For the record, thanks a lot for doing this. I find it distasteful that someone publishes a *-sys package pretending that actually does not use the system library.

Ideally https://docs.rs/system-deps/latest/system_deps/ would make even more streamlined dealing with this kind of dependencies, once this land it can be iterated over.

Rebased on master.

Another option is the solution adopted in zstd-rs: a feature to opt-in to pkg-config, otherwise build the local source by default. That's probably my current preference, mainly my use cases prefer this approach.

I definitely don't want the linkage decision to depend on the whether or not a pkg-config query succeeds.

The system_deps solution is interesting: it would change the default to using pkg-config, and it uses an env var to select a internal build. The env var could be put into the config.toml of a project that wants an internal build, but that feels slightly less discoverable, given how most of the other crates work.

Any of those approaches could work.

If there is an opt-in feature, we would carry a downstream patch in Fedora’s rust-lz4-sys patch to make the pkg-config dependency and query unconditional (because distribution packages always need to link the system libraries). That’s feasible, and we often have to carry that kind of patch in -sys packages.

If pkg-config is the default and an environment variable is used to select an internal lz4, that’s OK for us because the default is what we want. The problem with environment variables, though, is that they have to be set in everything that directly or indirectly depends on the crate, so if the default were using the internal lz4, we would again be patching our rust-lz4-sys package to use pkg-config unconditionally.