Fix UnionN::clone to check alignment of the cloned pointer

crates/ptr-union/src/lib.rs

@@ -558,9 +558,14 @@ macro_rules! impl_union {
         {
             paste::paste! {
                 fn clone(&self) -> Self {
-                    let builder = unsafe { <$Builder<$($A,)*>>::new_unchecked() };
+                    #[cold]
+                    #[inline(never)]
+                    fn clone_error<A>() -> ! {
+                        panic!("Tried to clone {} in a {}, but the cloned pointer wasn't sufficiently aligned", core::any::type_name::<A>(), stringify!($Union))
+                    }
+
                     None
-                        $(.or_else(|| self.[<clone_ $a>]().map(|this| builder.$a(this))))*
+                        $(.or_else(|| self.[<clone_ $a>]().map(|this| Self::[<new_ $a>](this).unwrap_or_else(|_| clone_error::<$A>()))))*
                         .unwrap_or_else(|| unsafe { unreachable_unchecked() })
                 }
             }

crates/ptr-union/tests/clone_unaligned.rs

@@ -0,0 +1,67 @@
+//! This is a regression test for https://github.com/CAD97/pointer-utils/issues/89
+//!
+//! The idea here is to have a Box like pointer which we can control the alignment for to
+//! ensure that the tests are stable (doesn't depend on allocator shenanigans).
+
+use std::{ptr::NonNull, sync::atomic::AtomicUsize};
+
+use ptr_union::Union8;
+
+#[derive(Debug)]
+struct MyBox {
+    ptr: NonNull<u8>,
+}
+
+// SAFETY:
+// * MyBox doesn't have any shared mutability
+// * the address of the returned pointer doesn't depend on the address of MyBox
+// * MyBox doesn't implement Deref
+unsafe impl erasable::ErasablePtr for MyBox {
+    fn erase(this: Self) -> erasable::ErasedPtr {
+        this.ptr.cast()
+    }
+
+    unsafe fn unerase(this: erasable::ErasedPtr) -> Self {
+        Self { ptr: this.cast() }
+    }
+}
+
+static OFFSET: AtomicUsize = AtomicUsize::new(8);
+
+impl MyBox {
+    fn new() -> Self {
+        let offset = OFFSET.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+        MyBox {
+            ptr: NonNull::new(offset as _).unwrap(),
+        }
+    }
+}
+
+impl Clone for MyBox {
+    fn clone(&self) -> Self {
+        Self::new()
+    }
+}
+
+type Union = Union8<
+    MyBox,
+    NonNull<u8>,
+    NonNull<u8>,
+    NonNull<u8>,
+    NonNull<u8>,
+    NonNull<u8>,
+    NonNull<u8>,
+    NonNull<u8>,
+>;
+
+#[test]
+#[allow(clippy::redundant_clone)]
+#[should_panic = "but the cloned pointer wasn't sufficiently aligned"]
+fn test_clone_unaligned() {
+    let bx = MyBox::new();
+    // this can't fail since the first `MyBox` is created at address 8, which is aligned to 8 bytes
+    let x = Union::new_a(bx).unwrap();
+
+    // this clone should panic, since the next `MyBox` is created at address 9, which is not aligned to 8 bytes
+    let _y = x.clone();
+}