The title is a bit misleading.

LaTeX is a macro package written using the TeX macro system, so I doubt that LaTeX itself is being rewritten in Rust. TeX, created by Donald Knuth, is now over 40 years old. It has a number of remarkable characteristics:

* TeX is essentially bug free. It can be a challenging program to use, but in my experience I was always consoled by the knowledge that if something wasn't working it was my fault (in contrast to my use of other document processing systems and word processors). See [1].

* The TeX program is extraordinarily well documented. Knuth has an entire book which is the literate program comprises TeX. See [2].

* TeX was originally written in a version of Pascal, the source code was written using a literate style and processed using tools the Knuth wrote (Tangle and Weave). This was later machine translated into C.

* TeX hasn't changed. It reached version 3 in 1989 and Knuth essentially froze it at this point. One of its great values is that documents written in TeX will always produce the same output, whether written last century, this century, or next century. We will always be able to see what the original author wanted.

* TeX is unusually capable. Knuth is obviously a perfectionist. Unhappy with the state of digital typography when he wrote TeX he designed an entire toolchain for creating fonts call MetaFont. I used it to design my company's first unique logo back in the 90's. (I only needed five characters.) See [3]. Knuth did this before Postscipt was invented.

* TeX is public domain. Before open source was a thing, Knuth put TeX in the public domain.

* TeX is the Lingua Franca used by Mathematicians, Physicists, Computer Scientists and others for books, academic papers, and even presentations; nothing else comes close.

* TeX has a number of widely used forks, notably XeTeX and LuaTeX. XeTeX added Unicode support and LuaTeX is an attempt to make TeX and it's companion LaTeX more easily programmable. See [4] and [5].

* TeX has a powerful macro system. This is how packages are added to TeX. The most widely used such package is LaTeX itself. LaTeX is not written in C or Pascal, it is written as a complex set of macros.

* Tex's macro system is great for its intended purpose, but not so great for how it is currently used. Knuth originally used the macros to define layouts of books and papers. Things like line spacing font choice for page numbers and such can be easily encapsulated as TeX macros. TeX's macro system is powerful enough that even complex packages like LaTeX could be written in it. Today, there are packages like TikZ/Pgf that are very sophisticated and complex programs written entirely in TeX macros. TikZ/Pgf is a powerful graphics, drawing, and even animation system. It's manual is 1271 pages long. See [6]

Where does this leave the originally linked project called Tectonic? It's not obvious from the github repository, but the user web site is a bit more informative, see [7]. This "rewrite" seems to rely on XeTeX and provides some command line operations that streamline the invocation of XeTeX. I don't understand its value proposition.

For me, what's really needed is a better language than TeX's macro system for writing extensions. A few years ago I was writing TeX code (macros) to format spine labels for the books in my personal library. I used the number of pages in the book to adjust the size and layout of the call number and 2D barcode printed on the label. All of this was quite challenging in TeX's macro system, so I wrote a simple Python program that generated the LaTeX output that resulted in a page of labels that were easy to print. Maybe I could have done it in LuaTeX using Lua, but at the time LuaTeX was so poorly documented (how ironic) that I couldn't figure out how to do it.

The adoption of some programming language that could guarantee backward compatibility indefinitely and a software package written in it that was compatible with LaTeX2e or LaTeX3 and replaced them. This language should not be TeX's macro system. The right language, perhaps something like ML or Scheme, would make programmed typesetting so much easier.

[1] https://onlinelibrary.wiley.com/doi/abs/10.1002/spe.43801907..., behind a paywall but copies are easily available.

[2] https://www-cs-faculty.stanford.edu/~knuth/abcde.html

[3] https://en.wikipedia.org/wiki/Metafont

[4] https://en.wikipedia.org/wiki/XeTeX

[5] https://en.wikipedia.org/wiki/LuaTeX

[6] http://mirrors.ibiblio.org/CTAN/graphics/pgf/base/doc/pgfman...

[7] https://tectonic-typesetting.github.io/en-US/

See Also:

* Wikipedia entry for TeX: https://en.wikipedia.org/wiki/TeX

* Wikipedia entry for MetaFont: https://en.wikipedia.org/wiki/Metafont

* Wikipedia entry for LaTeX: https://en.wikipedia.org/wiki/LaTeX