Thanks. Is there any code to look at? (Couldn't easily find anything relevant on that repo…) I didn't mention it earlier, but I'm collecting a list of alternative TeX implementations[1] so if there's even (say) a dozen lines of WEB that have been converted to Rust, I'd be very eager to take a look and compare — it would be illuminating.

[1]: https://tex.stackexchange.com/questions/507846

Edit: To answer my own question. The relevant source (corresponding to the main part of xetex.web) is here: https://github.com/crlf0710/tectonic/blob/2580c55/engine/src... I've posted a comparison of some example code first from the official xetex.web listing, and then from the “Rust” code here: https://gist.github.com/shreevatsa/627399d0150e66d211a264bc0...

You can draw your own conclusions from comparing the code samples, but to point out a few obvious differences:

• What has the symbolic name “fi_or_else” in the WEB code has become the magic number “108” in the Rust code. (This is because the author of this project decided to have their Rust code start from the autogenerated C code which has already lost this symbolic information.)

• What is simply “if tracing_ifs>0” in the WEB code is 29 lines of Rust code, involving a magic offset into eqtb.

• The comments from the original are gone.

• Something like “cur_if:=subtype(p);” becomes “cur_if = (*mem.offset(p as isize)).b16.s0 as small_number;”.

I wonder how maintainable such Rust code will be. These problems are not insurmountable and the code can always be cleaned up later I guess… my point is simply that at the moment it is not idiomatic Rust code, for instance.