Add support for Swift 5.1 (5.0 impossible due to compiler bug)

[slashmo]

Since we plan on introducing Baggage as a dependency for Logging (apple/swift-log#235) we want to lower our minimum supported Swift version to 5.0 to match Logging.

[ktoso]

@yim-lee could we ask you to help us out and add 5.0 and 5.1 CI to this project? We want to match what swift-log supports, even it supporting 5.0 is a bit ridiculous at this point 😆

[yim-lee]

Added CI for 5.0 and 5.1. @swift-server-bot test this please

[slashmo]

Added CI for 5.0 and 5.1.

Thank you 🙏

[slashmo]

5.0 is failing because of what I think is a compiler bug. We have a test which contains conditionally compiled code (#if swift(>=5.5) && canImport(_Concurrency)), but it still fails to compile on 5.0:

/code/Tests/InstrumentationBaggageTests/BaggageTests.swift:81:17: error: expected numeric value following '$'
        Baggage.$current.withValue(baggage, operation: exampleFunction)
                        ^

CI Run: https://ci.swiftserver.group/job/swift-distributed-tracing-baggage-swift50-prb/4/console
Unit Test in question: https://github.com/apple/swift-distributed-tracing-baggage/blob/8ea12077aafca36aa28500adead394b7789b8f5c/Tests/InstrumentationBaggageTests/BaggageTests.swift#L66

[ktoso]

Can't seem to be able to workaround the 5.0 issue, even compiler(>=5.5) does not help. We may need to drop 5.0 support tbh.

[ktoso]

Talked a lot with the swift team about the bug here -- there does not seem to be a workaround, we can't support 5.0 basically -- even with the compiler guards it still will attempt to lex the task local but will fail doing so.

[ktoso]

Discussed this in depth with the Swift team, it is impossible for a library to support any task-local usage and "#if-out" its usage without breaking Swift 5.0, since it will still attempt and fail to lex this piece of "iffed out" code. As such, we cannot support 5.0, nor can any library which optionally (!!!) supports task-locals, because of this compiler bug in 5.0.