Clarify nullability handling in CastExpr and improve comments for struct casting validation

Author: kosiew

datafusion/physical-expr/src/expressions/cast.rs

@@ -243,6 +243,12 @@ impl PhysicalExpr for CastExpr {
     }
 
     fn nullable(&self, input_schema: &Schema) -> Result<bool> {
+        // Runtime nullability is determined solely by the child expression.
+        // The target field's nullability is metadata used during planning and
+        // is surfaced via `return_field()`; it should not influence the
+        // physical runtime check.  Keeping this method child-driven avoids
+        // misinforming optimizers about possible null outputs while still
+        // preserving logical nullability separately.
         self.expr.nullable(input_schema)
     }
 
@@ -336,13 +342,13 @@ pub(crate) fn cast_with_target_field_and_options(
     }
 
     let is_valid_cast = match (&expr_type, cast_type) {
-            // Allow struct-to-struct casts that pass name-based compatibility validation.
-            // This validation is applied at planning time (now) to fail fast, rather than
-            // deferring errors to execution time. The name-based casting logic will be
-            // executed at runtime via ColumnarValue::cast_to.
-            (Struct(_), Struct(_)) => can_cast_struct_types(&expr_type, cast_type),
-            _ => can_cast_types(&expr_type, cast_type),
-        };
+        // Allow struct-to-struct casts that pass name-based compatibility validation.
+        // This validation is applied at planning time (now) to fail fast, rather than
+        // deferring errors to execution time. The name-based casting logic will be
+        // executed at runtime via ColumnarValue::cast_to.
+        (Struct(_), Struct(_)) => can_cast_struct_types(&expr_type, cast_type),
+        _ => can_cast_types(&expr_type, cast_type),
+    };
 
     if !is_valid_cast {
         not_impl_err!("Unsupported CAST from {expr_type} to {cast_type}")