What does [1] do over [2] ? I looked at the readme of [1] but it's just a fork of [2], so I'm not sure why [2] needs a rewrite? Is [1] just a rewrite of [2] in Rust?
Tectonic[2] is the Rust wrapper around C and C++ code of XeTeX. The fork rewrote everything, also aims to split it into separate crates that might be reused elsewhere, or to use some external crate instead of the own reimplementation.