dev-dependencies are not allowed to be optional

[gnzlbg]

Because dev-dependencies are not allowed to be optional (cc @ehuss), I have to make serde_test an unconditional dev-dependency, which means that it will always be part of the dependency graph of my crate when building it, and because serde_test requires std, this means that building my crate ends up always linking against the standard library.

My crate is a #[no_std] crate, so I cannot accept that, therefore, I cannot use serde_test for testing.

The only workaround I can think of, is to make serde_test an optional dependency of serde, and for serde to re-export it when this optional dependency is enabled. This way, I can optionally depend on serde, without any features, and I can add a cargo feature to my crate that is enabled only for testing, and that enabled the serde_test feature of serde. This would allow me to pull in serde_test optionally only when testing, allowing my application to only link against the standard library there, and not unconditionally (as it would be caused by an unconditional dev-dependency to serde_test).

[gnzlbg]

I suppose that, alternatively, I can add serde_test as an optional, non-dev dependency. Hmm. Don't know what will work best.

[dtolnay]

Currently serde_test is designed to depend on serde so serde can't also depend on serde_test. I would recommend giving your crate an optional ordinary dependency on serde_test and following up with Cargo to improve the flexibility on their end.

[Anders429]

Ran into this same issue, and wanted to add here that using -Z features=dev_dep on nightly was sufficient to resolve this problem.