Consider turning on NonisolatedNonsendingByDefault

[Cyberbeni]

I am currently testing this upcoming feature and noticed that having dependencies without this option enabled can cause #SendingRisksDataRace errors that require you to bubble up the @concurrent annotation in those call chains. If I understand it correctly, having this turned on won't cause any issues to consumers of the package.

Sadly it seems to not work correctly for local functions, so those have to be manually annotated. (2 places in the current codebase, might also need some #if compiler checks since this feature is Swift 6.2+ only. I only have 6.2.3 installed, so I didn't check this.)

I don't know yet if there are benefits other than not blocking the consumers of the package to use this upcoming feature themselves.

Edit: We should also benchmark it before commiting to turning it on.

[sliemeobn]

thanks for the issue, it is definitely something to consider. I also saw your forum post - it is funky if local functions are not included in the changed default.

as you wrote, since it is a 6.2+ feature and it may have subtle implications I want to be rather cautious here. I see no reason to rush into this - do you have an actual use case for this?

[Cyberbeni]

Any possible performance gain from this is pretty much just micro-optimization, so that aspect is not important at all.

Since this is part of "Approachable Concurrency", I thought that enabling it would help people, who never used Swift, trying to extend the functionality with some super simple code not get smacked with a cryptic error that they can't easily understand/resolve. (like this one: #79) But it seems like in this case where dependencies are not using this flag, it's even harmful. I also haven't found a case where this prevents cryptic errors.