Swap PredicateObligation to ThinVec

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -2,6 +2,7 @@ use std::marker::PhantomData;
 use std::mem;
 use std::ops::ControlFlow;
 
+use rustc_data_structures::thinvec::ExtractIf;
 use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::solve::{CandidateSource, GoalSource, MaybeCause};
@@ -81,7 +82,8 @@ impl<'tcx> ObligationStorage<'tcx> {
             // we get all obligations involved in the overflow. We pretty much check: if
             // we were to do another step of `select_where_possible`, which goals would
             // change.
-            self.overflowed.extend(self.pending.extract_if(|o| {
+            // FIXME: <https://github.com/Gankra/thin-vec/pull/66> is merged, this can be removed.
+            self.overflowed.extend(ExtractIf::new(&mut self.pending, |o| {
                 let goal = o.clone().into();
                 let result = <&SolverDelegate<'tcx>>::from(infcx)
                     .evaluate_root_goal(goal, GenerateProofTree::No)

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -14,6 +14,7 @@ use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, Binder, Const, GenericArgsRef, TypeVisitableExt};
+use thin_vec::ThinVec;
 use tracing::{debug, debug_span, instrument};
 
 use super::project::{self, ProjectAndUnifyResult};
@@ -28,7 +29,7 @@ use crate::traits::normalize::normalize_with_depth_to;
 use crate::traits::project::{PolyProjectionObligation, ProjectionCacheKeyExt as _};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 
-pub(crate) type PendingPredicateObligations<'tcx> = Vec<PendingPredicateObligation<'tcx>>;
+pub(crate) type PendingPredicateObligations<'tcx> = ThinVec<PendingPredicateObligation<'tcx>>;
 
 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
     /// Note that we include both the `ParamEnv` and the `Predicate`,