test(Export): add UI test

gdnative-derive/src/export.rs

@@ -1,5 +1,5 @@
 use proc_macro2::TokenStream as TokenStream2;
-use syn::DeriveInput;
+use syn::{DeriveInput, Fields};
 
 pub(crate) fn derive_export(input: &DeriveInput) -> syn::Result<TokenStream2> {
     let derived_enum = match &input.data {
@@ -17,11 +17,23 @@ pub(crate) fn derive_export(input: &DeriveInput) -> syn::Result<TokenStream2> {
 }
 
 fn impl_export(enum_ty: &syn::Ident, data: &syn::DataEnum) -> syn::Result<TokenStream2> {
-    let mappings = data.variants.iter().map(|variant| {
-        let key = &variant.ident;
-        let val = quote! { #enum_ty::#key as i64 };
-        quote! { (stringify!(#key).to_string(), #val) }
-    });
+    let mappings = {
+        let mut m = Vec::with_capacity(data.variants.len());
+
+        for variant in &data.variants {
+            if !matches!(variant.fields, Fields::Unit) {
+                return Err(syn::Error::new(
+                    variant.ident.span(),
+                    "#[derive(Export)] only supports fieldless enums",
+                ));
+            }
+            let key = &variant.ident;
+            let val = quote! { #enum_ty::#key as i64 };
+            m.push(quote! { (stringify!(#key).to_string(), #val) });
+        }
+
+        m
+    };
     let impl_block = quote! {
         impl ::gdnative::export::Export for #enum_ty {
             type Hint = ::gdnative::export::hint::IntHint<i64>;

gdnative/tests/ui.rs

@@ -40,6 +40,9 @@ fn ui_tests() {
     t.compile_fail("tests/ui/from_variant_fail_07.rs");
     t.compile_fail("tests/ui/from_variant_fail_08.rs");
     t.compile_fail("tests/ui/from_variant_fail_09.rs");
+
+    // Export
+    t.compile_fail("tests/ui/export_fail_01.rs");
 }
 
 // FIXME(rust/issues/54725): Full path spans are only available on nightly as of now

gdnative/tests/ui/export_fail_01.rs

@@ -0,0 +1,8 @@
+use gdnative::prelude::*;
+
+#[derive(Export, ToVariant)]
+pub enum Foo {
+    Bar(String),
+}
+
+fn main() {}

gdnative/tests/ui/export_fail_01.stderr

@@ -0,0 +1,5 @@
+error: #[derive(Export)] only supports fieldless enums
+ --> tests/ui/export_fail_01.rs:5:5
+  |
+5 |     Bar(String),
+  |     ^^^