[macros] prepare generic arguments for replacement in macros

[ktoso]

Swift-syntax changes necessary of allowing the following code:

@freestanding(expression)
public macro checkGeneric_root<DAS>() = #externalMacro(module: "MacroDefinition", type: "GenericToVoidMacro")

@freestanding(expression)
public macro checkGeneric<DAS>() = #checkGeneric_root<DAS>()

I didn't quite implement it entirely right and need to add a test here as well.

Resolves rdar://122004157

[ktoso]

@swift-ci please smoke test

[ktoso]

Thanks folks! I added the notes, and a test covering the new functionality as well.

Please have a look 👀

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci Please test

[ktoso]

Thanks a lot for the review! I'll follow up shortly

[ktoso]

@swift-ci Please test

[ktoso]

swiftlang/swift#71271
@swift-ci Please test

[ahoppen]

Is this a leftover todo?

[ktoso]

Wasn't sure what to do here, but actually best might be to just .skipChildren and call it a day

[ahoppen]

Just an open question: Would it make sense to specialize macros with types that aren’t part of the second-level macro? Ie. would the following make sense?

macro gen<T>(a: T) = #externalMacro …
macro genString(a: String) = #gen<String>(a: a)

And then #genString(a: "x") would have the intermediate expansion step #gen<String>(a: "x")

---

Similarly, would it make sense to allow generic nesting in a second-level macro definition? Ie. would something like the following make sense?

macro gen<T>(a: T) = #externalMacro …
macro genArray<T>(a: [T]) = #gen<Array<T>>(a: a)

And then #genArray<Int>(a: [1]) would have the intermediate expansion step #gen<Array<Int>>(a: [1])

AFAICT neither of those are currently supported.

[ktoso]

Yeah these may be worth supporting as well -- I made a ticket for it #2476

Thanks for the thorough review Alex. I think we're good now, if CI gods are willing 😇

[ktoso]

swiftlang/swift#71271
@swift-ci Please smoke test

[ktoso]

swiftlang/swift#71271
@swift-ci Please test