Add a fallback extension none os dylibs

[rauhul]

Adds a fake extension for none os dynamic libraries to avoid a SwiftPM crash during build planning when cross compiling to none OS with a dylib product in the graph.

Fixes #7893

[rauhul]

@swift-ci test

[MaxDesiatov]

Any reason this is preferred over .so?

[rauhul]

I don't really like the idea of the extension for "macho" being "so". I think that would be kinda surprising to the user.

I was considering this default branch determining the extension based on the object file format instead, thought?

[xtremekforever]

What is the behavior if building on a non-macho platform like Linux? Would this still work?

[rauhul]

This code path is only used to plan the build of a dynamic library product for an os none triple. Actually doing this build is nonsensical because the dynamic library would require a specific dynamic loader provided by the OS and by definition, os none does not have a dynamic loader.

One alternative option is to use the object format e.g.:

switch os {
case _ where isDarwin():
    return ".dylib"
case .linux, .openbsd:
    return ".so"
case .win32:
    return ".dll"
case .wasi:
    return ".wasm"
default:
  break
}

guard let objectFormat = self.objectFormat else {
    fatalError("Cannot create dynamic libraries unknown object format.")
}

switch objectFormat {
case .coff:
    return ".coff"
case .elf:
    return ".elf"
case .macho:
    return ".macho"
case .wasm:
    return ".wasm"
case .xcoff:
    return ".xcoff"
}

[xtremekforever]

So this basically just spoofs the "dynamic library" on a none OS but doesn't actually generate anything? That's kind of weird. But if that's the case then I guess it's just arbitrary what you pick there.

[rauhul]

yeah its not a great thing to do, but fixing the underlying issue with build planning is something the swift pm team is working on

[xtremekforever]

Well, I'm okay with this if it fixes the none OS issue and if it doesn't break anything!

[dschaefer2]

I think doing this based on object format looks better.

[rauhul]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows