diff --git a/objc-sys/Cargo.toml b/objc-sys/Cargo.toml
index c2adc6fb8..6ba032f47 100644
--- a/objc-sys/Cargo.toml
+++ b/objc-sys/Cargo.toml
@@ -47,6 +47,9 @@ winobjc = ["gnustep-1-8"]
 # TODO
 objfw = []
 
+# Marks opaque types as !Sized
+unstable_extern_types = []
+
 [package.metadata.docs.rs]
 default-target = "x86_64-apple-darwin"
 
diff --git a/objc-sys/src/lib.rs b/objc-sys/src/lib.rs
index 95b8f035b..e8c432a87 100644
--- a/objc-sys/src/lib.rs
+++ b/objc-sys/src/lib.rs
@@ -13,6 +13,7 @@
 //! [objc4-mirror]: https://github.com/madsmtm/objc4-mirror.git
 
 #![no_std]
+#![cfg_attr(feature = "unstable_extern_types", feature(extern_types))]
 #![allow(clippy::upper_case_acronyms)]
 #![allow(non_camel_case_types)]
 #![allow(non_upper_case_globals)]
@@ -26,8 +27,11 @@
 // See https://github.com/japaric/cty/issues/14.
 extern crate std;
 
-use core::cell::UnsafeCell;
-use core::marker::{PhantomData, PhantomPinned};
+#[cfg(not(feature = "unstable_extern_types"))]
+use core::{
+    cell::UnsafeCell,
+    marker::{PhantomData, PhantomPinned},
+};
 
 mod class;
 mod constants;
@@ -63,4 +67,9 @@ pub use various::*;
 /// (It's also less of a breaking change on our part if we re-add these).
 ///
 /// TODO: Replace this with `extern type` to also mark it as `!Sized`.
+#[cfg(not(feature = "unstable_extern_types"))]
 type OpaqueData = PhantomData<(UnsafeCell<()>, *const UnsafeCell<()>, PhantomPinned)>;
+#[cfg(feature = "unstable_extern_types")]
+extern "C" {
+    type OpaqueData;
+}
diff --git a/objc2-foundation/Cargo.toml b/objc2-foundation/Cargo.toml
index b5883150e..18a882c6a 100644
--- a/objc2-foundation/Cargo.toml
+++ b/objc2-foundation/Cargo.toml
@@ -23,6 +23,8 @@ default = ["block"]
 # Provided as a way to cut down on dependencies
 block = ["block2"]
 
+unstable_extern_types = ["objc2/unstable_extern_types", "objc-sys/unstable_extern_types"]
+
 [dependencies]
 # Exact dependencies, because block2 and objc2 are expected to change
 block2 = { path = "../block2", version = "=0.2.0-alpha.1", optional = true }
diff --git a/objc2-foundation/src/lib.rs b/objc2-foundation/src/lib.rs
index ee4e95ed0..d6810e81d 100644
--- a/objc2-foundation/src/lib.rs
+++ b/objc2-foundation/src/lib.rs
@@ -1,4 +1,5 @@
 #![no_std]
+#![cfg_attr(feature = "unstable_extern_types", feature(extern_types))]
 #![warn(elided_lifetimes_in_paths)]
 #![deny(non_ascii_idents)]
 #![warn(unreachable_pub)]
diff --git a/objc2/Cargo.toml b/objc2/Cargo.toml
index 55d447cb5..6b322b67f 100644
--- a/objc2/Cargo.toml
+++ b/objc2/Cargo.toml
@@ -37,6 +37,7 @@ malloc = ["malloc_buf"]
 
 # Uses nightly features to make AutoreleasePool zero-cost even in debug mode
 unstable_autoreleasesafe = []
+unstable_extern_types = ["objc-sys/unstable_extern_types"]
 
