Because hello_world returns an IronResult, which in turn is just a simple Result type from Rust. Note that the status::Ok is different from the Ok(...).
Iron::new expects a function that returns a Result type, so it has to be there.
Iron::new expects a function that returns a Result type, so it has to be there.
let _ = something_returning_result()
The fact that you must acknowledge the existence of the error value before accessing the meaty success value (either with a match, an unwrap, or a try) is a great feature in Rust.
must_use forcing you to handle errors for things you don't need the meaty success value of is just icing on the cake.
If the caller does not want to use the success value (esp. in cases which return Result<(),E>, i.e. return an optional error), there is a lint which tries to ensure that the error value is handled, though as steve says you can circumvent it (which is done in an explicit way so that it's pretty obvious that there is an error being ignored).