[57_maintenance] Revert "Seal Array trait (#9092)", mark Array as unsafe (#9234) (#9313)

- Part of #9240
- Related to #9184

This is a backport of the following PR to the 57 line
- #9234 from @gabotechs  

As discussed in #9184, rather
than sealing the Array trait which broke several downstream crates, this
PR markes the trait as `unsafe` so that projects have time to find
alternatives for their use-cases that are now satisfied by a custom
Array implementation.

Co-authored-by: Gabriel <45515538+gabotechs@users.noreply.github.com>

Author: alamb

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
     }

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
     }

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
     }

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
     }

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
     }

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
     }

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
     }

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
     }

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
     }

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)
     }