More efficient try!() alternative

[dtolnay]

Our generated code uses the standard try! macro heavily.

match $expr {
    Ok(val) => val,
    Err(err) => {
        return Err(From::from(err))
    }
}

I think practically none of our uses of try! rely on the From::from conversion (i.e. it resolves to the impl From<T> for T). Let's experiment with generating code that omits the conversion and see whether that improves the compile times of the generated code.

[oli-obk]

Do you want a(n allow-by-default) lint for that?

[dtolnay]

Thanks! Let's wait until we find out that this helps compile time measurably.

(Or do you mean as a way to help me find all the try! that need to be changed? I'm not worried about that, I can just replace them all and then change back the few that fail to compile.)

[oli-obk]

I meant more to not regress in the future

[dtolnay]

Testing against our json-benchmark with --bin json-benchmark --no-default-features --features parse-struct,all-files,lib-serde it looks like the more efficient try!() results in 15% faster "item-bodies checking", 25% faster "borrow checking", 2% faster debug compile overall, 0.6% smaller debug binary, and 0.2% small release binary.

[dtolnay]

Maybe the most impactful number: 7% faster cargo check time.