Better engine detection

[Neved4]

LaTeX3 has some convenient tools for package writers. In l3kernel -> l3sys we have \c_sys_engine_str, that holds the name of the engine (luatex, pdftex, xetex... etc); and \sys_if_engine_xetex_p:, a boolean that is true on XeTeX-based engines.
In LaTeX2 world, people resort to the iftex package.

As tectonic evolves, it'll differentiate itself from XeTeX. Having a way to detect XeTeX-based is nice, and I'm not aware if we have a way of detecting tectonic unequivocally. That would come handy for situations where writing tectonic-specific conditional code is needed.

Part of this is outside the project direct control: both LaTeX3 and other packages would need to add options to detect tectonic, but we could keep this issue to keep tabs on those.

Internally the question is about how we could bridge that detection of tectonic: an internal macro only present when compiled with the engine? That would suffice for developers, regardless of other packages adding tectonic to their features list.

If suitable, I'm open to converting this into a Discussion.

[pkgw]

Yup, this is something to think about! I have a couple of quick comments. For the foreseeable future, I think that engine selectors like \c_sys_engine_str should report whatever they do for XeTeX: I don't want to force package authors to have to write the equivalent of \ifxetex || \iftectonic everywhere.

Also, Tectonic does provide a new primitive called \TectonicCodaTokens that could be used to implement a hypothetical \iftectonic.