Rust implementation of adjective replacement

trait AdjectiveGenerator {
    fn random_adjective(&self) -> Box<str>;
}

struct FixedAdjectiveGenerator {}
impl AdjectiveGenerator for FixedAdjectiveGenerator {
    fn random_adjective(&self) -> Box<str> {
        Box::from("ADJ")
    }
}

fn replace_adjectives_in<A: AdjectiveGenerator>(generator: Box<A>, review_contents: &str) -> Box<str> {
    let mut string_builder = String::new();
    for segment in review_contents.split("*") {
        if string_builder.is_empty() {
            // First segment before any asterisk
            string_builder.push_str(segment);
            continue;
        }
        let words_in_segment = segment.split(" ");
        let remaining_words = words_in_segment.skip(1);

        let random_adjective = generator.random_adjective();
        string_builder.push_str(&random_adjective);
        for remaining_word in remaining_words {
            string_builder.push(' ');
            string_builder.push_str(remaining_word);
        }
    }
    return string_builder.into_boxed_str();

}

#[cfg(test)]
mod tests {
    use crate::totp::totp::{replace_adjectives_in, FixedAdjectiveGenerator, AdjectiveGenerator};
    use std::borrow::Borrow;
    use std::ops::Deref;

    #[test]
    fn replace_adjectives_in_test1() {
        let generator = Box::new(FixedAdjectiveGenerator {});
        assert_eq!(
            replace_adjectives_in(generator, "this is a *spicy review"),
            Box::from("this is a ADJ review"));
    }

    #[test]
    fn replace_adjectives_in_test2() {
        let generator = Box::new(FixedAdjectiveGenerator {});
        assert_eq!(
            replace_adjectives_in(generator, "this is a *spicy review with lots of *salty salt"),
            Box::from("this is a ADJ review with lots of ADJ salt"));
    }
}