Add the #[skip] attribute for skipping fields in FromVarargs.

This is useful mainly for skipping marker fields in generic code.

Author: chitoyuu

gdnative-derive/src/lib.rs

@@ -493,7 +493,21 @@ pub fn derive_from_variant(input: TokenStream) -> TokenStream {
 ///     #[opt] baz: Option<Ref<Node>>,
 /// }
 /// ```
-#[proc_macro_derive(FromVarargs, attributes(opt))]
+///
+/// ## Field attributes
+///
+/// Attributes can be used to customize behavior of certain fields. All attributes are optional.
+///
+/// ### `#[opt]`
+///
+/// Marks an argument as optional. Required arguments must precede all optional arguments.
+/// Default values are obtained through `Default::default`.
+///
+/// ### `#[skip]`
+///
+/// Instructs the macro to skip a field. Skipped fields do not affect the signature of the
+/// argument list. They may be located anywhere. Values are obtained through `Default::default`.
+#[proc_macro_derive(FromVarargs, attributes(opt, skip))]
 pub fn derive_from_varargs(input: TokenStream) -> TokenStream {
     let derive_input = syn::parse_macro_input!(input as syn::DeriveInput);
     match varargs::derive_from_varargs(derive_input) {

gdnative-derive/src/varargs.rs

@@ -48,7 +48,13 @@ pub(crate) fn derive_from_varargs(input: DeriveInput) -> Result<TokenStream2, sy
 
         let mut required = Vec::new();
         let mut optional = Vec::new();
+        let mut skipped = Vec::new();
         for field in fields {
+            if field.attrs.iter().any(|attr| attr.path.is_ident("skip")) {
+                skipped.push(field);
+                continue;
+            }
+
             let is_optional = field.attrs.iter().any(|attr| attr.path.is_ident("opt"));
             if !is_optional && !optional.is_empty() {
                 return Err(syn::Error::new(
@@ -111,6 +117,16 @@ pub(crate) fn derive_from_varargs(input: DeriveInput) -> Result<TokenStream2, sy
             .map(|field| format!("{}", field.ty.to_token_stream()))
             .collect::<Vec<_>>();
 
+        let skipped_var_idents = skipped
+            .iter()
+            .enumerate()
+            .map(|(n, field)| {
+                field.ident.clone().unwrap_or_else(|| {
+                    Ident::new(&format!("__skipped_arg_{}", n), Span::call_site())
+                })
+            })
+            .collect::<Vec<_>>();
+
         Ok(quote! {
             #derived
             impl #generics ::gdnative::export::FromVarargs for #ident #generics #where_clause {
@@ -147,9 +163,14 @@ pub(crate) fn derive_from_varargs(input: DeriveInput) -> Result<TokenStream2, sy
                         let #req_var_idents = #req_var_idents.unwrap();
                     )*
 
+                    #(
+                        let #skipped_var_idents = core::default::Default::default();
+                    )*
+
                     std::result::Result::Ok(#ident {
                         #(#req_var_idents,)*
                         #(#opt_var_idents,)*
+                        #(#skipped_var_idents,)*
                     })
                 }
             }