By default, cargo will treat any optional dependency as a feature. As of cargo 1.60, these can be disabled by declaring a feature that activates the optional dependency as dep:<name> (see RFC #3143).

On the next edition, cargo will stop exposing optional dependencies as features implicitly, requiring users to add foo = ["dep:foo"] if they still want it exposed.


While implicit features offer a low overhead way of defining features,

  • It is easy to overlook using dep: when the optional dependency is not intended to be exposed
    • Making it easy for crate authors to use the wrong syntax and be met with errors (rust-lang/cargo#10125)
    • Potentially breaking people if they are later removed (rust-lang/cargo#12687))
    • Leading to confusing choices when cargo add lists features that look the same (e.g. cargo add serde showing derive and serde_derive)
    • Leading to confusing errors for callers when they reference the dependency, instead of the feature, and things don’t work right
  • Tying feature names to dependency names is a code smell because it ties the API to the implementation
  • It requires people and tools to deal with this special case (rust-lang/cargo#10543)
    • Granted, anything having to deal with old editions will still have to deal with this

Guide-level explanation

Updated documentation:

Optional Dependencies

(Replaces the same section in the book)

Dependencies can be marked “optional”, which means they will not be compiled by default.

For example, let’s say that our 2D image processing library uses an external package to handle GIF images. This can be expressed like this:

gif = ["dep:giffy"]

giffy = { version = "0.11.1", optional = true }

This means that this dependency will only be included if the gif feature is enabled.

Note: Prior to the 202X edition, features were implicitly created for optional dependencies not referenced via dep:.

Note: Another way to optionally include a dependency is to use [platform-specific dependencies]. Instead of using features, these are conditional based on the target platform.

Reference-level explanation

Existing editions

As is expected for edition changes, cargo fix --edition will add foo = ["dep:foo"] features as needed. Where undesired, users can remove these and switch their references to the dependency from foo to dep:foo, dealing with the potential breaking changes.

Ideally, this will be accomplished by cargo emitting an allow-by-default warning when parsing a workspace member’s package when an optional dependency is not referenced via dep: in the features (rust-lang/cargo#9088) using the planned warning control system (rust-lang/cargo#12235). The warning will be named something like cargo::implicit_feature and be part of the cargo::rust-202X-compatibility group.

Suggested text:

implicit features for optional dependencies is deprecated and will be unavailable in the 202X edition.

This would be machine applicable with a suggestion to add foo = ["dep:foo"]. cargo fix would then insert this feature.

If that system is not ready in time, we can always hard code the change in cargo fix.

Next edition

On the next edition, this warning will be a hard error.

Suggested text:

unused optional dependency `foo`.  To use it, a feature must activate it with `dep:foo` syntax

This could be machine applicable with a suggestion to add foo = ["dep:foo"].


To help users through this, cargo add will be updated so that cargo add foo --optional will create a foo = ["dep:foo"] if its not already referenced by another features (rust-lang/cargo#11010).


  • Some boilerplate is needed for all features, rather than just a subset
  • Extra ecosystem churn on the next edition update

Rationale and alternatives

Instead of a cargo::rust-202X-compatibility group, we could put this in rust-202X-compatibility so there is one less group to enable to prepare for the next edition at the risk of user confusion over a “rust” lint controlling cargo.

Instead of an error, optional dependencies could be

  • Make optional dependencies private features (RFC #3487)
    • Seems like the need for this would be relatively low and be less obvious
    • We could still transition to this in the future
  • Allow access to the feature via #[cfg(accessible)] (RFC #2523)

Prior art

Unresolved questions

Future possibilities