src/test/ui/union/union-lint-dead-code.rs

#![deny(dead_code)]

union Foo {
    x: usize,
    b: bool, //~ ERROR: field is never used
    _unused: u16,
}

fn field_read(f: Foo) -> usize {
    unsafe { f.x }
}

fn main() {
    let _ = field_read(Foo { x: 2 });
}