What benefits are there with making println a macro?

Question

In this code, there is a ! after the println:

fn main() {
    println!("Hello, world!");
}

In most languages I have seen, the print operation is a function. Why is it a macro in Rust?

Answer 1

By being a procedural macro, println!() gains the ability to:

1. Automatically reference its arguments. For example this is valid:

   let x = "x".to_string();
   println!("{}", x);
   println!("{}", x); // Works even though you might expect `x` to have been moved on the previous line.
   

2. Accept an arbitrary number of arguments.

3. Validate, at compile time, that the format string placeholders and arguments match up. This is a common source of bugs with C's printf().

None of those are possible with plain functions or methods.

See also:

• Does println! borrow or own the variable?
• How can I create a function with a variable number of arguments?
• Is it possible to write something as complex as `print!` in a pure Rust macro?
• What is the difference between macros and functions in Rust?