diff --git a/source/opt/fold.cpp b/source/opt/fold.cpp
index 2fc600f4d..e7ff18143 100644
--- a/source/opt/fold.cpp
+++ b/source/opt/fold.cpp
@@ -193,7 +193,7 @@ bool InstructionFolder::FoldInstructionInternal(opt::Instruction* inst) const {
   std::vector<const analysis::Constant*> constants =
       const_manager->GetOperandConstants(inst);
 
-  for (FoldingRule rule : GetFoldingRules().GetRulesForOpcode(opcode)) {
+  for (const FoldingRule& rule : GetFoldingRules().GetRulesForOpcode(opcode)) {
     if (rule(inst, constants)) {
       return true;
     }
@@ -230,7 +230,7 @@ uint32_t InstructionFolder::FoldScalars(
 }
 
 bool InstructionFolder::FoldBinaryIntegerOpToConstant(
-    opt::Instruction* inst, std::function<uint32_t(uint32_t)> id_map,
+    opt::Instruction* inst, const std::function<uint32_t(uint32_t)>& id_map,
     uint32_t* result) const {
   SpvOp opcode = inst->opcode();
   opt::IRContext* context = inst->context();
@@ -414,7 +414,7 @@ bool InstructionFolder::FoldBinaryIntegerOpToConstant(
 }
 
 bool InstructionFolder::FoldBinaryBooleanOpToConstant(
-    opt::Instruction* inst, std::function<uint32_t(uint32_t)> id_map,
+    opt::Instruction* inst, const std::function<uint32_t(uint32_t)>& id_map,
     uint32_t* result) const {
   SpvOp opcode = inst->opcode();
   opt::IRContext* context = inst->context();
@@ -463,7 +463,7 @@ bool InstructionFolder::FoldBinaryBooleanOpToConstant(
 }
 
 bool InstructionFolder::FoldIntegerOpToConstant(
-    opt::Instruction* inst, std::function<uint32_t(uint32_t)> id_map,
+    opt::Instruction* inst, const std::function<uint32_t(uint32_t)>& id_map,
     uint32_t* result) const {
   assert(IsFoldableOpcode(inst->opcode()) &&
          "Unhandled instruction opcode in FoldScalars");
diff --git a/source/opt/fold.h b/source/opt/fold.h
index 967e4821f..c4e0dbc20 100644
--- a/source/opt/fold.h
+++ b/source/opt/fold.h
@@ -134,24 +134,24 @@ class InstructionFolder {
   // folded, the resulting value is returned in |*result|.  Valid result types
   // for the instruction are any integer (signed or unsigned) with 32-bits or
   // less, or a boolean value.
-  bool FoldBinaryIntegerOpToConstant(opt::Instruction* inst,
-                                     std::function<uint32_t(uint32_t)> id_map,
-                                     uint32_t* result) const;
+  bool FoldBinaryIntegerOpToConstant(
+      Instruction* inst, const std::function<uint32_t(uint32_t)>& id_map,
+      uint32_t* result) const;
 
   // Returns true if |inst| is a binary operation on two boolean values, and
   // folds
   // to a constant boolean value when the ids have been replaced using |id_map|.
   // If |inst| can be folded, the result value is returned in |*result|.
-  bool FoldBinaryBooleanOpToConstant(opt::Instruction* inst,
-                                     std::function<uint32_t(uint32_t)> id_map,
-                                     uint32_t* result) const;
+  bool FoldBinaryBooleanOpToConstant(
+      Instruction* inst, const std::function<uint32_t(uint32_t)>& id_map,
+      uint32_t* result) const;
 
   // Returns true if |inst| can be folded to an constant when the ids have been
   // substituted using id_map.  If it can, the value is returned in |result|. If
   // not, |result| is unchanged.  It is assumed that not all operands are
   // constant.  Those cases are handled by |FoldScalar|.
-  bool FoldIntegerOpToConstant(opt::Instruction* inst,
-                               std::function<uint32_t(uint32_t)> id_map,
+  bool FoldIntegerOpToConstant(Instruction* inst,
+                               const std::function<uint32_t(uint32_t)>& id_map,
                                uint32_t* result) const;
 
   // Folding rules used by |FoldInstructionToConstant| and |FoldInstruction|.