refactor: CastExpr queries CastRulesRegistry before operator methods (#17012)

Summary:

CastExpr::applyPeeled() now checks CastRulesRegistry::canCast() before falling back to the per-operator isSupportedFromType()/isSupportedToType() methods. This makes the registry the primary source of truth for cast validation, with operator methods as a fallback for types not yet fully migrated to the registry (e.g., JSON container types like ARRAY<X>→JSON that require recursive type checking).

This is a no-op behavioral change: the registry answers match what operators would answer for all registered types. The fallback path ensures JSON container casts and any future types with complex validation continue to work.

Differential Revision: D99232107

Author: natashasehgal

velox/expression/CastExpr.cpp

@@ -26,6 +26,7 @@
 #include "velox/expression/ScopedVarSetter.h"
 #include "velox/external/tzdb/time_zone.h"
 #include "velox/functions/lib/RowsTranslationUtil.h"
+#include "velox/type/CastRegistry.h"
 #include "velox/type/Type.h"
 #include "velox/type/tz/TimeZoneMap.h"
 #include "velox/vector/ComplexVector.h"
@@ -782,15 +783,19 @@ void CastExpr::applyPeeled(
     const TypePtr& toType,
     VectorPtr& result) {
   auto castFromOperator = getCastOperator(fromType);
-  if (castFromOperator && !castFromOperator->isSupportedToType(toType)) {
-    VELOX_USER_FAIL(
-        "Cannot cast {} to {}.", fromType->toString(), toType->toString());
-  }
-
   auto castToOperator = getCastOperator(toType);
-  if (castToOperator && !castToOperator->isSupportedFromType(fromType)) {
-    VELOX_USER_FAIL(
-        "Cannot cast {} to {}.", fromType->toString(), toType->toString());
+
+  // Check CastRulesRegistry first — it has all registered cast rules.
+  // If no rule exists (e.g., JSON container types), fall back to operator.
+  if (!CastRulesRegistry::instance().canCast(fromType, toType)) {
+    if (castFromOperator && !castFromOperator->isSupportedToType(toType)) {
+      VELOX_USER_FAIL(
+          "Cannot cast {} to {}.", fromType->toString(), toType->toString());
+    }
+    if (castToOperator && !castToOperator->isSupportedFromType(fromType)) {
+      VELOX_USER_FAIL(
+          "Cannot cast {} to {}.", fromType->toString(), toType->toString());
+    }
   }
 
   if (castFromOperator || castToOperator) {

velox/expression/CastExpr.h

@@ -29,11 +29,13 @@ class CastOperator {
   virtual ~CastOperator() = default;
 
   /// Determines whether the cast operator supports casting to the custom type
-  /// from the other type.
+  /// from the other type. CastRulesRegistry is checked first; this method is
+  /// only called as a fallback when no registry rule exists.
   virtual bool isSupportedFromType(const TypePtr& other) const = 0;
 
   /// Determines whether the cast operator supports casting from the custom type
-  /// to the other type.
+  /// to the other type. CastRulesRegistry is checked first; this method is
+  /// only called as a fallback when no registry rule exists.
   virtual bool isSupportedToType(const TypePtr& other) const = 0;
 
   /// Casts an input vector to the custom type. This function should not throw