Remove `posix_spawn*` on tvOS and watchOS?

[madsmtm]

Apple's spawn.h header marks the posix_spawn* symbols as __API_UNAVAILABLE(watchos, tvos), so we probably shouldn't expose them on those platforms.

Currently though, the standard library uses these symbols, so it seems that this is not enforced very strictly? We should ask Apple (I'll file a FB when I get the time), but in any case, the standard library will have to be updated before we can cfg-gate these away.

Split out from #5158.

[tgross35]

What's the correct way to spawn processes on these platform? Given fork+exec isn't available and posix_spawn is marked unavailable.

I think it would make sense to add #[deprecated] in any case, feel free to send that.

[madsmtm]

The correct way is "don't".

(IIUC it's actually the same story on iOS, it's just a bit less enforced there).

[tgross35]

Huh, that's rather interesting - does it actually work or do we consistently error? If it doesn't work then I wonder if we should just return an error without bothering with the call. (in std)

[madsmtm]

It works (and is useful) in some environments, like the simulator and Mac Catalyst. So I wouldn't unconditionally return an error, at least not without inspecting the state of the program first.

I haven't tested yet how it works on device (but my vague suspicion is that it does work).

[tgross35]

Maybe worth a section on platform-specific behavior in Command saying it's recommended against but may work. Something similar to https://doc.rust-lang.org/nightly/std/fs/fn.rename.html#platform-specific-behavior.

[madsmtm]

I still want to ask Apple for clarification first, but yeah, will do something like that.