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

  • @Solemarc
    2•9 months ago

    I’m not sure if the rules are different with macros, I’ve never written one but this lint is generally caused because you set a var to a value and then overwrite that value before you use it. e.g.

    let mut a = 1; a = 2; println!(“{}”, a);

    This will throw the same warning because 1 is never used, this could’ve just been:

    let a = 2; println!(“{}”, a);

    So first I’d double check that I NEED last at all. Maybe try:

    cargo clippy

    See if it can tell you how to fix it.

    If that doesn’t work, it’s sometimes necessary to skip certain lints. E.g. if you make a library, most of the code will be flagged as dead code because it isn’t used and you can use an #[allow(dead_code)] to stop the linter warning. You might be able to use #[allow(this_linting_rule)].

    Hope something here helps.

    • @[email protected]OPEnglish
      2•9 months ago

      Clippy didn’t tell anything about the macro.

      warning: dereferencing a tuple pattern where every element takes a reference
  --> src/lib.rs:13:9
   |
13 |         &Some(ref cons_rc) => {
   |         ^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrowed_reference
   = note: `#[warn(clippy::needless_borrowed_reference)]` on by default
help: try removing the `&` and `ref` parts
   |
13 -         &Some(ref cons_rc) => {
13 +         Some(cons_rc) => {
   |

      To put #[allow(this_linting_rule)] like this:

          [ $x:expr, $( $y:expr ),* ] => {
	#[allow(unused_assignments)]
	{

      I got error[E0658]: attributes on expressions are experimental.

      To put it like this:

      #[macro_export]
#[allow(unused_assignments)]
macro_rules! list {
    () => {
	None

      It doesn’t work.