100-exercises-to-learn-rust/exercises/06_ticket_management/11_mutable_slices/src/lib.rs

// TODO: Define a function named `lowercase` that converts all characters in a string to lowercase,
//  modifying the input in place.
//  Does it need to take a `&mut String`? Does a `&mut str` work? Why or why not?

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn empty() {
        let mut s = String::from("");
        lowercase(&mut s);
        assert_eq!(s, "");
    }

    #[test]
    fn one_char() {
        let mut s = String::from("A");
        lowercase(&mut s);
        assert_eq!(s, "a");
    }

    #[test]
    fn multiple_chars() {
        let mut s = String::from("Hello, World!");
        lowercase(&mut s);
        assert_eq!(s, "hello, world!");
    }

    #[test]
    fn mut_slice() {
        let mut s = "Hello, World!".to_string();
        lowercase(s.as_mut_str());
        assert_eq!(s, "hello, world!");
    }
}