Reuse allocation for Vec<Candidate>

Author: tesuji

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -58,9 +58,9 @@ impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
         }
 
         let ccx = ConstCx::new(tcx, body);
-        let (mut temps, all_candidates) = collect_temps_and_candidates(&ccx);
+        let (mut temps, mut all_candidates) = collect_temps_and_candidates(&ccx);
 
-        let promotable_candidates = validate_candidates(&ccx, &mut temps, &all_candidates);
+        let promotable_candidates = validate_candidates(&ccx, &mut temps, &mut all_candidates);
 
         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
         self.promoted_fragments.set(promoted);
@@ -688,18 +688,15 @@ impl<'tcx> Validator<'_, 'tcx> {
     }
 }
 
-fn validate_candidates(
+fn validate_candidates<'a>(
     ccx: &ConstCx<'_, '_>,
     temps: &mut IndexSlice<Local, TempState>,
-    candidates: &[Candidate],
-) -> Vec<Candidate> {
+    candidates: &'a mut Vec<Candidate>,
+) -> &'a [Candidate] {
     let mut validator = Validator { ccx, temps, promotion_safe_blocks: None };
 
-    candidates
-        .iter()
-        .copied()
-        .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
-        .collect()
+    candidates.retain(|&candidate| validator.validate_candidate(candidate).is_ok());
+    &candidates[..]
 }
 
 struct Promoter<'a, 'tcx> {
@@ -965,7 +962,7 @@ fn promote_candidates<'tcx>(
     body: &mut Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     mut temps: IndexVec<Local, TempState>,
-    candidates: Vec<Candidate>,
+    candidates: &[Candidate],
 ) -> IndexVec<Promoted, Body<'tcx>> {
     // Visit candidates in reverse, in case they're nested.
     debug!(promote_candidates = ?candidates);
@@ -978,7 +975,7 @@ fn promote_candidates<'tcx>(
     let mut promotions = IndexVec::new();
 
     let mut extra_statements = vec![];
-    for candidate in candidates.into_iter().rev() {
+    for candidate in candidates.iter().copied().rev() {
         let Location { block, statement_index } = candidate.location;
         if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
         {