How to handle `Self` in Rust procedual macros?

Question

I'm trying to create a derive macro in Rust to create instances, as an example, which should implement following trait:

pub trait MyTrait {
    fn my_default() -> Option<Self>;
}

I use syn and quote to parse and generate the AST, as following:

#[proc_macro_derive(MyTrait)]
pub fn my_default_derive(input: TokenStream) -> TokenStream {
    // Use `syn` to parse
    // ...

    let gen = quote! {
        impl #impl_generics_strait::MyTrait for #name #ty_generics #where_clause {
            fn my_default() -> Option<Self>{
                // ...
            }
        }
    };

    gen.into()
}

As the compiler doest not know what Self is at compile time, foregoing will cause error:

the size for values of type `Self` cannot be known at compilation time

My questions are:

• Is it possible to use Self in proc_macro_derive?
• How?

BTW, I'm trying to use ident instead, and will post progress here later.

Update:

• It turns out, the error is caused by the trait declaration, not the proc_macro_derive. To fix, Sized should be added to the trait name.