mac::format_if!

macro_rules! format_if {
    ($enabled:expr, $borrowed:expr, $fmt:expr, $($args:expr),*) => {
        if $enabled {
            ::std::borrow::Cow::Owned(format!($fmt, $($args),*))
        } else {
            ::std::borrow::Cow::Borrowed($borrowed)
        }
    }
}

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");
assert!(formatted.is_owned());

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");
assert!(not_formatted.is_borrowed())