(Almost) every `cargo` invocation rebuilds bindgen and all its downstream dependencies

I'm really not sure where to go with this one - is it a cargo issue? a bindgen issue? an issue with my local setup? If it's the last, I'm not sure how I'd begin debugging it - I haven't seen if there's anything within cargo that'll allow you to ask it to tell you why a crate is rebuilding (besides https://github.com/rust-lang/cargo/issues/2904, which doesn't really seem to give me much to go on).

All that said, I have a project that depends on rocksdb, which depends on librocksdb-sys, which uses bindgen, and having to recompile librocksdb-sys every time I run cargo test is getting to be quite a lot.

Start by reproducing the reported project setup: a project depending on rocksdb, librocksdb-sys, and bindgen, with repeated cargo test runs. Investigate why librocksdb-sys and bindgen's downstream dependencies rebuild, and review cargo issue 2904 for available rebuild diagnostics. Done means identifying whether the cause is in cargo, bindgen, or the local setup and documenting a clear reproduction or fix direction.