I'd like to see something powerful for safety and speed in Crates.io and Cargo:

The ability for crates to specify whether they use certain features of Rust:

- unsafe

- arena allocation or other future "useful" stuff

- proc_macros, eg. serde (a huge compile speed hit)

- deep transitive dependency (total #, or depth)

- some proxy for the "number of compiler steps" (compiler workload complexity)

We should be able to set a crate-wide or workspace-wide policy that can ban other crates that violate these rules.

We could make this hand-written early on, but eventually enforced by the compiler. You could then easily guarantee that no downstream dependency uses unsafe code, has a crazy number of dependencies, or has crazy compile times.

You could opt-in or opt-out to these things without having to think too much about them or constantly (manually) be on the lookout.

This would be hugely beneficial to the Rust ecosystem, safety, code quality, and developer productivity / happiness.