pub static UNUSED_EXTERN_CRATES: &'static Lint
unused_extern_crates lint guards against
extern crate items
that are never used.
extern crate proc_macro;
extern crate items that are unused have no effect and should be
removed. Note that there are some cases where specifying an
extern crate is desired for the side effect of ensuring the given crate is
linked, even though it is not otherwise directly referenced. The lint
can be silenced by aliasing the crate to an underscore, such as
extern crate foo as _. Also note that it is no longer idiomatic to
extern crate in the 2018 edition, as extern crates are now
automatically added in scope.
This lint is “allow” by default because it can be noisy, and produce
false-positives. If a dependency is being removed from a project, it
is recommended to remove it from the build configuration (such as
Cargo.toml) to ensure stale build entries aren’t left behind.