Function format 

pub unsafe fn format(
    format: *const c_char,
    args: &mut VaListTag,
    handler: impl FnMut(Argument<'_>) -> c_int,
) -> c_int

Parse a format parameter and write it somewhere.

Safety

The args walk is as unsafe as any va_list: the arguments the caller passed must match the passed format, which must be a valid printf format string.