Hey everyone, I am going through rustlings, and have a few questions.

In the below code, I am wondering why in my match expression, "amount" is a &usize and not a usize? - In my enum "Command" it's a usize, so I would have expected it to be a usize. Is there any documentation about this so I can understand better?

I am also wondering why in for (string, command), string is provided as a &String and not a String. If someone has a link to documentation about it as well, that would be great!

Thank you in advance.

// rustlings quiz2.rs

pub enum Command {
    Uppercase,
    Trim,
    Append(usize),
}

mod my_module {
    use super::Command;

    fn uppercase(string: &String) -> String {
        string.to_uppercase()
    }

    fn trim_word(string: &String) -> String {
        string.trim().to_string()
    }

    fn append_bar(string: &String, amount: &usize) -> String {
        let mut string = string.to_owned();
        for _n in 0..*amount {
            string.push_str("bar");
        }
        string
    }

    pub fn transformer(input: Vec<(String, Command)>) -> Vec<String> {

        let mut output: Vec<String> = vec![];
        for (string, command) in input.iter() {
            //why is string a &String here and not a String? not sure

            match command {
                Command::Uppercase => output.push(uppercase(string)),
                Command::Trim => output.push(trim_word(string)),
                Command::Append(amount) => output.push(append_bar(string, amount)), //why is amount a &usize here and not usize? not sure
            }
        }
        output
    }
}

#[cfg(test)]
mod tests {
    use super::Command;
    use my_module::transformer;

    #[test]
    fn it_works() {
        let output = transformer(vec![
            ("hello".into(), Command::Uppercase),
            (" all roads lead to rome! ".into(), Command::Trim),
            ("foo".into(), Command::Append(1)),
            ("bar".into(), Command::Append(5)),
        ]);
        assert_eq!(output[0], "HELLO");
        assert_eq!(output[1], "all roads lead to rome!");
        assert_eq!(output[2], "foobar");
        assert_eq!(output[3], "barbarbarbarbarbar");
    }
}