So, I have a struct Panel which contains a data widget which implements the trait Widget
I have to implement a function for Panel that uses another function that requires a type that implements Widget.
I tried Box<T>, Rc<T>, Box<dyn Widget, &T, but nothing, always compiler errors.
It's really not possible to answer your question with the information you provided. Just saying "there's an error" doesn't tell us much.
You need to include actual code and the full error messages. Ideally, create a minimal example in the Rust playground that reproduces the issue and then link it: https://play.rust-lang.org/