From 49642b4b11e7ca28725c112b8d9381edaf8373c6 Mon Sep 17 00:00:00 2001
From: Raphael Taylor-Davies <r.taylordavies@googlemail.com>
Date: Fri, 2 Jan 2026 16:02:32 +0000
Subject: [PATCH 1/3] Seal Array trait

---
 arrow-array/src/array/boolean_array.rs           |  2 ++
 arrow-array/src/array/byte_array.rs              |  2 ++
 arrow-array/src/array/byte_view_array.rs         |  2 ++
 arrow-array/src/array/dictionary_array.rs        |  4 ++++
 arrow-array/src/array/fixed_size_binary_array.rs |  2 ++
 arrow-array/src/array/fixed_size_list_array.rs   |  2 ++
 arrow-array/src/array/list_array.rs              |  2 ++
 arrow-array/src/array/list_view_array.rs         |  2 ++
 arrow-array/src/array/map_array.rs               |  2 ++
 arrow-array/src/array/mod.rs                     | 10 +++++++++-
 arrow-array/src/array/null_array.rs              |  2 ++
 arrow-array/src/array/primitive_array.rs         |  2 ++
 arrow-array/src/array/run_array.rs               |  4 ++++
 arrow-array/src/array/struct_array.rs            |  2 ++
 arrow-array/src/array/union_array.rs             |  2 ++
 15 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arrow-array/src/array/boolean_array.rs b/arrow-array/src/array/boolean_array.rs
index 530121ea7853..acea680ae374 100644
--- a/arrow-array/src/array/boolean_array.rs
+++ b/arrow-array/src/array/boolean_array.rs
@@ -286,6 +286,8 @@ impl BooleanArray {
     }
 }
 
+impl super::private::Sealed for BooleanArray {}
+
 impl Array for BooleanArray {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/byte_array.rs b/arrow-array/src/array/byte_array.rs
index fbd8458846fc..bd85bffcfe44 100644
--- a/arrow-array/src/array/byte_array.rs
+++ b/arrow-array/src/array/byte_array.rs
@@ -462,6 +462,8 @@ impl<T: ByteArrayType> std::fmt::Debug for GenericByteArray<T> {
     }
 }
 
+impl<T: ByteArrayType> super::private::Sealed for GenericByteArray<T> {}
+
 impl<T: ByteArrayType> Array for GenericByteArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/byte_view_array.rs b/arrow-array/src/array/byte_view_array.rs
index f677c4ae6757..b31c76ab5a27 100644
--- a/arrow-array/src/array/byte_view_array.rs
+++ b/arrow-array/src/array/byte_view_array.rs
@@ -854,6 +854,8 @@ impl<T: ByteViewType + ?Sized> Debug for GenericByteViewArray<T> {
     }
 }
 
+impl<T: ByteViewType + ?Sized> super::private::Sealed for GenericByteViewArray<T> {}
+
 impl<T: ByteViewType + ?Sized> Array for GenericByteViewArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/dictionary_array.rs b/arrow-array/src/array/dictionary_array.rs
index 5243218392f6..be7703b13c5c 100644
--- a/arrow-array/src/array/dictionary_array.rs
+++ b/arrow-array/src/array/dictionary_array.rs
@@ -697,6 +697,8 @@ impl<'a, T: ArrowDictionaryKeyType> FromIterator<&'a str> for DictionaryArray<T>
     }
 }
 
+impl<T: ArrowDictionaryKeyType> super::private::Sealed for DictionaryArray<T> {}
+
 impl<T: ArrowDictionaryKeyType> Array for DictionaryArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
@@ -856,6 +858,8 @@ impl<'a, K: ArrowDictionaryKeyType, V> TypedDictionaryArray<'a, K, V> {
     }
 }
 
+impl<K: ArrowDictionaryKeyType, V: Sync> super::private::Sealed for TypedDictionaryArray<'_, K, V> {}
+
 impl<K: ArrowDictionaryKeyType, V: Sync> Array for TypedDictionaryArray<'_, K, V> {
     fn as_any(&self) -> &dyn Any {
         self.dictionary
diff --git a/arrow-array/src/array/fixed_size_binary_array.rs b/arrow-array/src/array/fixed_size_binary_array.rs
index d13cecb18027..b94e168cfe7c 100644
--- a/arrow-array/src/array/fixed_size_binary_array.rs
+++ b/arrow-array/src/array/fixed_size_binary_array.rs
@@ -602,6 +602,8 @@ impl std::fmt::Debug for FixedSizeBinaryArray {
     }
 }
 
