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

[epage]

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

[epage]

This will be needed for #12826 once we no longer allow optional dependencies to be unreferenced. I'd propose we go ahead and implement it now to help raise awareness of dep: and prepare for that future.

[epage]

The contrib guide will soon have some guidance on how we should be handling formatting. See #12836

[LuuuXXX]

Looks like nobody's handling it. I'd like to try it. @rustbot claim