Macro format_if

macro_rules! format_if {
    ($enabled:expr, $borrowed:expr, $fmt:expr, $($args:expr),*) => { ... };
}

Conditionally perform string formatting.

If $enabled is true, then do the formatting and return a Cow::Owned.

Otherwise, just return the borrowed (often 'static) string $borrowed.

When $enabled is false, this avoids the overhead of allocating and writing to a buffer, as well as any overhead or side effects of the format arguments.

Example

You can use format_if to implement a detailed error logging facility that can be enabled at runtime.

let formatted = format_if!(true, "Vague error", "Error code {:?}", 3);

assert_eq!(&formatted[..], "Error code 3");
if let Borrowed(_) = formatted {
    panic!("Wrong!")
}

let not_formatted = format_if!(false, "Vague error", "Error code {:?}", {
    // Note that the argument is not evaluated.
    panic!("oops");
});

assert_eq!(&not_formatted[..], "Vague error");
if let Owned(_) = not_formatted {
    panic!("Wrong!")
}