Min example: ``` trait Zoom {} struct Foo; struct Bar; impl Zoom for Foo {} ``` https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f9122e0d0aaecb171c6fded1d8589f19 Warns Bar is unused but not Foo. ``` Compiling playground v0.0.1 (/playground) warning: struct `Bar` is never constructed --> src/lib.rs:4:8 | 4 | struct Bar; | ^^^ | = note: `#[warn(dead_code)]` on by default warning: `playground` (lib) generated 1 warning Finished dev [unoptimized + debuginfo] target(s) in 0.34s ```