Commits on Mar 27, 2021

1. Always preserve None-delimited groups in a captured TokenStream …

   Previously, we would silently remove any `None`-delimiters when
   capturing a `TokenStream`, 'flattenting' them to their inner tokens.
   This was not normally visible, since we usually have
   `TokenKind::Interpolated` (which gets converted to a `None`-delimited
   group during macro invocation) instead of an actual `None`-delimited
   group.
   
   However, there are a couple of cases where this becomes visible to
   proc-macros:
   1. A cross-crate `macro_rules!` macro has a `None`-delimited group
      stored in its body (as a result of being produced by another
      `macro_rules!` macro). The cross-crate `macro_rules!` invocation
      can then expand to an attribute macro invocation, which needs
      to be able to see the `None`-delimited group.
   2. A proc-macro can invoke an attribute proc-macro with its re-collected
      input. If there are any nonterminals present in the input, they will
      get re-collected to `None`-delimited groups, which will then get
      captured as part of the attribute macro invocation.
   
   Both of these cases are incredibly obscure, so there hopefully won't be
   any breakage. This change will allow more agressive 'flattenting' of
   nonterminals in rust-lang#82608 without losing `None`-delimited groups.

   

   Aaron1011 committed Mar 27, 2021
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