build: Allow specifying RUST_PROTOBUF_SRC_PROTOC env variable

[ParkMyCar]

This PR allows users to specify a path to an existing instance of protoc, which will skip building the vendored version. This is useful for build processes where we don't want to use the build script (e.g. Bazel) or for processes that want to provide their own version of protoc.

[benesch]

I don’t think this is the right approach! The point of the protobuf-src crate is to build the vendored copy. If someone wants to opt out I think that should happen at a higher level. Ie, don’t depend on protobuf-src.

[ParkMyCar]

I don’t think this is the right approach! The point of the protobuf-src crate is to build the vendored copy. If someone wants to opt out I think that should happen at a higher level. Ie, don’t depend on protobuf-src.

I get that! But I think this change is the only way to handle the scenario where for local dev you want to build a vendored version of protoc and maybe for CI or some other process you rely on an already built version. An alternative option would be in all of the crates that depend on protobuf-src you could make it an optional dependency and in your build script have something like:

#[cfg(feature = "protobuf-src")]
{
    std::env::set_var("PROTOC", protobuf_src::protoc());
}

but even this approach doesn't totally work since a transitive dependency could pull in protobuf-src and require it get built.

[ParkMyCar]

Hmmmm wait, another thought I just had is I could make a wrapping crate, e.g. mz-protoc which includes all of this logic in its own build script. Still doesn't get around the transitive dependency issue though, but maybe that doesn't matter in practice 🤔

Edit: Never mind this wouldn't work, because even when building mz-protoc you would still need to build protobuf-src since it's a dependency. I don't think there's anyway to skip builing a dependency based on an env var

[benesch]

Just noting for posterity that we chatted about this on Slack and I believe we found another viable path!

[ParkMyCar]

Closing out because we found a different path!