 [dependencies]
 malloc_buf = { version = "1.0", optional = true }
diff --git a/objc2/src/cache.rs b/objc2/src/cache.rs
index 0fd2cb950..ec47a6930 100644
--- a/objc2/src/cache.rs
+++ b/objc2/src/cache.rs
@@ -39,7 +39,7 @@ impl CachedSel {
 /// Allows storing a [`Class`] reference in a static and lazily loading it.
 #[doc(hidden)]
 pub struct CachedClass {
-    ptr: AtomicPtr<Class>,
+    ptr: AtomicPtr<c_void>,
 }
 
 impl CachedClass {
@@ -56,7 +56,7 @@ impl CachedClass {
     #[doc(hidden)]
     pub unsafe fn get(&self, name: &str) -> Option<&'static Class> {
         // `Relaxed` should be fine since `objc_getClass` is thread-safe.
-        let ptr = self.ptr.load(Ordering::Relaxed);
+        let ptr = self.ptr.load(Ordering::Relaxed) as *const Class;
         if ptr.is_null() {
             let cls = unsafe { ffi::objc_getClass(name.as_ptr() as *const _) } as *const Class;
             self.ptr.store(cls as *mut _, Ordering::Relaxed);
diff --git a/objc2/src/declare.rs b/objc2/src/declare.rs
index 9bdb02f3a..479db5cf8 100644
--- a/objc2/src/declare.rs
+++ b/objc2/src/declare.rs
@@ -36,7 +36,6 @@ use alloc::format;
 use alloc::string::ToString;
 use core::mem;
 use core::mem::ManuallyDrop;
-use core::ptr;
 use std::ffi::CString;
 
 use crate::runtime::{Bool, Class, Imp, Object, Protocol, Sel};
@@ -137,7 +136,7 @@ unsafe impl Sync for ClassDecl {}
 impl ClassDecl {
     fn with_superclass(name: &str, superclass: Option<&Class>) -> Option<ClassDecl> {
         let name = CString::new(name).unwrap();
-        let super_ptr = superclass.map_or(ptr::null(), |c| c) as _;
+        let super_ptr = superclass.map_or(0 as *const _, |c| c) as _;
         let cls = unsafe { ffi::objc_allocateClassPair(super_ptr, name.as_ptr(), 0) };
         if cls.is_null() {
             None
diff --git a/objc2/src/exception.rs b/objc2/src/exception.rs
index cc72649e1..f388ab291 100644
--- a/objc2/src/exception.rs
+++ b/objc2/src/exception.rs
@@ -45,7 +45,7 @@ extern "C" {
 pub unsafe fn throw(exception: Option<&Id<Object, Shared>>) -> ! {
     let exception = match exception {
         Some(id) => &**id as *const Object as *mut ffi::objc_object,
-        None => ptr::null_mut(),
+        None => ptr::from_raw_parts_mut(ptr::null_mut(), ()),
     };
     unsafe { ffi::objc_exception_throw(exception) }
 }
@@ -63,7 +63,7 @@ unsafe fn try_no_ret<F: FnOnce()>(closure: F) -> Result<(), Option<Id<Object, Sh
     let mut closure = Some(closure);
     let context = &mut closure as *mut _ as *mut c_void;
 
-    let mut exception = ptr::null_mut();
+    let mut exception = ptr::from_raw_parts_mut(ptr::null_mut(), ());
     let success = unsafe { rust_objc_try_catch_exception(f, context, &mut exception) };
 
     if success == 0 {
diff --git a/objc2/src/lib.rs b/objc2/src/lib.rs
index 70cedd365..412a9ce95 100644
--- a/objc2/src/lib.rs
+++ b/objc2/src/lib.rs
@@ -63,6 +63,7 @@ The bindings can be used on Linux or *BSD utilizing the
     feature = "unstable_autoreleasesafe",
     feature(negative_impls, auto_traits)
 )]
+#![cfg_attr(feature = "unstable_extern_types", feature(ptr_metadata))]
 #![warn(elided_lifetimes_in_paths)]
 #![warn(missing_docs)]
 #![deny(non_ascii_idents)]
diff --git a/objc2/src/message/mod.rs b/objc2/src/message/mod.rs
index 8c994875f..5104a29cb 100644
--- a/objc2/src/message/mod.rs
+++ b/objc2/src/message/mod.rs
@@ -399,7 +399,7 @@ mod tests {
     #[cfg(not(feature = "verify_message"))]
     #[test]
     fn test_send_message_nil() {
-        let nil: *mut Object = ::core::ptr::null_mut();
+        let nil: *mut Object = 0 as *mut Object;
 
         let result: *mut Object = unsafe { msg_send![nil, description] };
         assert!(result.is_null());
@@ -447,7 +447,7 @@ mod tests {
         };
         assert_eq!(result, 4);
 
