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

Depender (with downloads and own rev deps) mozbuild version
1.8K 3 ohttp optional build ^0.1
1.0K 1 nss-gk-api optional build ^0.1
350 stylo optional build ^0.1
320 bitcoin-ohttp optional build ^0.1