Essence parser in Rust about vip HOT 3 OPEN

@niklasdewally mentioned tree-sitter as a good candidate for this project.

https://tree-sitter.github.io/tree-sitter/creating-parsers

From what I have seen it looks very good!

We do want the parser to interleave name-resolution, type-checking, category checking etc as well for the best possible error messages. Tree-sitter seems to work well with syntax highlighting, and LSP-like functionality which is great.

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This may be a bad suggestion, but I wonder if we could do "error-driven development"? In my experience, getting a parse which produces the correct answer for correct inputs isn't too hard, but getting good error messages out is very hard, and also hard to do afterwards, when your parser is already written.

The difficulty (as I understand it) is choosing when to 'commit' to a partial parse, else your parser ends up saying "I tried to parse this whole document, and I failed".

An early version of conjure used to almost always print (I think?) 'unexpected g' whenever it was given invalid input, and this made using Conjure very difficult.

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I can talk about the 'unexpected g' fame at lenght if people are interested :)

On the bright side Chris, we now have a good bunch of well-formed and otherwise Essence files. And we can compare the error messages to those of Conjure's existing parser. So I think it'll be a bit easier this time, given a good parsing library.

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