Unused imports like use libc;.
One could use this regex

^\s*use\s+[a-z][_a-zA-Z0-9]*;

or try https://grep.app/search?q=%5E%5Cs%2Ause%5Cs%2B%5Ba-z%5D%5B_a-zA-Z0-9%5D%2A%3B&regexp=true&case=true&filter[lang][0]=Rust&filter[repo][0]=rust-lang/rust.

This issue has been assigned to @Yuqio via this comment.