The compiler doesn't execute at runtime

src/ch03-05-control-flow.md

@@ -387,9 +387,9 @@ will reach a value of `5` at some point, the loop stops executing before trying
 to fetch a sixth value from the array.
 
 But this approach is error prone; we could cause the program to panic if the
-index length is incorrect. It’s also slow, because the compiler needs to
-perform the conditional check on every element on every iteration through the
-loop.
+index length is incorrect. It’s also slow, because the compiler adds
+runtime code to perform the conditional check on every element on every
+iteration through the loop.
 
 As a more efficient alternative, you can use a `for` loop and execute some code
 for each item in a collection. A `for` loop looks like this: