How to handle `Self` in Rust procedual macros?

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.