Macro eprint 

macro_rules! eprint {
    ($($arg:tt)*) => { ... };
}

Prints to the standard error.

Similar to [std::eprint], except it:

• Writes atomically, up to the greatest length supported on the platform.
• Doesn’t use locks (in userspace).
• Preserve libc’s errno and Windows’ last-error code value.

This allows it to be used to debug allocators, multi-threaded code, synchronization routines, startup code, and more.