+impl super::private::Sealed for FixedSizeBinaryArray {}
+
 impl Array for FixedSizeBinaryArray {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/fixed_size_list_array.rs b/arrow-array/src/array/fixed_size_list_array.rs
index fca92a64812c..3d5e8a0787c2 100644
--- a/arrow-array/src/array/fixed_size_list_array.rs
+++ b/arrow-array/src/array/fixed_size_list_array.rs
@@ -462,6 +462,8 @@ impl From<FixedSizeListArray> for ArrayData {
     }
 }
 
+impl super::private::Sealed for FixedSizeListArray {}
+
 impl Array for FixedSizeListArray {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/list_array.rs b/arrow-array/src/array/list_array.rs
index 32add1abf557..225be14ae365 100644
--- a/arrow-array/src/array/list_array.rs
+++ b/arrow-array/src/array/list_array.rs
@@ -525,6 +525,8 @@ impl<OffsetSize: OffsetSizeTrait> GenericListArray<OffsetSize> {
     }
 }
 
+impl<OffsetSize: OffsetSizeTrait> super::private::Sealed for GenericListArray<OffsetSize> {}
+
 impl<OffsetSize: OffsetSizeTrait> Array for GenericListArray<OffsetSize> {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/list_view_array.rs b/arrow-array/src/array/list_view_array.rs
index 867dcf955be7..52c88d581d20 100644
--- a/arrow-array/src/array/list_view_array.rs
+++ b/arrow-array/src/array/list_view_array.rs
@@ -415,6 +415,8 @@ impl<OffsetSize: OffsetSizeTrait> ArrayAccessor for &GenericListViewArray<Offset
     }
 }
 
+impl<OffsetSize: OffsetSizeTrait> super::private::Sealed for GenericListViewArray<OffsetSize> {}
+
 impl<OffsetSize: OffsetSizeTrait> Array for GenericListViewArray<OffsetSize> {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/map_array.rs b/arrow-array/src/array/map_array.rs
index b5e611a92b57..86608d586f34 100644
--- a/arrow-array/src/array/map_array.rs
+++ b/arrow-array/src/array/map_array.rs
@@ -361,6 +361,8 @@ impl MapArray {
     }
 }
 
+impl super::private::Sealed for MapArray {}
+
 impl Array for MapArray {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/mod.rs b/arrow-array/src/array/mod.rs
index bb114be95045..40d9732fb7a6 100644
--- a/arrow-array/src/array/mod.rs
+++ b/arrow-array/src/array/mod.rs
@@ -78,8 +78,14 @@ pub use list_view_array::*;
 
 use crate::iterator::ArrayIter;
 
+mod private {
+    pub trait Sealed {}
+
+    impl<T: Sealed> Sealed for &T {}
+}
+
 /// An array in the [arrow columnar format](https://arrow.apache.org/docs/format/Columnar.html)
-pub trait Array: std::fmt::Debug + Send + Sync {
+pub trait Array: std::fmt::Debug + Send + Sync + private::Sealed {
     /// Returns the array as [`Any`] so that it can be
     /// downcasted to a specific implementation.
     ///
@@ -341,6 +347,8 @@ pub trait Array: std::fmt::Debug + Send + Sync {
 /// A reference-counted reference to a generic `Array`
 pub type ArrayRef = Arc<dyn Array>;
 
+impl private::Sealed for ArrayRef {}
+
 /// Ergonomics: Allow use of an ArrayRef as an `&dyn Array`
 impl Array for ArrayRef {
     fn as_any(&self) -> &dyn Any {
diff --git a/arrow-array/src/array/null_array.rs b/arrow-array/src/array/null_array.rs
index 72556a92a3bc..b682466b6738 100644
--- a/arrow-array/src/array/null_array.rs
+++ b/arrow-array/src/array/null_array.rs
@@ -76,6 +76,8 @@ impl NullArray {
     }
 }
 
+impl super::private::Sealed for NullArray {}
+
 impl Array for NullArray {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/primitive_array.rs b/arrow-array/src/array/primitive_array.rs
index e71f4d47193f..457c2428145e 100644
--- a/arrow-array/src/array/primitive_array.rs
+++ b/arrow-array/src/array/primitive_array.rs
@@ -1190,6 +1190,8 @@ impl<T: ArrowPrimitiveType> From<PrimitiveArray<T>> for ArrayData {
     }
 }
 
