That signifies a macro. It&#x27;s meant to visually distinguish macros from ordinary function calls because macros may have arbitrary semantics (they can expand into any code at all). The stdlib defines `println` as a macro because it allows for a neat compile-time parsing of format strings.<p>(See [1] and the linked source if curious.)<p>[1] <a href="https:&#x2F;&#x2F;doc.rust-lang.org&#x2F;std&#x2F;macro.println!.html" rel="nofollow">https:&#x2F;&#x2F;doc.rust-lang.org&#x2F;std&#x2F;macro.println!.html</a>