Up to this point, we've used println! for outputting text. This is fine, but consider what println! does. Whenever you see a ! mark, it symbolizes a macro. Macros are used when some part of the function needs to be executed at compile time, rather than at runtime.

Consider the following:

println!("{}", a); 
Console.WriteLine("{0}", a); 

In C#, the preceding snippet will output a line with the value of a on the line. In this case, a can be of any type that supports conversion to a formatted output. The same applies to Rust. A line is output with the value of a.

The println! macro is actually implemented in the Rust standard library.