RFC: Extend manifest dependencies with used

[epage]

Extend Cargo's dependency specifications so users can mark that a dependency is used, independent of what unused externs says.

Important

When responding to RFCs, try to use inline review comments (it is possible to leave an inline review comment for the entire file at the top) instead of direct comments for normal comments and keep normal comments for procedural matters like starting FCPs.

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Rendered

[bjorn3]

Maybe we could create a [peer-dependencies] section for that just like npm? This would also improve compile time by avoiding a need to block compilation of the current crate on said dependency. And cargo could get the option to warn if a peer-dependency is not used by any other crate (aka pinning a transitive dependency has no effect).

[epage]

Maybe we could create a [peer-dependencies] section for that just like npm?

There is an ecosystem cost to adding new dependencies tables. We'd have to weigh the cost against the benefits.

This would also improve compile time by avoiding a need to block compilation of the current crate on said dependency.

For the two use cases we have, compilation will already be blocked on them. Do we know of any use cases where this would be a concern?

[epage]

Reading up on peerDependencies, they sounds like target."cfg(false)".dependencies which helps with version requirements but not with feature activation.