"threads" not being used

The first warning states that the “threads” field is not being used. It looks used to me. am I wrong ?

You assign to it but you never use it.

Isn’t it being used on lines 10 and 17 ?

The unused warning is on the field of ThreadPool at line 3, the threads local variable at line 10 is a totally different thing.
And at line 17 you assign the local variable threads to the struct field but you never access it, that’s why the compiler give you a warning.

I guess the tutorial from the rust book hasn’t gotten to that step yet. probably will use it in main.rs