`i` is just getting closed over, it's an integer filter is from the standard lib apply is defined as fn apply<A, B, C, F, G>(mut f: F, a: A) -> impl FnMu

`i` is just getting closed over, it's an integer filter is from the standard lib apply is defined as fn apply<A, B, C, F, G>(mut f: F, a: A) -> impl FnMut(&B) -> C where F: FnMut(&B) -> G, G: FnMut(A) -> C, A: Clone { move |b| f(b)(a.clone()) } second comes from https://github.com/Stebalien/tool-rs/blob/master/src/sequence.rs#L30-L33 https://github.com/Stebalien/tool-rs/blob/master/src/sequenc... which means you have to track down its implementation somewhere in macros