Specifying features of dependencies on the command line

[dhardy]

From #3058 it sounds like one is not supposed to be able to specify features of dependencies on the command-line, though it's not clear since the example involves transitive features (two slashes).

Attempt to replicate that fails, as expected:

$ git clone https://github.com/kas-gui/kas.git && cd kas
$ cargo check --features kas-widgets/kas-core/winit
error: multiple slashes in feature `kas-widgets/kas-core/winit` is not allowed

However, using a single slash is currently functional:

$ cd crates/kas-widgets
$ cargo test --features kas/winit,kas/wayland
[...]
test result: ok.

Should this be supported?

Aside: in this particular case, this approach feels justifiable since kas-widgets is only ever intended to be used in addition to kas-core (here renamed kas), thus it makes no sense to re-export its features under kas-widgets.

This came up on docs.rs: it would appear that their build process does not support correct dependency retrieval for features enabled in this way.

[ehuss]

Thanks for the report!

Features of packages in a workspace can be specified with the / syntax. That is documented here, that is the expected behavior.

When building a standalone package (like kas-resvg) outside of the workspace (like what docs.rs does), dependencies like kas-core are no longer workspace members. It is not intended that you can specify features like kas/winit from the command-line if nothing else enables that feature when that is a registry dependency. This is a known bug that it is allowed in some circumstances (#3629).

I would recommend configuring your dependencies (like kas-resvg) to enable the necessary features on kas-core, either in the kas-core dependency declaration, or in the [features] table. That will tell cargo that you need the dependencies that those features enable.

[dhardy]

Thanks for the clarification...

... however, docs.rs is building from the workspace, right?

In particular, the feature resolution should never be a problem in any other circumstance I can foresee since it is never intended that kas-widgets be used without also having a direct dependency on kas-core. Of course, docs.rs does not support specification of additional dependencies, thus to me it seems that it should support feature specification of workspace members like this.

[ehuss]

... however, docs.rs is building from the workspace, right?

I do not think so. I believe it just downloads and extracts the .crate file from crates.io and builds it directly.

[dhardy]

Thanks @ehuss; that concludes this issue.