hello.rs

fn main() {
  do RoutedServer::serve |config, router| {
    config.listen("127.0.0.1", 1337);

    router.get("/hello", |_,res| res.write("hello world"));

    router.get("/posts/:id", |req,res| {
      res.write(format!("post {}", req.params["id"]))
    });
  }
}

I think he's just confused by (inconsistent?) rules for semicolon usage, i.e. no trailing semicolon on lines 3 & 8, but there is one on lines 5 & 9.

No semicolon changes it from a statement to an expression. Leaving off the semicolon means you want to return the value of the expression.

"foo"
return "foo";

The convention is to only use return of you want an early exit.

Line 3 was a bug. Line 8 probably should have a semicolon but write() returns () so it doesn't matter.