> (This is not real Rust code. Quick challenge for the curious Rustacean, can you explain why we cannot rewrite the above code like this, even if we import all of the symbols?)

Um, you can:

        #![feature(import_trait_associated_functions)]
        use Iterator::{collect, map, filter};
        
        fn get_ids2(data: Vec<usize>) -> Vec<usize> {
            collect(map(filter(<[_]>::iter(&data), |v| ...), |v| ...))
        }

and you can because it's lazy, which is also the same reason you can write it the other way.. in rust. I think the author was getting at an ownership trap, but that trap is avoided the same way for both arrangements, the call order is the same in both arrangements. If the calls were actually a pipeline (if collect didn't exist and didn't need to be called) then other considerations show up.