From 8ba03e600ce7407e04a9166ac8d86a02a2161ac1 Mon Sep 17 00:00:00 2001
From: Gabriel <45515538+gabotechs@users.noreply.github.com>
Date: Mon, 26 Jan 2026 22:52:36 +0100
Subject: [PATCH] Revert "Seal Array trait (#9092)", mark `Array` as `unsafe`
 (#9234)

This reverts commit 721f373fba7493c873a64421e32f1f67114ac130.

# Which issue does this PR close?

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- Discussed in https://github.com/apache/arrow-rs/issues/9184
- Closes https://github.com/apache/arrow-rs/issues/9184

# Rationale for this change

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in the issue then this section is not needed.
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your changes and offer better suggestions for fixes.
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As discussed in Discussed in
https://github.com/apache/arrow-rs/issues/9184, rather than sealing the
`Array` trait, it's going to potentially be marked `unsafe` so that
projects have time to find alternatives for their use-cases that are now
satisfied by a custom `Array` implementation.


# What changes are included in this PR?

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is sometimes worth providing a summary of the individual changes in this
PR.
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Clean revert of https://github.com/apache/arrow-rs/pull/9092

# Are these changes tested?

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Not applicable here

# Are there any user-facing changes?

Yes, users can no implement the `Array` trait in their codebases again.

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---
 arrow-array/src/array/boolean_array.rs        |  5 ++--
 arrow-array/src/array/byte_array.rs           |  5 ++--
 arrow-array/src/array/byte_view_array.rs      |  5 ++--
 arrow-array/src/array/dictionary_array.rs     |  9 +++----
 .../src/array/fixed_size_binary_array.rs      |  5 ++--
 .../src/array/fixed_size_list_array.rs        |  5 ++--
 arrow-array/src/array/list_array.rs           |  5 ++--
 arrow-array/src/array/list_view_array.rs      |  5 ++--
 arrow-array/src/array/map_array.rs            |  5 ++--
 arrow-array/src/array/mod.rs                  | 25 ++++++++-----------
 arrow-array/src/array/null_array.rs           |  5 ++--
 arrow-array/src/array/primitive_array.rs      |  5 ++--
 arrow-array/src/array/run_array.rs            | 10 +++-----
 arrow-array/src/array/struct_array.rs         |  5 ++--
 arrow-array/src/array/union_array.rs          |  5 ++--
 15 files changed, 42 insertions(+), 62 deletions(-)

diff --git a/arrow-array/src/array/boolean_array.rs b/arrow-array/src/array/boolean_array.rs
index acea680ae374..a2a7a6c8f38c 100644
--- a/arrow-array/src/array/boolean_array.rs
+++ b/arrow-array/src/array/boolean_array.rs
@@ -286,9 +286,8 @@ impl BooleanArray {
     }
 }
 
-impl super::private::Sealed for BooleanArray {}
-
-impl Array for BooleanArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 bd85bffcfe44..599b140cf6ae 100644
--- a/arrow-array/src/array/byte_array.rs
+++ b/arrow-array/src/array/byte_array.rs
@@ -462,9 +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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 b31c76ab5a27..91836054ebea 100644
--- a/arrow-array/src/array/byte_view_array.rs
+++ b/arrow-array/src/array/byte_view_array.rs
@@ -854,9 +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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 be7703b13c5c..a68ddb22a5bf 100644
--- a/arrow-array/src/array/dictionary_array.rs
+++ b/arrow-array/src/array/dictionary_array.rs
@@ -697,9 +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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe impl<T: ArrowDictionaryKeyType> Array for DictionaryArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
     }
@@ -858,9 +857,7 @@ 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> {
+unsafe 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 b94e168cfe7c..18757fc6aceb 100644
--- a/arrow-array/src/array/fixed_size_binary_array.rs
+++ b/arrow-array/src/array/fixed_size_binary_array.rs
@@ -602,9 +602,8 @@ impl std::fmt::Debug for FixedSizeBinaryArray {
     }
 }
 
-impl super::private::Sealed for FixedSizeBinaryArray {}
-
-impl Array for FixedSizeBinaryArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 3d5e8a0787c2..070e6617f350 100644
--- a/arrow-array/src/array/fixed_size_list_array.rs
+++ b/arrow-array/src/array/fixed_size_list_array.rs
@@ -462,9 +462,8 @@ impl From<FixedSizeListArray> for ArrayData {
     }
 }
 
