Make non-Trace types work with unsafe_skip_trace

Author: Techcable

libs/derive/src/derive.rs

@@ -753,7 +753,11 @@ impl TraceDeriveInput {
         let traced_field_types = self.determine_field_types(false);
         let all_field_types = self.determine_field_types(true);
         let needs_trace = traced_field_types.iter().map(|ty| quote_spanned!(ty.span() => <#ty as zerogc::Trace>::NEEDS_TRACE));
-        let needs_drop = all_field_types.iter().map(|ty| quote_spanned!(ty.span() => <#ty as zerogc::Trace>::NEEDS_DROP));
+        let needs_drop = all_field_types.iter().map(|ty| if traced_field_types.contains(ty) {
+            quote_spanned!(ty.span() => <#ty as zerogc::Trace>::NEEDS_DROP)
+        } else {
+            quote_spanned!(ty.span() => core::mem::needs_drop::<#ty>())
+        });
         let assoc_constants = if !immutable {
             Some(quote! {
                 const NEEDS_TRACE: bool = #(#needs_trace || )* false /* NOTE: Default to *false* if we have no GC types inside */;

libs/derive/tests/basic.rs

@@ -99,9 +99,15 @@ struct UnsafeSkipped<'gc> {
     s: &'static str,
     i: i32,
     #[zerogc(unsafe_skip_trace)]
-    wow: Gc<'gc, i32, EpsilonCollectorId>
+    wow: Gc<'gc, i32, EpsilonCollectorId>,
+    #[zerogc(unsafe_skip_trace)]
+    not_impld: NotImplTrace
 }
 
+
+/// A type that doesn't implement `Trace`
+struct NotImplTrace;
+
 #[derive(Trace)]
 #[zerogc(ignore_lifetimes("'a"), immutable, collector_ids(EpsilonCollectorId))]
 #[allow(unused)]