Implicit features out of dependencies

[ignatenkobrain]

Let's take rand's Cargo.toml, it has

[dependencies]
rustc-serialize = { version = "0.3", optional = true }
serde = { version = "1.0.16", optional = true }
rand = { version = "0.3", optional = true }
sha1 = { version = "0.4", optional = true }
md5 = { version = "0.3", optional = true }

[dev-dependencies]
serde_test = "1.0.19"

[features]
std = []
use_std = ["std"] # deprecated, equivalent to `std`
v1 = ["rand"]
v3 = ["md5"]
v4 = ["rand"]
v5 = ["sha1"]

So strictly speaking it doesn't have serde and rustc-serialize" features, right?

There is tool called stratisd which has

[dependencies.uuid]
version = "0.5"
features = ["serde", "v4"]

in it. Cargo doesn't reject this, but happily passes that feature (serde) and code in uuid handles that just fine.

Does cargo automatically create feature for each dependency (only optional ones?, only non-dev/non-build?), is it expected or should we force people to explicitly specify them? What are the rules here?

This came out of my RPM packaging efforts where we do Provides for each feature crate provides and Requires for everything what is mentioned on the other side. So in this case we get unsatisfied requirement.

[KodrAus]

Does cargo automatically create feature for each dependency

That's right, cargo will create a feature for each optional dependency in your Cargo.toml. So the set of features available to uuid as per the file above are:

• rustc-serialize
• serde
• rand
• sha1
• md5
• std
• use_std
• v1
• v2
• v3
• v4
• v5

Once a dependency has been added to a Cargo.toml file its name can't be used as a feature anymore, even if that dependency is required. Once a dependency is made optional then its name becomes a feature, and specifying that feature will include the dependency.

[ignatenkobrain]

@KodrAus but... why? what's the reason for this? Is it something what it is supposed to do or accidental bug which many developers use?

I can't find any docs on this, tbh.

[KodrAus]

It's definitely intentional, I'm not actually sure where it first came about though. It's a convenient way to toggle optional dependencies when running a build, but unfortunately means you can't just look at the set of features in the manifest to find the ones available to the crate.

You could use a library like cargo_metadata to parse a manifest and collect the set of features and optional dependencies:

extern crate cargo_metadata;

fn main() {
    let metadata = cargo_metadata::metadata(Some("/path/to/Cargo.toml".as_ref())).unwrap();
    let manifest = &metadata.packages[0];

    let features = manifest.dependencies
        .iter()
        .filter_map(|dep| {
            if dep.optional {
                Some(&dep.name)
            }
            else {
                None
            }
        })
        .chain(manifest.features.keys())
        .collect::<Vec<_>>();

    println!("features: {:?}", features);
}

With a Cargo.toml like:

[dependencies]
rand = "*"
uuid = { version = "*", optional = true }

[features]
a = []

You'll get an output of:

features: ["uuid", "a"]

[ignatenkobrain]

@KodrAus at this point, rust2rpm is still written in python, so I am relying on cargo read-manifest output... and it doesn't list all those features, so if this is really intentional we need to fix output of read-manifest I suppose.

Also, what's the exact rules here? All optional deps? Or only those which are not included in any other feature? What about build/dev-dependencies?

[KodrAus]

As far as I know the rules are:

• All optional dependencies have a feature to toggle them
• Regular dependencies and build dependencies can be optional
• Dev dependencies can't be optional right now (this shouldn't matter though because read-manifest puts all dependencies in the one array)

I think you'll have to parse the json from read-manifest and use the same idea as my snippet above to concat the names of optional dependencies with the crate features to get the set of 'actual' features available.

[ghost]

I can't find any docs on this, tbh.

@ignatenkobrain
I've been reading about this recently, docs here: https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section

Hope that helps.

[ignatenkobrain]

@xftroxgpx after reading those sentences for 30 mins I found mention of that... So I think from this issue I want improvement of those words to mark that optional deps are also "features".

[ghost]

I mean

:D

EDIT: hey look, I found the last time something like this happened to me, right here: rust-lang/rust#34444 :)

[stale]

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[stale]

As I didn't see any updates in 30 days I'm going to close this. Please see the previous comment for more information!