Function rustc_expand::mbe::macro_rules::is_in_follow

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fn is_in_follow(tok: &TokenTree, kind: NonterminalKind) -> IsInFollow
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Returns true if frag can legally be followed by the token tok. For fragments that can consume an unbounded number of tokens, tok must be within a well-defined follow set. This is intended to guarantee future compatibility: for example, without this rule, if we expanded expr to include a new binary operator, we might break macros that were relying on that binary operator as a separator.