message: Fix nightly-only warning in wasmer-derive
diff: @@ -76,6 +76,7 @@ fn derive_struct_fields(data: &DataStruct) -> (TokenStream, TokenStream) {
  Fields::Unnamed(fields) => fields.unnamed.iter().cloned().collect(),
  };
  for (field_num, f) in fields.into_iter().enumerate() {
  + let field_idx = syn::Index::from(field_num);
  let name = f.ident.clone();
  let top_level_ty: &Type = &f.ty;
  touched_fields.push(name.clone());
  @@ -177,7 +178,7 @@ fn derive_struct_fields(data: &DataStruct) -> (TokenStream, TokenStream) {
  f.span()=>
  match #access_expr {
  Ok(#local_var) => {
  - self.#field_num.initialize(#local_var);
  + self.#field_idx.initialize(#local_var);
  },
  Err(_) => (),
  }
  @@ -186,7 +187,7 @@ fn derive_struct_fields(data: &DataStruct) -> (TokenStream, TokenStream) {
  quote_spanned! {
  f.span()=>
  let #local_var: #inner_type = #access_expr?;
  - self.#field_num.initialize(#local_var);
  + self.#field_idx.initialize(#local_var);
  }
  }
  } else {
  