How to return a new String from a declarative macro?

So here I am, trucking along with Rustlings, until I get broadsided with test 4.

It wants me to write a macro that will satisfy the following code:

fn main() {
    if my_macro!("world!") != "Hello world!" {
        panic!("Oh no! Wrong output!");
    }
}

So, I wrote this:

macro_rules! my_macro {
    ($val:expr) => {
        println!("Hello {}", $val);
    }
}

And Rustlings spat this out:

error[E0308]: mismatched types
  --> exercises/test4.rs:15:31
   |
15 |     if my_macro!("world!") != "Hello world!" {
   |                               ^^^^^^^^^^^^^^ expected (), found reference
   |
   = note: expected type `()`
              found type `&'static str`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

Which, you know. I get. I understand what the problem is, but I don't understand how to write a macro that will satisfy the code. I can change the code I'm testing against, but that's not what the test wants me to do. I'm only to write a macro. I'm stumped. I also don't understand how encapsulating the macro in a module is meant to help, but the test says it's a test on modules as well as macros.

println! will print to the stdout. Instead, you just want to format the string and return it from the macro. Use format! instead, and drop the ; so that it will return the expression instead of ():

macro_rules! my_macro {
    ($val:expr) => {
        format!("Hello {}", $val)
    }
}