• Checks that the lhs contains no repetition which could match an empty token tree, because then the matcher would hang indefinitely.
  • check_rhs 🔒
  • Converts a macro item into a syntax extension.
  • Expands the rules based macro defined by lhses and rhses for a given input arg.
  • Returns true if a fragment of type frag can be followed by any sort of token. We use this (among other things) as a useful approximation for when frag can be followed by a repetition like $(...)* or $(...)+. In general, these can be a bit tricky to reason about, so we adopt a conservative position that says that any fragment specifier which consumes at most one token tree can be followed by a fragment specifier (indeed, these fragments can be followed by ANYTHING without fear of future compatibility hazards).
  • 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.
  • Try expanding the macro. Returns the index of the successful arm and its named_matches if it was successful, and nothing if it failed. On failure, it’s the callers job to use track accordingly to record all errors correctly.