lib.rs

struct Thing {
    inner: Inner,
}

struct Inner {
    name: Composite,
}

struct Composite {
    t1: String,
    t2: String,
}

pub fn boop() {
    let mut thing = Thing{
        inner: Inner {
            name: Composite {
                t1: "t1".into(),
                t2: "t2".into(),
            }
        }
    };
    thing.rename("butts".into());
}

impl Thing {
    fn rename(&mut self, newname: String) {
        let old = self.inner.name;
        self.inner.name = Composite {
            t1: old.t1,
            t2: newname,
        }
    }
}