It's used at build time in 25 crates (of which 5 optionally, 3 directly).

Depender (with downloads and own rev deps) tectonic_cfg_support version
700 2 tectonic_pdf_io build >=0.1.2, <1
700 1 tectonic_engine_xetex build >=0.1.2, <1
700 2 tectonic_xetex_layout build >=0.1.3, <1