Enable features on an indirect dependency exposed via public API

[jhpratt]

Describe the problem you are trying to solve
Enable a feature on an indirect dependency (that is exposed in public API)

Describe the solution you'd like
Something like the following?

sha2 = { version = "0.9", features = ["digest/more_lengths"] }

This would require explicit knowledge of the dependencies. This is admittedly not the best of ideas, but could be limited to situations where the type is only in the public API. I've no idea how that would interact with Cargo, however.

Notes
This would be analogous to enabling features of direct dependencies in a crate's features listing, though it wouldn't require the intermediate crate to re-export features.

[ehuss]

I think for now it is intentional that this is not allowed. Features of transitive dependencies are intended to be private implementation details, not exposed as part of the API unless explicitly re-exported by the package.

In this example, if you want to enable features on the digest package, you can explicitly list it as a dependency within your own package.

Closing as this is intended behavior, and there are other ways to express the desired features. I realize controlling features of deep dependencies can sometimes be awkward, but I don't think this is the approach we would likely go with.
cc #2821 #3058

[bergkvist]

I just ran into this issue because I wanted to build a project as wasm. The project had the following dependency:

rand_core = { version = "0.6.4", features = ["getrandom"] }

This gives me the following error:

Compiling getrandom v0.2.10
error: the wasm*-unknown-unknown targets are not supported by default, you may need to enable the "js" feature. For more information see: https://docs.rs/getrandom/#webassembly-support

Naturally, I then try to enable the js-feature in getrandom:

rand_core = { version = "0.6.4", features = ["getrandom", "getrandom/js"] }

But this gives me the error message:

error: failed to parse manifest at `/home/tobias/unrepoed/threshold-ecdsa-dkls-wasm/Cargo.toml`

Caused by:
  feature `getrandom/js` in dependency `rand_core` is not allowed to contain slashes
  If you want to enable features of a transitive dependency, the direct dependency needs to re-export those features from the `[features]` table.

Most package maintainers won't check what features exist in all their dependencies so that they can re-export the useful ones.

[bergkvist]

While the above would be an intuitive and convenient way of specifying transitive dependency features, the workaround in my case was to explicitly specify the transitive dependency:

rand_core = { version = "0.6.4", features = ["getrandom"] }
getrandom = { version = "*", features = ["js"] }

[epage]

FYI part of this confusion comes from a dependencies being implicitly treated as features. We are going to be removing support for that in the 2024 edition. See rust-lang/rfcs#3491