What Slice Definitions Should swift:attribute be Allowed on?

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Swift supports a swift:attribute metadata, for adding attributes into the generated code. But, there's a major inconsistency in it.

The code-gen checks for this metadata on struct, but the validation rejects it on structs.
So, if you put swift:attribute on a struct, the validation removes it and warns you, even though the code-gen could of used it.

And the code-gen ignores this metadata on interface, but the validation allows it on interface.
So, if you put swift:attribute on an interface, the validation is happy, but it does absolutely nothing.

The documentation for this metadata says it's only supported on class, exception, and enum?
There is no mention of either struct or interface.

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I'm not familiar with Swift, but only allowing it on class, exception, and enum feels like a weird list.
Should this be supported on struct?, on interface?, on anything else?