Make Whatever thread-safe by default (add markers `Send + Sync`)

[zardini123]

Allows Whatever's to be sent between threads by making them thread-safe by marking the source std::error::Error with Send + Sync.

For more discussion on why, see issue #446.

Resolves #446.

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[failable]

News?

[Enet4]

Well, since my comment on https://github.com/shepmaster/snafu/issues/446#issuecomment-2033809122, no one has expressed concern over this change, but this also doesn't mean that someone out there is depending on Whatever not having to be Send or Sync.

At some point I hope we can move forward with this, since it does seem desirable and makes the error type more usable. It will imply a semver bump in any case.

[Stargateur]

Still think sync is not needed

[failable]

So can you share the best practices when using snafu in async Rust? Thanks!

[Yevgnen]

Is it recommended to use Whatever in an async context or multi-threaded environment?

[JakubKoralewski]

would an additional WhateverAsync be possible so no semver bump would be needed?

[shepmaster]

an additional WhateverAsync be possible

Yep! You can do this today by copy-pasting the Whatever type into your code and modifying it. Stealing from this PR:

// You pick if you want `+ Send`, `+ Sync`, both, or neither! Change it in all 3 places.

#[derive(Debug, Snafu)]
#[snafu(whatever)]
#[snafu(display("{message}"))]
#[snafu(provide(opt, ref, chain, dyn std::error::Error + Send + Sync => source.as_deref()))]
pub struct WhateverAsync {
    #[snafu(source(from(Box<dyn std::error::Error + Send + Sync>, Some)))]
    #[snafu(provide(false))]
    source: Option<Box<dyn std::error::Error + Send + Sync>>,
    message: String,
    backtrace: Backtrace,
}