#[must_use] instead of drop_bomb? about lang HOT 1 CLOSED

I only realized while I was typing up a reply to you that this wouldn’t quite work.

Let’s start from the simplest case – suppose we’ve just written p.start() somewhere randomly. With drop_bomb we get a panic at runtime, while with #[must_use] we get a warning telling us that the Marker value can’t just be discarded like that.

Next, suppose we’ve created a marker with let m = p.start(), but forget to call m.complete(p, kind) – with drop_bomb we again get a panic, but with #[must_use] we get a warning about m being an unused local. We would’ve gotten the same warning even without #[must_use], because from the compiler’s perspective the marker is “used” by it being stored in that m local, regardless of whether the local is ever accessed.

In this scenario, had we actually used m at some point (and never called Marker::complete), we wouldn’t get a warning with #[must_use] where we would’ve panicked with drop_bomb. In some complex scenarios you can actually end up needing to pass markers around to grammar functions, so it isn’t like this is a hypothetical. I’ve never had to do this in any of the parsers I’ve written, but it can happen.

Also there’s no need to apologize, I’m happy to see people are still finding this useful :D

from lang.