The `macro_rules!` parser was written to match the series of rules using
the macros-by-example (MBE) engine and a hand-written equivalent of the
left-hand side of a MBE macro. This was complex to read, difficult to
extend, and produced confusing error messages. Because it was using the
MBE engine, any parse failure would be reported as if some macro was
being applied to the `macro_rules!` invocation itself; for instance,
errors would talk about "macro invocation", "macro arguments", and
"macro call", when they were actually about the macro *definition*.
And in practice, the `macro_rules!` parser only used the MBE engine to
extract the left-hand side and right-hand side of each rule as a token
tree, and then parsed the rest using a separate parser.
Rewrite it to parse the series of rules using a simple loop, instead.
This makes it more extensible in the future, and improves error
messages. For instance, omitting a semicolon between rules will result
in "expected `;`" and "unexpected token", rather than the confusing "no
rules expected this token in macro call".
This work was greatly aided by pair programming with Vincenzo Palazzo
and Eric Holk.
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