After what I know has been a very long wait, I have a beta version of an updated TeXLive 2020.0 bundle ready for testing. The short instructions are that you can test it with the following command-line argument:
tectonic -w https://ttassets.z13.web.core.windows.net/tlextras-2020.0r0.tar ...
I have bitten the bullet and started hosting the files off of a cloud storage that I’m paying for, rather than using free services; you should notice a significant speed increase when pulling down new files.
I recommend that you test this bundle using the just-released Tectonic version 0.3, which updates the engine implementations to TeXLive 2020.0. The bundle files should work with older versions, but the LaTeX format generation depends on some of the new primitives introduced in TL2020.0, so the internals will have substantial differences depending on whether you process documents using 0.3 or an older version. (In particular, expl3 related pieces won’t be enabled unless you upgrade.)
I have extracted the code to generate the bundle from the “staging” repository to a new tectonic-texlive-bundles repo, which should be a lot easier to understand because all of the reference-sources support is gone. I started a very primitive test suite in
tests/ to test out compilations with new versions of the bundle.
Cross-posted on GitHub as tectonic-typesetting/tectonic#669.