Features on Dependencies

[psionic-k]

We have a dependency that we pass through features to.

nats = ["other-crate/nats"]

This doesn't pass through and instead we must change the dependency when doing a production build:

other-crate = {version = "0.1.0", features = ["nats"]

other-crate is a central repo that contains a lot of code and exposes features to cut the compile times. The leaf services use that central crate as a dependency and expose those same features for CLI usage to switch between full and mocked implementations. This style is just a productivity concern.

During container builds, we always enable the same feature set with no variability. We could pass through the feature through extra cargo args, but that might collide with some other dependency's features.

I'm thinking patching our top level cargo.toml might be the simplest approach. During development, we typically don't want complex default features, which are a pain to deactivate via CLI, but we could tolerate something that activates them directly on the dependency during the Nix build.

[ipetkov]

Typically cargo only enables feature flags on the top-level crates being built, so something like cargo build -p foo --features bar would effectively turn on foo's bar feature (but if foo depends on baz, it would need to "forward" its own bar feature to baz/bar).

Might be something to try if you want to be more targeted with which features are being enabled. Alternatively you could try naming your features something different/unique to minimize colliding with other features used across other workspace dependencies

[psionic-k]

it would need to "forward" its own bar feature to baz/bar

This is what I'm doing with cargo.

Ah, now I remember. I had turned on the feature in the build command. However, that obviously won't propagate into the dependents. The question becomes how to activate bar on foo where bar is not selected in the Cargo.toml and must be specified on the command line.

If that's not possible, we can add bar to default features and just deal with it in the CLI, passing --no-default-features when building without Nix.

[ipetkov]

You can add an optional dependency on an-otherwise-transitive-package for the sake of forwarding features:

[features]
nats = ["my-dep/nats"]

[dependencies]
# Not used directly, but declared here for feature forwarding
my-dep = { version = "*", optional = true }

[psionic-k]

That's what I'm doing, passing through features. The slight problem is that the features must be turned on in the Cargo.toml and can't be specified at evaluation time.

By default, features like "prod" are off. The most common usage is local development. If I could specify this in Nix for evaluation time only, it would work with that default.

[ipetkov]

The slight problem is that the features must be turned on in the Cargo.toml and can't be specified at evaluation time.

This is the part I'm not sure I'm following, what needs to know feature flags at evaluation time? Unless by that you mean while building inside of a Nix derivation? In which case I'd suggest setting cargoExtraArgs = "--features nats"; which would allow you to turn on non-default features