Fix parse errors in procedural derive for enums

Wow this is why I should actually unit test stuff -_-

Author: Techcable

libs/derive/src/lib.rs

@@ -554,22 +554,22 @@ fn impl_trace(target: &DeriveInput, info: &GcTypeInfo) -> Result<TokenStream, Er
                                 )
                             },
                         };
-                        quote!(ref mut #ident)
+                        quote!(#ident)
                     }
                 );
                 let pattern = match variant.fields {
                     Fields::Named(ref fields) => {
                         let names = fields.named.iter()
                             .map(|field| field.ident.as_ref().unwrap());
-                        quote!({ #(#names,)* })
+                        quote!({ #(ref mut #names,)* })
                     },
                     Fields::Unnamed(ref fields) => {
                         let names = (0..fields.unnamed.len())
                             .map(|index| Ident::new(
                                 &format!("field{}", index),
                                 Span::call_site()
                             ));
-                        quote!(#(#names,)*)
+                        quote!(( #(ref mut #names,)* ))
                     },
                     Fields::Unit => quote!(),
                 };
@@ -783,6 +783,7 @@ fn add_trait_bounds(
 }
 
 fn debug_derive(key: &str, message: &dyn Display, value: &dyn Display) {
+    // TODO: Use proc_macro::tracked_env::var
     match ::std::env::var_os("DEBUG_DERIVE") {
         Some(var) if var == "*" ||
             var.to_string_lossy().contains(key) => {

libs/derive/tests/basic.rs

@@ -18,6 +18,29 @@ pub struct BasicCopy<'gc, Id: CollectorId> {
     basic: Option<Gc<'gc, Basic<'gc, Id>, Id>>
 }
 
+
+#[derive(Copy, Clone, Trace)]
+#[zerogc(copy, ignore_params(Id))]
+pub enum BasicEnum<'gc, Id: CollectorId> {
+    Unit,
+    Tuple(i32),
+    First {
+        all: i32
+    },
+    Second {
+        you: Gc<'gc, String, Id>,
+        need: bool
+    },
+    Third {
+        is: (),
+        love: Gc<'gc, BasicEnum<'gc, Id>, Id>
+    },
+    Fifth(
+        Gc<'gc, BasicEnum<'gc, Id>, Id>,
+        Gc<'gc, Basic<'gc, Id>, Id>
+    )
+}
+
 fn assert_copy<T: Copy>() {}
 fn assert_null_trace<T: NullTrace>() {}
 fn check_id<'gc, Id: CollectorId>() {