rust - println! error: expected a literal / format argument must be a string literal

Question

This extremely simple Rust program:

fn main() {
    let c = "hello";
    println!(c);
}

throws the following compile-time error:

error: expected a literal
 --> src/main.rs:3:14
  |
3 |     println!(c);
  |              ^

In previous versions of Rust, the error said:

error: format argument must be a string literal.
     println!(c);
              ^

Replacing the program with:

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

Works fine.

The meaning of this error isn't clear to me and a Google search hasn't really shed light on it. Why does passing c to the println! macro cause a compile time error? This seems like quite unusual behaviour.

Answer 1

This should work:

fn main() {
    let c = "hello";
    println!("{}", c);
}

The string "{}" is a template where {} will be replaced by the next argument passed to println!.