`@convention(thin)` function pointers

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So this is sneakily a proposal for direct/indirect. :-) I do have some questions/concerns:

  • Does directness + ownership fully capture the calling convention for all types, and are we sure that will be true in the future?

  • Will the compiler have to diagnose types that can't be passed directly? Up until now I think that hasn't been a Sema-level (roughly, "type-checking time") concept, and maybe it's still something that can be deferred to SIL diagnostics (roughly, "flow-sensitive diagnostics that run later in compilation than type-checking"). But I do worry about people trying to use direct NonFrozenStruct outside the original module and getting weird errors from the compiler, and I myself don't know whether direct Any or indirect NSObject are valid concepts or not.

  • Having @symbolName without @cdecl worries me: I think people will start exposing Swift functions to C [even more than they already do], and have things mostly work but then cause extremely weird errors in the corner cases. (But while @cdecl isn't that much more work, it's a lot more than this…)

  • The dlsym code is only correct if both sides use the same ABI, which could be a cause of weird issues on non-stable-ABI platforms if people don't think about it. But there is an unsafeBitCast there, so maybe it's ""fine"".

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