From f82a35702cb85da7ace298960024b773a2da6d85 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Wed, 17 Jun 2026 15:29:22 +0200
Subject: feat(engine): add tagged union info to enum reflection

---
 engine-macros/src/reflection/enum_impl.rs | 25 +++++++++++--------------
 engine-reflection/src/lib.rs              | 23 +++++++++++++++++++++--
 2 files changed, 32 insertions(+), 16 deletions(-)

diff --git a/engine-macros/src/reflection/enum_impl.rs b/engine-macros/src/reflection/enum_impl.rs
index 611da6d..5dbb176 100644
--- a/engine-macros/src/reflection/enum_impl.rs
+++ b/engine-macros/src/reflection/enum_impl.rs
@@ -219,23 +219,21 @@ pub fn generate(input: syn::ItemEnum) -> proc_macro2::TokenStream
         &engine_crate_path,
     );
 
-    let discriminant_layout = if is_unit_only {
-        quote! {
-            std::alloc::Layout::new::<Self>()
-        }
-    } else {
-        quote! {
-            std::alloc::Layout::new::<#mod_name::Discriminant>()
-        }
-    };
-
-    let fields_layout = if is_unit_only {
+    let tagged_union = if is_unit_only {
         quote! {
             None
         }
     } else {
         quote! {
-            Some(std::alloc::Layout::new::<#mod_name::Fields>())
+            Some(#engine_crate_path::reflection::EnumTaggedUnion {
+                discriminant_layout: std::alloc::Layout::new::<#mod_name::Discriminant>(),
+                discriminant_byte_offset:
+                    std::mem::offset_of!(#mod_name::Equivalent #type_generics, tag),
+                fields_layout:
+                    std::alloc::Layout::new::<#mod_name::Fields #type_generics>(),
+                fields_byte_offset:
+                    std::mem::offset_of!(#mod_name::Equivalent #type_generics, payload),
+            })
         }
     };
 
@@ -256,8 +254,7 @@ pub fn generate(input: syn::ItemEnum) -> proc_macro2::TokenStream
                         #engine_crate_path::reflection::Enum {
                             variants: &[#(#variants),*],
                             is_unit_only: #is_unit_only,
-                            discriminant_layout: #discriminant_layout,
-                            fields_layout: #fields_layout,
+                            tagged_union: #tagged_union,
                             get_default_value: || {
                                 #get_default_value_fn
                             }
diff --git a/engine-reflection/src/lib.rs b/engine-reflection/src/lib.rs
index 4e5e8d8..3bd6f97 100644
--- a/engine-reflection/src/lib.rs
+++ b/engine-reflection/src/lib.rs
@@ -119,8 +119,7 @@ pub struct Enum
     /// The enum only contains unit variants.
     pub is_unit_only: bool,
 
-    pub discriminant_layout: Layout,
-    pub fields_layout: Option<Layout>,
+    pub tagged_union: Option<EnumTaggedUnion>,
 
     pub get_default_value: fn() -> Option<DefaultValueFn>,
 }
@@ -140,6 +139,15 @@ impl Enum
     }
 }
 
+#[derive(Debug, Clone)]
+pub struct EnumTaggedUnion
+{
+    pub discriminant_layout: Layout,
+    pub discriminant_byte_offset: usize,
+    pub fields_layout: Layout,
+    pub fields_byte_offset: usize,
+}
+
 #[derive(Debug, Clone)]
 pub struct EnumVariant
 {
@@ -164,6 +172,17 @@ pub enum EnumVariantFields
     },
 }
 
+impl EnumVariantFields
+{
+    pub fn fields(&self) -> &'static [Field]
+    {
+        match self {
+            Self::Named { fields } => fields,
+            Self::Unnamed { fields } => fields,
+        }
+    }
+}
+
 /// The discriminant of a enum variant.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct EnumDiscriminant
-- 
cgit v1.2.3-18-g5258