sol.rs

trait Builder {
    // I want this trait to return a trait object
    fn commits(&self) -> Box<dyn Commit>;

    fn finish(&self);
}

trait Commit {
}

struct FooBuilder {
}

struct FooCommit {
}

impl Builder for FooBuilder {
    fn commits(&self) -> Box<dyn Commit> {
        Box::new(FooCommit{ })
    }

    fn finish(&self) {
    }
}

impl Commit for FooCommit {
}

fn get_commits(b: &dyn Builder) {
    // trait object returns a trait
    let c = b.commits();
}

fn main() {
    let b = FooBuilder{};
    get_commits(&b);
    b.finish();
}