[swift6] Add Sendable conformance to structs and enums

[x-sheep]

Swift 6 mandates types to conform to the Sendable protocol to be used in async contexts. This PR will only add conformance when it can be proved by the compiler, or if a locking mechanism is in place.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11) @dydus0x14 (2023/06)

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[4brunu]

Hi @x-sheep, thanks for creating this PR.
So basically we can add sendable conformance to structs and enums, but not classes, am I right?

[x-sheep]

A class can only be safely marked Sendable if it's immutable, or if it handles concurrent access itself. While the generated code won't cause data races, it's possible that consumers of this code do. So they might need to add an extension with @unchecked Sendable themselves, and make sure that classes aren't concurrently modified.

I could add a case where Sendable is added to classes if the readonlyProperties config is set to true.

[4brunu]

That's a good idea.
Could you please add that?
By the way, what's your opinion on the best default value for read only properties?
Currently it's disabled by default, although I think it would be a good idea by default to enable it.
What are your thoughts on this?

[x-sheep]

In some of my projects, I do extensively mutate the structs retrieved from an external API.

I believe Swift's handling of mutating structs is robust enough. The compiler will still stop possible race conditions, even when the data is marked as Sendable. So I think the current defaults (read-only off, classes off) are suitable for the majority of cases.

[x-sheep]

I've now added Sendable conformance to classes with read-only properties.

Maybe it's a good idea to add a sample project to the repo using these settings?

[4brunu]

Maybe it's a good idea to add a sample project to the repo using these settings?

That's a good idea, but right now we have a problem that we have some many samples that sometimes CI failes due to timeout.
Could you add those configurations to some existing samples please? This way we would increate the CI times.
For example, here we test a lot of configurations.
https://github.com/OpenAPITools/openapi-generator/blob/master/bin/configs/swift6-urlsessionLibrary.yaml

[x-sheep]

I've added readonlyProperties to the urlsessionLibrary sample. The vapor sample already uses classes without readonly.

[4brunu]

Thanks for the contribution 👍