--- src/earley/perf.rs
+++ src/earley/perf.rs
@@ -143,3 +143,3 @@
     for (i, c) in s.chars().enumerate() {
         // Insert a comma once we've passed 'offset' and every 3 digits after that.
-        if i != 0 && i >= offset && (i - offset).is_multiple_of(3) {
+        if i != 0 && i >= offset && (i - offset) % 3 == 0 {
             result.push(',');
         }