可通过 #[cfg(debug_assertions)] 和 #[cfg(not(debug_assertions))] 来控制

Reference: https://doc.rust-lang.org/reference/conditional-compilation.html#debug_assertions

fn main() {
    #[cfg(not(debug_assertions))]
    println!("release mode build");
    #[cfg(debug_assertions)]
    println!("debug mode build");
}

# 输出: release mode build
cargo r --release

# 输出: debug mode build
cargo r