It's used at build time in 5 crates (of which 5 optionally, 4 directly).

Depender (with downloads and own rev deps) mozbuild version
1.9K 3 ohttp optional build ^0.1
1.4K 1 nss-gk-api optional build ^0.1
430 bitcoin-ohttp optional build ^0.1
370 stylo optional build ^0.1