feat(ecs-macros): add component derive macro drop_last flag

1 files changed, 63 insertions, 2 deletions

diff --git a/ecs-macros/src/lib.rs b/ecs-macros/src/lib.rs
index 6fe343f..8e801f9 100644
--- a/ecs-macros/src/lib.rs
+++ b/ecs-macros/src/lib.rs
@@ -1,18 +1,26 @@
 use proc_macro::TokenStream;
 use quote::{quote, ToTokens};
 use syn::spanned::Spanned;
-use syn::{parse, Ident, Item, ItemEnum, ItemStruct, ItemUnion};
+use syn::{parse, Attribute, Ident, Item, ItemEnum, ItemStruct, ItemUnion};
 
-#[proc_macro_derive(Component)]
+#[proc_macro_derive(Component, attributes(component))]
 pub fn component_derive(input: TokenStream) -> TokenStream
 {
     let item: TypeItem = parse::<Item>(input).unwrap().try_into().unwrap();
 
     let item_ident = item.ident();
 
+    let component_attr = item.component_attribute().unwrap_or_default();
+
+    let drop_last = component_attr.drop_last;
+
     quote! {
         impl ecs::component::Component for #item_ident
         {
+            fn drop_last(&self) -> bool {
+                #drop_last
+            }
+
             fn as_any_mut(&mut self) -> &mut dyn std::any::Any
             {
                 self
@@ -54,6 +62,29 @@ impl TypeItem
             Self::Union(union_item) => &union_item.ident,
         }
     }
+
+    fn component_attribute(&self) -> Option<ComponentAttribute>
+    {
+        let item_attrs = match &self {
+            Self::Struct(struct_item) => &struct_item.attrs,
+            &Self::Enum(enum_item) => &enum_item.attrs,
+            &Self::Union(union_item) => &union_item.attrs,
+        };
+
+        let mut component_attr: Option<&Attribute> = None;
+
+        for item_attr in item_attrs {
+            if component_attr.is_some() {
+                panic!("Expected only one component attribute");
+            }
+
+            if item_attr.path().get_ident()? == "component" {
+                component_attr = Some(item_attr);
+            }
+        }
+
+        Some(ComponentAttribute::from_attribute(component_attr?).unwrap())
+    }
 }
 
 impl TryFrom<Item> for TypeItem
@@ -85,3 +116,33 @@ impl ToTokens for TypeItem
         }
     }
 }
+
+#[derive(Debug, Default)]
+struct ComponentAttribute
+{
+    drop_last: bool,
+}
+
+impl ComponentAttribute
+{
+    fn from_attribute(attribute: &Attribute) -> Result<Self, syn::Error>
+    {
+        let mut drop_last = false;
+
+        attribute.parse_nested_meta(|meta| {
+            if meta
+                .path
+                .get_ident()
+                .is_some_and(|flag| flag == "drop_last")
+            {
+                drop_last = true;
+
+                return Ok(());
+            }
+
+            Err(meta.error("Unrecognized token"))
+        })?;
+
+        Ok(Self { drop_last })
+    }
+}