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

Depender (with downloads and own rev deps) mozbuild version
3.9K 1 mtu optional build ^0.1
1.9K 3 stylo optional build ^0.1
1.7K 2 ohttp optional build ^0.1
1.0K 3 bitcoin-ohttp optional build ^0.1
250 1 nss-gk-api optional build ^0.1