tuples containing #[must_use] types should be #[must_use]

[cramertj]

fn main() {
    Ok::<(), ()>(());
}

gives a warning, but

fn main() {
    (Ok::<(), ()>(()),);
}

does not.

[Centril]

@joshtriplett suggested that we test this out with crater to see if this would generate much substantial noise and how common it would be.

[cramertj]

I'd expect it'd be a pretty substantial source of warnings, but that's exactly why I think we should introduce it. People compiling with #![deny(warnings)] are asking for these sorts of bugs to be caught, and we should serve them appropriately ;)

In any case, running crater would require having an implementation to do a crater run on, so let's do it!

[scottmcm]

Let's do it. (I'd personally even like an attribute to allow people to opt-in to this for Option<T>, for example, when T is itself must_use. That way if I make a Vec<Result<..>>, I'll get a warning on pop that I normally wouldn't want but plausibly do now that I'm getting Results in the Options.)

[varkor]

I've opened a pull request to try this out: #61100.

[Centril]

We discussed this on the language team meeting; folks felt generally positive, but we'd like to do a crater run to see what the impact would be like and if there would be many false positives.