How can I use derive macro on a generic struct?

Question

I'm working on implementing dynamic listeners for the communication layer of my embedded program. I want to store a Vec<ReaderContext<T>> in my Reader structure. Since T can change, I need to hide it behind a trait: Vec<Box<dyn AnyReaderContext>> (following this SO question.)

I tried to follow the how-to-write-a-custom-derive-macro guide of the rust book, and I have the following minimal code:

• Crate any_reader_context

  # Cargo.toml
  [package]
  name = "any_reader_context"
  version = "0.1.0"
  edition = "2021"
  

  // lib.rs
  pub trait AnyReaderContext {
      fn my_derived_function();
  }
  

• Crate any_reader_context_derive

  # Cargo.toml
  [package]
  name = "any_reader_context_derive"
  version = "0.1.0"
  edition = "2021"
  
  [lib]
  proc-macro = true
  
  [dependencies]
  syn = "2.0.26"
  quote = "1.0"
  

  // lib.rs
  use proc_macro::TokenStream;
  use quote::quote;
  use syn;
  
  #[proc_macro_derive(AnyReaderContext)]
  pub fn any_reader_context_derive(input: TokenStream) -> TokenStream {
      let ast = syn::parse(input).unwrap();
  
      // Build the trait implementation
      impl_any_reader_context(&ast)
  }
  
  fn impl_any_reader_context(ast: &syn::DeriveInput) -> TokenStream {
      let name = &ast.ident;
      let gen = quote! {
          impl AnyReaderContext for #name {
              fn my_derived_function() {
                  println!("Hello, Macro! My name is {}!", stringify!(#name));
              }
          }
      };
      gen.into()
  }
  

• Main crate

  # Cargo.toml
  [package]
  name = "any_reader_test"
  version = "0.1.0"
  edition = "2021"
  
  [dependencies]
  any_reader_context = { path = "../any_reader_context" }
  any_reader_context_derive = { path = "../any_reader_context_derive" }
  

  // main.rs
  use any_reader_context_derive::AnyReaderContext;
  use any_reader_context::AnyReaderContext;
  
  #[derive(AnyReaderContext)]
  pub struct Reader<T> {
      value: T
  }
  
  impl<T> Reader<T> {
      fn new(value: T) -> Self {
          Self {value}
      }
  }
  
  fn main() {
      let readers: Vec<Box<dyn AnyReaderContext>> = vec![];
      readers.push(Reader::<u32>::new(42));
  }
  

any_reader_context and any_reader_context_derive crates compile properly.

Main crate has the following cryptic compilation error:

$ cargo build
   Compiling any_reader_context v0.1.0 (rust-sandbox/any_reader_context)
   Compiling any_reader_test v0.1.0 (rust-sandbox/any_reader_test)
error[E0107]: missing generics for struct `Reader`
 --> src/main.rs:5:12
  |
5 | pub struct Reader<T> {
  |            ^^^^^^ expected 1 generic argument
  |
note: struct defined here, with 1 generic parameter: `T`
 --> src/main.rs:5:12
  |
5 | pub struct Reader<T> {
  |            ^^^^^^ -
help: add missing generic argument
  |
5 | pub struct Reader<T><T> {
  |                  +++

I tried to add a second <T>, but obviously, the compiler didn't like it:

$ cargo build
   Compiling any_reader_test v0.1.0 (/home/tcravic/workspace/avionics/prototypes/rust-sandbox/any_reader_test)
error: expected `where`, `{`, `(`, or `;` after struct name, found `<`
 --> src/main.rs:5:21
  |
5 | pub struct Reader<T><T> {
  |                     ^ expected `where`, `{`, `(`, or `;` after struct name

Can anyone help with this one?

Answer 1

You completely ignore the generics which you simply can't do. You could simply forward them from the ast :

fn impl_any_reader_context(ast: &syn::DeriveInput) -> TokenStream {
    let name = &ast.ident;
    let (impl_generics, type_generics, where_clause) = ast.generics.split_for_impl();
    let gen = quote! {
        impl #impl_generics AnyReaderContext for #name #type_generics #where_clause {
            fn my_derived_function() {
                println!("Hello, Macro! My name is {}!", stringify!(#name));
            }
        }
    };
    gen.into()
}