Getting type metadata on non-Darwin platforms

Using Swift
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1

So now that Swift is ABI-stable on Darwin platforms, we can get all kinds of information by knowing the layout of type metadata. This is essentially what this library does.

For example, we could write a function that tells whether two generic types are the same nominal type:

func sameNominalTypes(_ lhs: Any.Type, _ rhs: Any.Type) -> Bool {
    let lhsMetadataPtr = unsafeBitCast(lhs, to: UnsafePointer<Int>.self)
    let rhsMetadataPtr = unsafeBitCast(rhs, to: UnsafePointer<Int>.self)
    return lhsMetadataPtr.successor().pointee == rhsMetadataPtr.successor().pointee
}

assert(sameNominalTypes(Array<Int>.self, Array<String>.self))
assert(!sameNominalTypes(Array<Int>.self, Optional<Int>.self))

Here we're taking advantage of the fact that metatype value is just a pointer to the type metadata, and that the nominal type descriptor pointer is stored at offset 1 in the metadata record.

So the question is can we rely on this on non-Darwin platforms, and if not, is there a better way to do this? Right now it works, but I'm not sure this won't change, since ABI stability is not guaranteed on non-Darwin platforms.
Maybe there are some functions in the Swift runtime to do this kind of querying?

2

Note this is only true of struct, enum, and foreign class metadata.

3

Thanks for pointing this out. But anyway struct is enough for me.