Should JoinGuard be must_use?

[shepmaster]

I’ve seen the same problem with using thread::scoped show up on IRC, Stack Overflow, even in the docs! The problem? Inserting a semicolon at the end of the call:

let guards: Vec<_> = (0..3).map(|_| {
    thread::scoped(|| {}); // Oh no!
}).collect();

sellibitze suggested that JoinGuard should be marked as #[must_use] - what do people think about this change?

[seanmonstar]

twenty characters

[huon]

It is already must_use:

rust-lang/rust/blob/cfea8ec41699e25c8fb524d625190f0cb860dc71/src/libstd/thread.rs#L638-L640

[huon]

(I suppose we should use must_use's optional message to provide more info, e.g. #[must_use = "code will not run in parallel ifJoinGuardis immediately dropped"]. The exact text could be better than my scratch example.)

[shepmaster]

It is already must_use:

Wow. Now what I want to know is why I didn’t look for that first, or notice those warnings when helping people. Sigh.

I suppose we should use must_use’s optional message to provide more info

I think I might just do that. Thanks!

[system]

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