How to implement custom Debug on a struct?

I have a generic function that operates on several different structs. On of these structs looks like this:

struct A {
    data: Vec<String>
}

When I try to use dbg! or println!("{:?}", my_instance), I get this error:

`T` cannot be formatted using `{:?}` because it doesn't implement `Debug`

I understand I need to write my own Debug impl, but I don't understand how to do that for vectors. How can I get my struct to just print? I can not figure it out. I've been trying to do something like this:

impl std::fmt::Debug for Datum {
    fn fmt(&self) -> String {
        format!("{:?}", self)
    }
}