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logicchains • today at 7:45 AM • 4 replies • view on HN

How does terrible code like this survive so long in such a key piece of infrastructure:

        let _ = self.poll_read(cx)?;
        let _ = self.poll_write(cx)?;
        let _ = self.poll_flush(cx)?;
Surely at the very least a linter should have flagged that the return values aren't handled.

Replies

fwlr • today at 8:38 AM

    let _ = …?
This is the Rust idiom for “I am intentionally ignoring this return value”. The linter would have caught

    self.poll_read()?;
and in fact one of the options the linter itself suggests in this case is exactly this “let underscore equals” idiom. (Arguably, this code exists because of the linter, not due to its absence!)

In any case, the return value is being “handled” - the question mark examines the result and breaks the loop if the result is not `Ok(…)`, ie if the call is not successful.

Intentionally ignoring the successful return value isn’t necessarily terrible, either - you could be calling the function for its side effect, and you don’t care what the specific result of that effect is, just as long as there is some effect. E.g. maybe you have a state machine, and this is the code that repeatedly drives it.

(Not coincidentally, polling is what you do to Futures, and Futures are state machines that you need to repeatedly drive…)

In conclusion, I do not think this is prima facie terrible code, nor is it an obvious bug. Async rust is subtle and complicated, and not always fully understood by those who nevertheless have to use it.

lifthrasiir • today at 8:12 AM

It is an explicit way to discard return values; `self.poll_read(cx)?` etc. alone would warn. Or in this case, `Poll<Result<(), Error>>` is unwrapped once and `Result<(), Error>` is being discarded. The decision to discard `Result<(), Error>` should have been intentional, albeit turned out to be not always the case.

➕ show 1 reply
QuantumNomad_ • today at 8:13 AM

Assigning to _ in Rust specifically means that you intentionally want to discard the value, and the clippy linter and the Rust compiler both know that.