The FromParam docs say:

If the conversion fails, the incoming request will be forwarded to the next matching route, if any.

Since there are no additional matching routes, this example will result in a 404 error for requests with invalid id values.

However, the actual behavior of such an endpoint when requested with a non-usize parameter is to return 422 Unprocessable Content:

#[rocket::get("/<id>")]
fn hello(id: usize) -> String {
    id.to_string()
}

#[rocket::launch]
async fn launch() -> _ {
    rocket::build()
    .mount("/", rocket::routes![
        hello,
    ])
}

Tested on Rocket commit 3bf9ef0.

Discussed in #2879