No longer possible to create syntax extensions that create syntax extensions.

[SiegeLord]

Prior to 677b7ca it used to be possible to register a macro that would itself expand into another syntax extension. Now, macros can only expand into items/expressions etc.

[SiegeLord]

I found a way to do this in an alternate way. All you do is manually insert your macro into ExtCtxt::syntax_env, and just return DummyResult::any() from the macro.