+impl<T: ArrowPrimitiveType> super::private::Sealed for PrimitiveArray<T> {}
+
 impl<T: ArrowPrimitiveType> Array for PrimitiveArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/run_array.rs b/arrow-array/src/array/run_array.rs
index ddc99f8e172d..5254a0ed3cdc 100644
--- a/arrow-array/src/array/run_array.rs
+++ b/arrow-array/src/array/run_array.rs
@@ -260,6 +260,8 @@ impl<R: RunEndIndexType> From<RunArray<R>> for ArrayData {
     }
 }
 
+impl<T: RunEndIndexType> super::private::Sealed for RunArray<T> {}
+
 impl<T: RunEndIndexType> Array for RunArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
@@ -519,6 +521,8 @@ impl<'a, R: RunEndIndexType, V> TypedRunArray<'a, R, V> {
     }
 }
 
+impl<R: RunEndIndexType, V: Sync> super::private::Sealed for TypedRunArray<'_, R, V> {}
+
 impl<R: RunEndIndexType, V: Sync> Array for TypedRunArray<'_, R, V> {
     fn as_any(&self) -> &dyn Any {
         self.run_array
diff --git a/arrow-array/src/array/struct_array.rs b/arrow-array/src/array/struct_array.rs
index 5b18bd35d026..6ad1ead0d250 100644
--- a/arrow-array/src/array/struct_array.rs
+++ b/arrow-array/src/array/struct_array.rs
@@ -401,6 +401,8 @@ impl TryFrom<Vec<(&str, ArrayRef)>> for StructArray {
     }
 }
 
+impl super::private::Sealed for StructArray {}
+
 impl Array for StructArray {
     fn as_any(&self) -> &dyn Any {
         self
diff --git a/arrow-array/src/array/union_array.rs b/arrow-array/src/array/union_array.rs
index 934107d075f7..e08542bc8638 100644
--- a/arrow-array/src/array/union_array.rs
+++ b/arrow-array/src/array/union_array.rs
@@ -738,6 +738,8 @@ impl From<UnionArray> for ArrayData {
     }
 }
 
+impl super::private::Sealed for UnionArray {}
+
 impl Array for UnionArray {
     fn as_any(&self) -> &dyn Any {
         self

From f6d9565ae8943ed723e56a0bbb0e60b63a98976d Mon Sep 17 00:00:00 2001
From: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>
Date: Mon, 5 Jan 2026 14:06:56 +0000
Subject: [PATCH 2/3] Update arrow-array/src/array/mod.rs

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>
---
 arrow-array/src/array/mod.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arrow-array/src/array/mod.rs b/arrow-array/src/array/mod.rs
index 40d9732fb7a6..6cc40e25314d 100644
--- a/arrow-array/src/array/mod.rs
+++ b/arrow-array/src/array/mod.rs
@@ -79,6 +79,7 @@ pub use list_view_array::*;
 use crate::iterator::ArrayIter;
 
 mod private {
+    /// Private marker trait to ensure [`Array`] can not be implemented outside this crate
     pub trait Sealed {}
 
     impl<T: Sealed> Sealed for &T {}

From 6d5172969e7332fa79e5ee7845b6e8e2638576ca Mon Sep 17 00:00:00 2001
From: Raphael Taylor-Davies <r.taylordavies@googlemail.com>
Date: Mon, 5 Jan 2026 14:13:28 +0000
Subject: [PATCH 3/3] Docs

---
 arrow-array/src/array/mod.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arrow-array/src/array/mod.rs b/arrow-array/src/array/mod.rs
index 6cc40e25314d..75e32d57e89c 100644
--- a/arrow-array/src/array/mod.rs
+++ b/arrow-array/src/array/mod.rs
@@ -79,13 +79,16 @@ pub use list_view_array::*;
 use crate::iterator::ArrayIter;
 
 mod private {
-    /// Private marker trait to ensure [`Array`] can not be implemented outside this crate
+    /// Private marker trait to ensure [`super::Array`] can not be implemented outside this crate
     pub trait Sealed {}
 
     impl<T: Sealed> Sealed for &T {}
 }
 
 /// An array in the [arrow columnar format](https://arrow.apache.org/docs/format/Columnar.html)
+///
+/// This trait is sealed as it is not intended for custom array types, rather only
+/// those defined in this crate.
 pub trait Array: std::fmt::Debug + Send + Sync + private::Sealed {
     /// Returns the array as [`Any`] so that it can be
     /// downcasted to a specific implementation.