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
650 1 tectonic_bridge_icu build >=0.1.0, <1
650 1 tectonic_bridge_freetype2 build >=0.1.0, <1
650 3 tectonic_bridge_graphite2 build >=0.1.0, <1
550 2 tectonic_pdf_io build >=0.1.0, <1
500 2 tectonic_bridge_harfbuzz build >=0.1.0, <1
500 2 tectonic_xetex_layout build >=0.1.0, <1