It's used at build time in 25 crates (of which 5 optionally, 6 directly).

Depender (with downloads and own rev deps) tectonic_dep_support version
950 1 tectonic_bridge_icu build >=0.1.0, <1
900 3 tectonic_bridge_graphite2 build >=0.1.0, <1
850 1 tectonic_bridge_freetype2 build >=0.1.0, <1
850 2 tectonic_bridge_harfbuzz build >=0.1.0, <1
850 2 tectonic_pdf_io build >=0.1.0, <1
750 2 tectonic_xetex_layout build >=0.1.0, <1