Cargo: Remove implicit features
🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
Summary
- Optional dependencies must now be explicitly specified in the
[features]table.
Details
In previous editions, when an optional dependency is specified, Cargo would automatically add an implicit feature of the same name as the dependency. For example:
[dependencies]
jpeg-decoder = { version = "0.3.1", optional = true }
This would automatically add a feature jpeg-decoder = ["dep:jpeg-decoder"] to provide a way to enable the dependency.
The dep: entries are specific syntax for referring to optional dependencies.
This implicit feature is only added if "dep:jpeg-decoder" is not specified in any other feature.
In the 2024 Edition, this implicit feature is no longer added, and you are required to explicitly specify the dependency in the [features] table.
For example, instead of exposing the particular internal name of some dependency, you may consider using a more general term for the feature name:
[features]
graphics = ["dep:jpeg-decoder"]
cargo add --optional <NAME> automatically adds a feature for the dependency to the [features] table if it isn't already there.
Motivation
One reason for requiring this to be explicit is that it encourages a conscious decision about the public exposure of the feature name, and makes it clearer when reading the [features] table which features exist.
This can help avoid tying the implementation details (the dependency names) to the public set of feature names.
Also, removing features is a SemVer incompatible change, which may not be obvious when removing an optional dependency that you thought was private.
Migration
When using cargo fix --edition, Cargo will automatically update your Cargo.toml file to include the implicit features if necessary.
If you would prefer to update your Cargo.toml manually, add a foo = ["dep:foo"] entry for each optional dependency named foo if dep:foo is not already specified anywhere in the [features] table.