormalizationStrategy, TypeRelating, TypeRelatingDelegate};
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
-use crate::infer::{InferCtxt, InferOk, InferResult, NllRegionVariableOrigin};
+use crate::infer::{InferCtxt, InferOk, InferResult, NllRegionVariableOrigin, TypeError};
 use crate::traits::query::{Fallible, NoSolution};
 use crate::traits::TraitEngine;
-use crate::traits::{Obligation, ObligationCause, PredicateObligation};
+use crate::traits::{Obligation, ObligationCause, PredicateObligation, PredicateObligations};
 use rustc_data_structures::captures::Captures;
 use rustc_index::vec::Idx;
 use rustc_index::vec::IndexVec;
@@ -219,7 +219,8 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         original_values: &OriginalQueryValues<'tcx>,
         query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
         output_query_region_constraints: &mut QueryRegionConstraints<'tcx>,
-    ) -> InferResult<'tcx, R>
+        obligations: &mut PredicateObligations<'tcx>
+    ) -> Result<R, TypeError<'tcx>>
     where
         R: Debug + TypeFoldable<'tcx>,
     {
@@ -229,7 +230,6 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         // Compute `QueryOutlivesConstraint` values that unify each of
         // the original values `v_o` that was canonicalized into a
         // variable...
-        let mut obligations = vec![];
 
         for (index, original_value) in original_values.var_values.iter().enumerate() {
             // ...with the value `v_r` of that variable from the query.
@@ -263,7 +263,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                             infcx: self,
                             param_env,
                             cause,
-                            obligations: &mut obligations,
+                            obligations: &mut *obligations,
                         },
                         ty::Variance::Invariant,
                     )
@@ -277,7 +277,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                             infcx: self,
                             param_env,
                             cause,
-                            obligations: &mut obligations,
+                            obligations: &mut *obligations,
                         },
                         ty::Variance::Invariant,
                     )
@@ -316,7 +316,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         let user_result: R =
             query_response.substitute_projected(self.tcx, &result_subst, |q_r| q_r.value.clone());
 
-        Ok(InferOk { value: user_result, obligations })
+        Ok(user_result)
     }
 
     /// Given the original values and the (canonicalized) result from

compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs

@@ -1,7 +1,7 @@
 use crate::infer::canonical::{
     Canonicalized, CanonicalizedQueryResponse, OriginalQueryValues, QueryRegionConstraints,
 };
-use crate::infer::{InferCtxt, InferOk};
+use crate::infer::{InferCtxt};
 use crate::traits::query::Fallible;
 use crate::traits::ObligationCause;
 use rustc_infer::infer::canonical::{Canonical, Certainty};
@@ -81,14 +81,14 @@ pub trait QueryTypeOp<'tcx>: fmt::Debug + Copy + TypeFoldable<'tcx> + 'tcx {
         query_key: ParamEnvAnd<'tcx, Self>,
         infcx: &InferCtxt<'_, 'tcx>,
         output_query_region_constraints: &mut QueryRegionConstraints<'tcx>,
+        obligations: &mut PredicateObligations<'tcx>,
     ) -> Fallible<(
         Self::QueryResponse,
         Option<Canonical<'tcx, ParamEnvAnd<'tcx, Self>>>,
-        PredicateObligations<'tcx>,
         Certainty,
     )> {
         if let Some(result) = QueryTypeOp::try_fast_path(infcx.tcx, &query_key) {
-            return Ok((result, None, vec![], Certainty::Proven));
+            return Ok((result, None, Certainty::Proven));
         }
 
         // FIXME(#33684) -- We need to use
@@ -101,16 +101,17 @@ pub trait QueryTypeOp<'tcx>: fmt::Debug + Copy + TypeFoldable<'tcx> + 'tcx {
             infcx.canonicalize_query_keep_static(query_key, &mut canonical_var_values);
         let canonical_result = Self::perform_query(infcx.tcx, canonical_self)?;
 
-        let InferOk { value, obligations } = infcx
+        let value = infcx
             .instantiate_nll_query_response_and_region_obligations(
                 &ObligationCause::dummy(),
                 old_param_env,
                 &canonical_var_values,
                 canonical_result,
                 output_query_region_constraints,
+                obligations,
             )?;
 
-        Ok((value, Some(canonical_self), obligations, canonical_result.value.certainty))
+        Ok((value, Some(canonical_self), canonical_result.value.certainty))
     }
 }
 
@@ -122,8 +123,9 @@ where
 
     fn fully_perform(self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
         let mut region_constraints = QueryRegionConstraints::default();
-        let (output, canonicalized_query, mut obligations, _) =
-            Q::fully_perform_into(self, infcx, &mut region_constraints)?;
+        let mut obligations = Vec::new();
+        let (output, canonicalized_query, _) =
+            Q::fully_perform_into(self, infcx, &mut region_constraints, &mut obligations)?;
 
         // Typically, instantiating NLL query results does not
         // create obligations. However, in some cases there
@@ -133,15 +135,15 @@ where
         while !obligations.is_empty() {
             trace!("{:#?}", obligations);
             let mut progress = false;
-            for obligation in std::mem::take(&mut obligations) {
-                let obligation = infcx.resolve_vars_if_possible(obligation);
+            for _ in 0..obligations.len() {
+                let obligation = infcx.resolve_vars_if_possible(obligations.swap_remove(0));
                 match ProvePredicate::fully_perform_into(
                     obligation.param_env.and(ProvePredicate::new(obligation.predicate)),
                     infcx,
                     &mut region_constraints,
+                    &mut obligations,
                 ) {
-                    Ok(((), _, new, certainty)) => {
-                        obligations.extend(new);
+                    Ok(((), _, certainty)) => {
                         progress = true;
                         if let Certainty::Ambiguous = certainty {
                             obligations.push(obligation);