Fix some possible performance issues found by clang-tidy

No obvious time differences when building AOSP, but these all seem like
reasonable changes.

1 files changed, 1 insertions, 1 deletions

diff --git a/rule.cc b/rule.cc
index 68ec7f5..6942fb5 100644
--- a/rule.cc
+++ b/rule.cc
@@ -54,7 +54,7 @@ Rule::Rule()
 }
 
 void ParseRule(Loc& loc, StringPiece line, char term,
-               function<string()> after_term_fn,
+               const function<string()> &after_term_fn,
                Rule** out_rule, RuleVarAssignment* rule_var) {
   size_t index = line.find(':');
   if (index == string::npos) {