Removing last will break my library.

#[macro_export]
macro_rules! list {
    () => {
	None
    };
    [ $x:expr, $( $y:expr ),* ] => {
	{
	    let mut first = cons($x, &None);
	    let mut last = &mut first;
	    $(
		let yet_another = cons($y, &None);
		if let Some(ref mut last_inner) = last {
		    let last_mut = Rc::get_mut(last_inner).unwrap();
		    last_mut.cdr = yet_another;
		    last = &mut last_mut.cdr;
		}
	    )*
	    first
	}
    }
}

This macro works as I expected because it can pass these tests.

    #[test]
    fn dolist() {
        let mut v = vec![];
        dolist!((i &cons(10, &list![20, 30, 40])) {
            v.push(i.car);
        });
        assert_eq!(v, vec![10, 20, 30, 40]);
    }

    #[test]
    fn turn_list_to_vec() {
        assert_eq!(list_to_vec(&list![1, 2, 3]), vec![1, 2, 3]);
    }

    #[test]
    fn count_elements() {
        assert_eq!(list_len(&list![10, 20, 30]), 3);
    }

However I got the warning “value assigned to last is never read.”

How can I avoid this warning?

P.S. Full code

  • sugar_in_your_tea
    4·
    10 months ago

    A hack would be to do a simple noop read, like:

    if last.is_some() {}
first

    The problem seems to be the last last = last_mut.cdr;, since you don’t use it on the last iteration.

    I’d personally do that with a comment until you (or someone else) can fix it, perhaps with a linter instruction at some point. That noop code should be eliminated during the optimization step, so it shouldn’t hurt performance or anything.

    Edit: don’t do this, just prefix the variable with an underscore as someone else mentioned.