Problem

This is inspired by my running cargo add and seeing a lot of features that might be there only because people have not updated to the new dep: syntax to disable unintended implicit features. For my crates, I'm aware of the syntax and plan to transition my crates. For others, they might not be aware the feature exists and by us cargo add turning it explicit, it would help raise visibility on these features to help lead more people to using it.

In addition, #10556 mentions the idea of cargo fix doing

possibly define a feature named "package" if it doesn't already exist and dep:package isn't already used

ie by convention moving away from implicit feature declarations.

Proposed Solution

cargo add --optional <dep> would create a <dep> = "dep:<dep> feature iff

• rust-version is unset or is new enough for the syntax
• dep:<dep> doesn't already exist