Future direction: Non-copyable `Span`

[stevapple]

Given that Span should be ended exactly once, and should not be shared between subprocesses or preserved after termination, it seems a potential client of the recently accepted SE-0390. With Swift 5.9+ we can mark Span as non-copyable, and only allow the operation to “borrow” the span. span.end() will consume the span. We can also add span.end() in deinit to automatically terminate the span.

Note that this is unfortunately very breaking. Haven’t thought about if there’s a way to land this safely in the 1.x period.

[ktoso]

Yeah it would be nice but I'm a bit worried about just doing it for 5.9+ for example...

Definitely worth keeping in mind so let's keep this issue open