Cargo: Reject unused inherited default-features

🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".

Summary

• default-features = false is no longer allowed in an inherited workspace dependency if the workspace dependency specifies default-features = true (or does not specify default-features).

Details

Workspace inheritance allows you to specify dependencies in one place (the workspace), and then to refer to those workspace dependencies from within a package. There was an inadvertent interaction with how default-features is specified that is no longer allowed in the 2024 Edition.

Unless the workspace specifies default-features = false, it is no longer allowed to specify default-features = false in an inherited package dependency. For example, with a workspace that specifies:

[workspace.dependencies]
regex = "1.10.4"

The following is now an error:

[package]
name = "foo"
version = "1.0.0"
edition = "2024"

[dependencies]
regex = { workspace = true, default-features = false }  # ERROR

The reason for this change is to avoid confusion when specifying default-features = false when the default feature is already enabled, since it has no effect.

If you want the flexibility of deciding whether or not a dependency enables the default-features of a dependency, be sure to set default-features = false in the workspace definition. Just beware that if you build multiple workspace members at the same time, the features will be unified so that if one member sets default-features = true (which is the default if not explicitly set), the default-features will be enabled for all members using that dependency.

Migration

When using cargo fix --edition, Cargo will automatically update your Cargo.toml file to remove default-features = false in this situation.

If you would prefer to update your Cargo.toml manually, check for any warnings when running a build and remove the corresponding entries. Previous editions should display something like:

warning: /home/project/Cargo.toml: `default-features` is ignored for regex,
since `default-features` was not specified for `workspace.dependencies.regex`,
this could become a hard error in the future