-        let obj: *const ManuallyDrop<Object> = (&**obj as *const Object).cast();
+        let obj = &**obj as *const Object as *const ManuallyDrop<Object>;
         let result: u32 = unsafe { msg_send![obj, foo] };
         assert_eq!(result, 4);
     }
diff --git a/objc2/src/rc/weak_id.rs b/objc2/src/rc/weak_id.rs
index 509e223b2..ed6be98c8 100644
--- a/objc2/src/rc/weak_id.rs
+++ b/objc2/src/rc/weak_id.rs
@@ -30,7 +30,7 @@ pub struct WeakId<T: ?Sized> {
     item: PhantomData<Id<T, Shared>>,
 }
 
-impl<T: Message> WeakId<T> {
+impl<T: Message + ?Sized + ptr::Thin> WeakId<T> {
     /// Construct a new [`WeakId`] referencing the given shared [`Id`].
     #[doc(alias = "objc_initWeak")]
     pub fn new(obj: &Id<T, Shared>) -> Self {
@@ -45,7 +45,8 @@ impl<T: Message> WeakId<T> {
     ///
     /// The object must be valid or null.
     unsafe fn new_inner(obj: *mut T) -> Self {
-        let inner = Box::new(UnsafeCell::new(ptr::null_mut()));
+        let null = ptr::from_raw_parts_mut(ptr::null_mut(), ());
+        let inner = Box::new(UnsafeCell::new(null));
         // SAFETY: `ptr` will never move, and the caller verifies `obj`
         let _ = unsafe { ffi::objc_initWeak(inner.get() as _, obj as _) };
         Self {
@@ -64,7 +65,8 @@ impl<T: Message> WeakId<T> {
     #[inline]
     pub fn load(&self) -> Option<Id<T, Shared>> {
         let ptr: *mut *mut ffi::objc_object = self.inner.get() as _;
-        let obj = unsafe { ffi::objc_loadWeakRetained(ptr) } as *mut T;
+        let obj = unsafe { ffi::objc_loadWeakRetained(ptr) } as *mut ();
+        let obj: *mut T = ptr::from_raw_parts_mut(obj, ());
         NonNull::new(obj).map(|obj| unsafe { Id::new(obj) })
     }
 }
@@ -77,12 +79,12 @@ impl<T: ?Sized> Drop for WeakId<T> {
     }
 }
 
-// TODO: Add ?Sized
-impl<T> Clone for WeakId<T> {
+impl<T: ?Sized + ptr::Thin> Clone for WeakId<T> {
     /// Makes a clone of the `WeakId` that points to the same object.
     #[doc(alias = "objc_copyWeak")]
     fn clone(&self) -> Self {
-        let ptr = Box::new(UnsafeCell::new(ptr::null_mut()));
+        let null = ptr::from_raw_parts_mut(ptr::null_mut(), ());
+        let ptr = Box::new(UnsafeCell::new(null));
         unsafe { ffi::objc_copyWeak(ptr.get() as _, self.inner.get() as _) };
         Self {
             inner: ptr,
@@ -91,14 +93,14 @@ impl<T> Clone for WeakId<T> {
     }
 }
 
-// TODO: Add ?Sized
-impl<T: Message> Default for WeakId<T> {
+impl<T: Message + ?Sized + ptr::Thin> Default for WeakId<T> {
     /// Constructs a new `WeakId<T>` that doesn't reference any object.
     ///
     /// Calling [`Self::load`] on the return value always gives [`None`].
     fn default() -> Self {
+        let null = ptr::from_raw_parts_mut(ptr::null_mut(), ());
         // SAFETY: The pointer is null
-        unsafe { Self::new_inner(ptr::null_mut()) }
+        unsafe { Self::new_inner(null) }
     }
 }
 
diff --git a/objc2/src/runtime.rs b/objc2/src/runtime.rs
index 3a3e388d9..d60f6ad20 100644
--- a/objc2/src/runtime.rs
+++ b/objc2/src/runtime.rs
@@ -341,7 +341,7 @@ impl Class {
         let name = CString::new(name).unwrap();
         let ivar = unsafe { ffi::class_getClassVariable(self.as_ptr(), name.as_ptr()) };
         // SAFETY: TODO
-        unsafe { ivar.cast::<Ivar>().as_ref() }
+        unsafe { (ivar as *const Ivar).as_ref() }
     }
 
     /// Describes the instance methods implemented by self.