Proposal: Grammar Working Group

[CAD97]

It has been mentioned that there are loose plans to potentially attempt this:

@eddyb

I hope we can replace the libsyntax parser with one generated from a CFG, with additional disambiguation rules on top, especially if we can easily make that incremental.

It could overlap or conflict with libsyntax2 plans. RLS would likely benefit from it, and could feed back information into it, about the delta from the last successful parse (improving error recovery).

We weren’t even planning to work on a Rust grammar for a longer while! But there seems to be some interest, and the benefits appear almost too good to be true.

We’ll have to see if performance is competitive with the hand-written parser, especially with the cost of additional features for error reporting and recovery . I expect you’ll hear more within the coming months, especially after the Rust 2018 edition release.

There seems to be a rough sentiment that GLL would be able to give the most benefit, providing both a canonical grammar and strong diagnostics.

(Pinging @eddyb)

I'd definitely be interested in helping out with this though (so long as I can resist the urge to write yet another parser generator for the project). It'd be interesting if the grammar generator chosen can support the libsyntax2 Syntax Tree format (ping @matklad) but I don't think it's necessary for a first formalism.

Note that error reporting doesn't have to be a first-pass focus; in fact, I'd argue that it shouldn't be.