-impl super::private::Sealed for FixedSizeListArray {}
-
-impl Array for FixedSizeListArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 225be14ae365..2b3e5122348b 100644
--- a/arrow-array/src/array/list_array.rs
+++ b/arrow-array/src/array/list_array.rs
@@ -525,9 +525,8 @@ impl<OffsetSize: OffsetSizeTrait> GenericListArray<OffsetSize> {
     }
 }
 
-impl<OffsetSize: OffsetSizeTrait> super::private::Sealed for GenericListArray<OffsetSize> {}
-
-impl<OffsetSize: OffsetSizeTrait> Array for GenericListArray<OffsetSize> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 52c88d581d20..713d8630f6ff 100644
--- a/arrow-array/src/array/list_view_array.rs
+++ b/arrow-array/src/array/list_view_array.rs
@@ -415,9 +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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 86608d586f34..4ea1b11725aa 100644
--- a/arrow-array/src/array/map_array.rs
+++ b/arrow-array/src/array/map_array.rs
@@ -361,9 +361,8 @@ impl MapArray {
     }
 }
 
-impl super::private::Sealed for MapArray {}
-
-impl Array for MapArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 75e32d57e89c..bb75768af360 100644
--- a/arrow-array/src/array/mod.rs
+++ b/arrow-array/src/array/mod.rs
@@ -78,18 +78,17 @@ pub use list_view_array::*;
 
 use crate::iterator::ArrayIter;
 
-mod private {
-    /// 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 {
+/// # Safety
+///
+/// Implementations of this trait must ensure that all methods implementations comply with
+/// the Arrow specification. No safety guards are placed and failing to comply with it can
+/// translate into panics or undefined behavior. For example, a value computed based on `len`
+/// may be used as a direct index into memory regions without checks.
+///
+/// Use it at your own risk knowing that this trait might be sealed in the future.
+pub unsafe trait Array: std::fmt::Debug + Send + Sync {
     /// Returns the array as [`Any`] so that it can be
     /// downcasted to a specific implementation.
     ///
@@ -351,10 +350,8 @@ pub trait Array: std::fmt::Debug + Send + Sync + private::Sealed {
 /// 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 {
+unsafe impl Array for ArrayRef {
     fn as_any(&self) -> &dyn Any {
         self.as_ref().as_any()
     }
@@ -433,7 +430,7 @@ impl Array for ArrayRef {
     }
 }
 
-impl<T: Array> Array for &T {
+unsafe impl<T: Array> Array for &T {
     fn as_any(&self) -> &dyn Any {
         T::as_any(self)
     }
diff --git a/arrow-array/src/array/null_array.rs b/arrow-array/src/array/null_array.rs
index b682466b6738..12fa6435c0fc 100644
--- a/arrow-array/src/array/null_array.rs
+++ b/arrow-array/src/array/null_array.rs
@@ -76,9 +76,8 @@ impl NullArray {
     }
 }
 
-impl super::private::Sealed for NullArray {}
-
-impl Array for NullArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 457c2428145e..0ef35202821f 100644
--- a/arrow-array/src/array/primitive_array.rs
+++ b/arrow-array/src/array/primitive_array.rs
@@ -1190,9 +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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 5254a0ed3cdc..3e722b72266c 100644
--- a/arrow-array/src/array/run_array.rs
+++ b/arrow-array/src/array/run_array.rs
@@ -260,9 +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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe impl<T: RunEndIndexType> Array for RunArray<T> {
     fn as_any(&self) -> &dyn Any {
         self
     }
@@ -521,9 +520,8 @@ 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> {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 6ad1ead0d250..0d86e6305c06 100644
--- a/arrow-array/src/array/struct_array.rs
+++ b/arrow-array/src/array/struct_array.rs
@@ -401,9 +401,8 @@ impl TryFrom<Vec<(&str, ArrayRef)>> for StructArray {
     }
 }
 
-impl super::private::Sealed for StructArray {}
-
-impl Array for StructArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe 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 e08542bc8638..f7adaa9568b5 100644
--- a/arrow-array/src/array/union_array.rs
+++ b/arrow-array/src/array/union_array.rs
@@ -738,9 +738,8 @@ impl From<UnionArray> for ArrayData {
     }
 }
 
-impl super::private::Sealed for UnionArray {}
-
-impl Array for UnionArray {
+/// SAFETY: Correctly implements the contract of Arrow Arrays
+unsafe impl Array for UnionArray {
     fn as_any(&self) -> &dyn Any {
         self
     }