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Handle `macro_rules!` tokens consistently across crates
When we serialize a `macro_rules!` macro, we used a 'lowered' `TokenStream` for its body, which has all `Nonterminal`s expanded in-place via `nt_to_tokenstream`. This matters when an 'outer' `macro_rules!` macro expands to an 'inner' `macro_rules!` macro - the inner macro may use tokens captured from the 'outer' macro in its definition.
This means that invoking a foreign `macro_rules!` macro may use a different body `TokenStream` than when the same `macro_rules!` macro is invoked in the same crate. This difference is observable by proc-macros invoked by a `macro_rules!` macro - a `None`-delimited group will be seen in the same-crate case (inserted when convering `Nonterminal`s to the `proc_macro` crate's structs), but no `None`-delimited group in the cross-crate case.
To fix this inconsistency, we now insert `None`-delimited groups when 'lowering' a `Nonterminal` `macro_rules!` body, just as we do in `proc_macro_server`. Additionally, we no longer print extra spaces for `None`-delimited groups - as far as pretty-printing is concerned, they don't exist (only their contents do). This ensures that `Display` output of a `TokenStream` does not depend on which crate a `macro_rules!` macro was invoked from.
This PR is necessary in order to patch the `solana-genesis-programs` for the upcoming hygiene serialization breakage (https://github.com/rust-lang/rust/pull/72121#issuecomment-646924847). The `solana-genesis-programs` crate will need to use a proc macro to re-span certain tokens in a nested `macro_rules!`, which requires us to consistently use a `None`-delimited group.
See `src/test/ui/proc-macro/nested-macro-rules.rs` for an example of the kind of nested `macro_rules!` affected by this crate.
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