Let grammars be defined in Rust's item positions (not only in functions)

[pczarn]

Allow top-level grammar definitions that can be used anywhere. For example,

grammar! MyGrammar {
    // ...
}

Then, parsers can be instantiated.

fn foo() {
    let parser = MyGrammar::new_parser();
    // ...
}

This won't work with stable syntax extensions, or at least with their first version.

[pczarn]

I think grammars in item positions shouldn't be cut off from all variables. Their semantic actions should have access to common values akin to self-passing. To this end, Rust code including variable definitions could be mixed with grammar rules.