It causes a panic if the write to stdout fails (e.g. if it's a pipe and the other side was closed, or if stdout is redirected to a file and the disk is full). T

It causes a panic if the write to stdout fails (e.g. if it's a pipe and the other side was closed, or if stdout is redirected to a file and the disk is full). The rust compiler requires handling this potential error in some way, or you get a compiler error. unwrap() "handles" the IO error by crashing the program with a panic, which is the simplest way of avoiding the compiler error. It's fine for such a toy program, but would be inappropriate for production code.