Enable Cargo feature in BUILD.bazel rust_test

[antjori]

Hey everyone!

I have a Cargo.toml file where I've defined a Cargo feature:

[features]
my-feature: []

I'm trying to enable this feature but only for an integration test without much success. I was expecting the following to work:

rust_library(
    name = "my-lib",
    srcs = [
        (...)
    ],
    deps = [
         (...)
    ],
)

rust_test(
    name = "integration-test",
    srcs = glob(["tests/**"]),
    crate_features = [
        "my-feature",
    ],
    deps = [
        ":my-lib",
    ],
)

If I add the crate_features attribute to the rust_library though, it works. Unfortunately, by doing it, the feature becomes available to the entire binary which is not intended.

What am I missing?

[scentini]

What is the error you're seeing when building what you spelled out above?

[antjori]

No error whatsoever. Only the Cargo feature isn't applied when the Bazel tests run. Whereas, if I move the attribute crate_features to the rust_library def and run the Bazel tests, the feature is taken into account.

[illicitonion]

This is working as intended - crate_feature applies to the specific code it's compiling and nothing else.

If you want to achieve what you're trying to do, you'd need a second copy of the rust_library which enables the feature, and to have the rust_test depend on that.

(Alternatively, you could potentially use a transition to alter the rust_library, but the effect is the same - effectively splitting the rust_library into two targets)

[antjori]

Thank you for your suggestion!

That was exactly what I tried to do but without any luck. That's why I opened this discussion.

Since the second copy of the rust_library cannot have the same name as the original one, and since the rust_test imports dependencies that reference the original rust_library, the rustc complains saying I should reference the second copy of the rust_library. But if I change the import dependencies from the original lib to the second copy, then the rustc complains as well because, for all that matters, the second copy's name isn't defined as being the lib.

I also tried creating the second copy of the rust_library with only the name and crate_features attributes, and then I'd add both libraries as deps of the rust_test, but Bazel complains saying that no source file was found in the second copy definition. I'm assuming the srcs attribute is required even though the documentation says it is Optional.

Is there a way to make both coexist given the details mentioned above?

[illicitonion]

You can use http://bazelbuild.github.io/rules_rust/flatten.html#rust_library-crate_name to change the name to be the same, but you're going to need to ensure somehow that a binary/test doesn't try to depend on both versions of the rust_library, or you'll end up with link-errors because of duplicate symbols.

[antjori]

Thank you so much, it worked! The test only depends on the secondary copy of the rust_library so no errors occurred. It was just that crate_name piece that was missing! 🙌🏼