dep-features can't reference dev-dependencies features

[mankinskin]

Say we have:

• app depends on awesome
• awesome dev-depends on cookie

activating a feature in the dev-dependency through a feature in awesome does not work:

# cookie/Cargo.toml
[features]
session = []

# awesome/Cargo.toml
[features]
session = ["cookie/session"]

[dev-dependencies.cookie]
path = "../cookie"

# app/Cargo.toml
[dependencies.awesome]
path = "../awesome"
features = ["session"]

The error message is this:

error: failed to select a version for `awesome`.
    ... required by package `app v0.1.0 (/home/$USER/git/app)`
versions that meet the requirements `=0.1.0` are: 0.1.0

the package `app` depends on `awesome`, with features: `cookie` but `awesome` does not have these features.

failed to select a version for `awesome` which could resolve this conflict

Notes

Output of cargo version: cargo 1.49.0-nightly (2af662e22 2020-11-12)

[mankinskin]

5139

[hvenev]

dev-dependencies only exist when building the crate that defines them, right? When building app the awesome crate does not have any dependency named cookie, not even an optional one.

[mankinskin]

Oh, that explains this. But then I can't use features of crates which conditionally enable features in their dev-dependencies, right? In this case, awesome might want to run different tests using cookie/session when the feature session is enabled. But when awesome/session is set by a downstream crate (app), cookie is not a dependency and cookie/session can't be set, resulting in this error.

I don't know if this approach should be encouraged, but the purpose of this is obviously to test the session feature, which depends on a feature in a dev-dependency.

I think we should either

• ignore dev-dependencies in features when the crate itself is a dependency (or dev-dependency), allowing authors to configure dev-dependencies with features
• provide an alternative approach to testing different features and disallow dev-dependency features in the "exported" features section

[mankinskin]

Maybe the problem becomes more clear when awesome/session does actually change something for downstream crates aswell:

# cookie/Cargo.toml
[features]
session = []

# awesome/Cargo.toml
[features]
session = ["cookie/session", "tide"]

[dependencies.tide]
path = "../tide"
optional = true

[dev-dependencies.cookie]
path = "../cookie"

# app/Cargo.toml
[dependencies.awesome]
path = "../awesome"
features = ["session"]

Now awesome behaves differently when session is enabled, and cookie/session is needed in the dev-dependencies to test this feature.

[Eh2406]

One workaround is for awesome to have two features session = ["tide"] for users to use, and session-testing = ["cookie/session", "tide"] for developers of awesome to use while testing.

[mankinskin]

This bug is now blocking me, because I need to enable a feature of a dependency which depends on my crate itself. This means I can only use dev-dependencies, because otherwise it would be a dependency cycle. But the features don't work correctly for dev-dependencies.

# riker
[features]
default = []
tokio_executor = ["riker-macros/tokio_executor"]

[dependencies]
riker-macros = { path = "riker-macros" }

# riker/riker-macros
[features]
default = []
tokio_executor = ["tokio", "riker-testkit/tokio_executor", "riker/tokio_executor"]

# ...

[dev-dependencies]
riker = { path = ".." }

error: failed to select a version for `riker`.
    ... required by package `riker-macros v0.2.0 (/home/#/git/riker/riker-macros)`
versions that meet the requirements `=0.4.2` are: 0.4.2

the package `riker-macros` depends on `riker`, with features: `riker-testkit` but `riker` does not have these features.

failed to select a version for `riker` which could resolve this conflict

[mankinskin]

I guess the solution to this would be to ignore "cookie/session" as a feature switch when building "awsome" as a dependency because it references a feature of a dev-dependency, which is not available when building as a dependency. I will see if I can make a PR for this.