Macro format_maybe 

macro_rules! format_maybe {
    ($s:literal $(,)?) => { ... };
    ($s:literal, $($args:tt)+) => { ... };
}

Formats a literal only when placeholders appear to be present.

With one argument, a literal containing both { and } is formatted; any other literal is converted directly through Into. With additional arguments, the first literal is always treated as a format string.