How to properly do one dependency with two sets of features?

[luciusmagn]

We have a library, which has feature A.

One crate that depends on it does not need feature A, and so it is not enabled.

A second crate does need the feature on the library, and so it is enabled.

Our current solution in Bazel is:

• Define the bazel rust_library target twice, one has the feature enabled and one doesn't, naming the one that does my_crate_feature
• On the first crate, we add //path/to/my_crate to deps
• On the second crate, we add //path/to/my_crate:my_crate_feature to deps
• And also add { "//path/to/my_crate:my_crate_feature" : "my_crate" } to aliases.

This works, but I have an inkling feeling that we are doing it wrong.

Is this the correct way? How else could we tackle this scenario?

Thanks for any advice,

[scentini]

I believe there is no better way to do this.

If we are talking about 2 separate builds that use different features, you could define a build setting and then select the right set of features based on what value you've passed to the build setting on the command line. This might make sense if you have some common theme for using features, eg if you're trying to do code size optimizations you could have a --//my_project:optimized_code_size=True and then select the correct set of features for every relevant crate. Having a separate build setting for each crate probably doesn't make sense though, because you'd need to specify many of those on the command line when bazel building.

If this is a single build in which you want your first crate to depend on the library with feature A enabled, and the second crate to not enable it, it is not possible to do that. Note that Cargo also doesn't support this - it will collect the union of the enabled features and build your library once, with A enabled: https://doc.rust-lang.org/cargo/reference/features.html#feature-unification