[BasicContainers] RigidArray,UniqueArray: Add Equatable conformance on Swift 6.4

This is currently conditional on the UnstableContainersPreview trait. (Swift 6.4 has not shipped yet, so we cannot unconditionally enable this without risking breakage.)

Author: lorentey

Sources/BasicContainers/RigidArray/RigidArray+Equatable.swift

@@ -25,7 +25,18 @@ extension RigidArray where Element: ~Copyable {
   }
 }
 
-
+#if compiler(>=6.4) && COLLECTIONS_UNSTABLE_CONTAINERS_PREVIEW
+@available(SwiftStdlib 5.0, *)
+extension RigidArray: Equatable where Element: Equatable & ~Copyable {
+  @inlinable
+  public static func ==(
+    left: borrowing Self,
+    right: borrowing Self
+  ) -> Bool {
+    left.span._elementsEqual(to: right.span)
+  }
+}
+#else
 @available(SwiftStdlib 5.0, *)
 extension RigidArray /*: Equatable */ where Element: Equatable /* & ~Copyable */ {
   @inlinable
@@ -36,5 +47,6 @@ extension RigidArray /*: Equatable */ where Element: Equatable /* & ~Copyable */
     left.span._elementsEqual(to: right.span)
   }
 }
+#endif
 
 #endif

Sources/BasicContainers/UniqueArray/UniqueArray+Equatable.swift

@@ -24,8 +24,9 @@ extension UniqueArray where Element: ~Copyable {
   }
 }
 
+#if compiler(>=6.4) && COLLECTIONS_UNSTABLE_CONTAINERS_PREVIEW
 @available(SwiftStdlib 5.0, *)
-extension UniqueArray /*: Equatable */ where Element: Equatable /* & ~Copyable */ {
+extension UniqueArray: Equatable where Element: Equatable & ~Copyable {
   @inlinable
   public static func ==(
     left: borrowing Self,
@@ -34,5 +35,17 @@ extension UniqueArray /*: Equatable */ where Element: Equatable /* & ~Copyable *
     left.span._elementsEqual(to: right.span)
   }
 }
+#else
+@available(SwiftStdlib 5.0, *)
+extension UniqueArray where Element: Equatable {
+  @inlinable
+  public static func ==(
+    left: borrowing Self,
+    right: borrowing Self
+  ) -> Bool {
+    left.span._elementsEqual(to: right.span)
+  }
+}
+#endif
 
 #endif

Sources/InternalCollectionsUtilities/Span+Extras.swift

@@ -38,6 +38,26 @@ extension Span where Element: ~Copyable {
   }
 }
 
+#if compiler(>=6.4) && COLLECTIONS_UNSTABLE_CONTAINERS_PREVIEW
+@available(SwiftStdlib 5.0, *)
+extension Span where Element: Equatable & ~Copyable {
+  @_alwaysEmitIntoClient
+  package func _elementsEqual(to other: borrowing Self) -> Bool {
+    return self.withUnsafeBufferPointer { a in
+      other.withUnsafeBufferPointer { b in
+        guard a.count == b.count else { return false }
+        guard a.baseAddress != b.baseAddress else { return true }
+        var i = 0
+        while i < self.count {
+          guard a[i] == b[i] else { return false }
+          i &+= 1
+        }
+        return true
+      }
+    }
+  }
+}
+#else
 @available(SwiftStdlib 5.0, *)
 extension Span where Element: Equatable /* & ~Copyable */ {
   @_alwaysEmitIntoClient
@@ -56,6 +76,7 @@ extension Span where Element: Equatable /* & ~Copyable */ {
     }
   }
 }
+#endif
 
 @available(SwiftStdlib 5.0, *)
 extension Span where Element: Hashable /* & ~Copyable */ {