I don't know which one yet, but I've seen a build_script_build invoking lib.exe. My guess is lmdb-sys. With chance, this only requires setting some environment variable in config/makefiles/rust.mk.

I was close enough. It was lmdb-rkv-sys, but also bzip2-sys.

The bad news is, AFAICT, the cc crate doesn't allow to override lib.exe from the environment.

Hm could you clarify what needs information for this? Is this a feature reuqest for cc to override lib.exe?

I guess this could be phrased like that. Would you prefer a github issue?

Ah yeah, a github issues would be good to track it!

Filed https://github.com/alexcrichton/cc-rs/issues/384

Filed https://github.com/alexcrichton/cc-rs/pull/387

This update from the official sources brings in the changes that we were
using glandium's fork for, as well as changes enabling us to tweak more
settings on Windows.

https://hg.mozilla.org/mozilla-central/rev/1b9e5f4b0589