What is "0is" notation in Rust?

Question

As seen in this repository:

https://github.com/ReactiveX/RxRust/blob/master/src/lib.rs#L110

let gen = move |:| {
    let it = range(0is, 20is);
    //             ~~~  ~~~~
    let q   = Box::new(Decoupler::new(dtx.clone()));
    let mut map1 = Box::new(Map::new(|i : isize| {i * 10}));
    let mut map2 = Box::new(Map::new(|i : isize| {i + 2}));
    let mut iter = Box::new(IterPublisher::new(it));


    map2.subscribe(q);
    map1.subscribe(map2);
    iter.subscribe(map1);
};

(squiggly emphasis mine)

I'm trying to figure out what is after a numeral is. The Book says about literal suffixes only briefly:

Note that all number literals except the byte literal allow a type suffix, such as 57u8, and _ as a visual separator, such as 1_000.

— https://doc.rust-lang.org/book/ch03-02-data-types.html#integer-types

And the compiler (1.53) only understands a set of specific suffixes, so I could not even get the original crate built on my machine:

invalid suffix `is`
help: the suffix must be one of the numeric types (`u32`, `isize`, `f32`, etc.)

Is it some sort of archaic syntax, or is it me missing something?