For a somewhat more "productive" hello world (as in a very simple application which does useful things), there's nh3 (https://github.com/messense/nh3) which is a relatively small wrapper around ammonia (https://github.com/rust-ammonia/ammonia), an HTML sanitisation library.

The lib.rs is all of 125 SLOC (and about half of that is owing to the handling of `attribute_filter` which is pretty demanding on the glue layer: https://github.com/messense/nh3/commit/72be3e6728b7ceb9185e7...).