Zero cost ivars

crates/objc2/src/__macro_helpers/declare_class.rs

@@ -107,6 +107,29 @@ impl<T: Message> MaybeOptionId for Option<Id<T>> {
     }
 }
 
+pub struct IvarStaticHelper {
+    offset: Cell<isize>,
+}
+
+impl IvarStaticHelper {
+    #[inline]
+    pub const fn new() -> Self {
+        Self {
+            offset: Cell::new(0),
+        }
+    }
+
+    pub fn set<T: IvarType>(&self, cls: &Class) {
+        let offset = ivar_offset(cls, T::NAME, &T::Type::__ENCODING);
+        self.offset.set(offset);
+    }
+
+    #[inline]
+    pub fn get(&self) -> isize {
+        self.offset.get()
+    }
+}
+
 /// Helper for ensuring that `ClassType::Mutability` is implemented correctly
 /// for subclasses.
 pub trait ValidSubclassMutability<T: mutability::Mutability> {}

crates/objc2/src/macros/__field_helpers.rs

@@ -202,13 +202,31 @@ macro_rules! __parse_fields {
                     __priv: (),
                 }
 
+                impl $field_name {
+                    #[inline]
+                    unsafe fn __offset_ptr() -> &'static $crate::__macro_helpers::IvarStaticHelper {
+                        static mut OFFSET: $crate::__macro_helpers::IvarStaticHelper
+                            = $crate::__macro_helpers::IvarStaticHelper::new();
+                        #[allow(unused_unsafe)]
+                        unsafe { &OFFSET }
+                    }
+                }
+
                 // SAFETY:
                 // - The ivars are in a type used as an Objective-C object.
                 // - The ivar is added to the class in `__objc2_declare_ivars`.
                 // - Caller upholds that the ivars are properly initialized.
                 unsafe impl $crate::declare::IvarType for $field_name {
                     type Type = IvarDrop<$ty>;
                     const NAME: &'static $crate::__macro_helpers::str = $ivar_name;
+
+                    #[inline]
+                    unsafe fn __offset(
+                        _ptr: $crate::__macro_helpers::NonNull<$crate::runtime::AnyObject>,
+                    ) -> $crate::__macro_helpers::isize {
+                        #[allow(unused_unsafe)]
+                        unsafe { Self::__offset_ptr().get() }
+                    }
                 }
             ) ($($ivar_type_name)* $field_name)
             (

crates/objc2/src/macros/declare_class.rs

@@ -607,6 +607,12 @@ macro_rules! __inner_declare_class {
                     }
 
                     let _cls = __objc2_builder.register();
+
+                    $(
+                        unsafe {
+                            $ivar::__offset_ptr().set::<$ivar>(_cls);
+                        }
+                    )*
                 });
 
                 // We just registered the class, so it should be available