The pub_use_of_private_extern_crate lint detects a specific situation of re-exporting a private extern crate:

extern crate core;
pub use core as reexported_core;

A public use declaration should not be used to publicly re-export a private extern crate. pub extern crate should be used instead.This was historically allowed, but is not the intended behavior according to the visibility rules.

This used to be tracked as part of #34537, but is only tangentially related so it got a new dedicated tracking issue.

Related PRs

• make pub_use_of_private_extern_crate show up in cargo's future breakage reports #127656 tried to make this a hard error, but there were too many regressions, mostly because several ancient versions of bitflags are still in use.