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When a `macro_rules!` macro expands to another `macro_rules!` macro, we
may see `None`-delimited groups in odd places when another crate
deserializes the 'inner' macro. This commit 'unwraps' an outer
`None`-delimited group to avoid breaking existing code.
See https://github.com/rust-lang/rust/pull/73569#issuecomment-650860457
for more details.
The proper fix is to handle `None`-delimited groups systematically
throughout the parser, but that will require significant work. In the
meantime, this hack lets us fix important hygiene bugs in macros
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