rustc/tests/ui/lint/force-warn/deny-by-default-lint.rs

// --force-warn $LINT causes $LINT (which is deny-by-default) to warn
//@ compile-flags: --force-warn mutable_transmutes
//@ check-pass

fn main() {
    unsafe {
        let y = std::mem::transmute::<&i32, &mut i32>(&5); //~WARN: undefined behavior
    }
}