MaxCardinality uint64
	// MessageExpression represents the cel Program that should be evaluated to generate an error message if the rule
	// fails to validate. If no MessageExpression was given, or if this expression failed to compile, this will be nil.
	MessageExpression cel.Program
	// MessageExpressionError represents an error encountered during compilation of MessageExpression. If no error was
	// encountered, this will be nil.

					warning.AddWarning(ctx, "", e.Error())
				} else {
					errs = append(errs, e)
				}
			}

			if compiled.MessageExpression != nil {
				messageExpression, newRemainingBudget, msgErr := evalMessageExpression(ctx, compiled.MessageExpression, rule.MessageExpression, activation, remainingBudget)
				if msgErr != nil {
					if msgErr.Type == cel.ErrorTypeInternal {

type StructuralValidationRule struct {
	rule, message, messageExpression, fieldPath string
}

func (s *StructuralValidationRule) Rule() string {
	return s.rule
}
func (s *StructuralValidationRule) Message() string {
	return s.message
}
func (s *StructuralValidationRule) FieldPath() string {
	return s.fieldPath
}
func (s *StructuralValidationRule) MessageExpression() string {
	return s.messageExpression
}

			messageExpression:     `"str with \na newline"`,
			costBudget:            celconfig.RuntimeCELCostBudget,
			expectedLogErr:        "messageExpression should not contain line breaks",
			expectedValidationErr: "message not messageExpression",
		},
		{
			name:                  "messageExpression is not allowed to generate messages >5000 characters",
			message:               "message not messageExpression",

			},
		},
		{
			name: "messageExpression inclusion",
			input: schema.Structural{
				Generic: schema.Generic{
					Type: "string",
				},
				ValidationExtensions: schema.ValidationExtensions{
					XValidations: apiextensions.ValidationRules{
						{
							Rule:              "self.startsWith('s')",
							MessageExpression: `"scoped field should start with 's'"`,
						},
					},

  optional string reason = 3;

  // messageExpression declares a CEL expression that evaluates to the validation failure message that is returned when this rule fails.
  // Since messageExpression is used as a failure message, it must evaluate to a string.
  // If both message and messageExpression are present on a validation, then messageExpression will be used if validation fails.

	// messageExpression declares a CEL expression that evaluates to the validation failure message that is returned when this rule fails.
	// Since messageExpression is used as a failure message, it must evaluate to a string.
	// If both message and messageExpression are present on a validation, then messageExpression will be used if validation fails.

						},
					},
					"messageExpression": {
						SchemaProps: spec.SchemaProps{

          "type": "string"
        },
        "messageExpression": {

- Added `messageExpression` field to `ValidationRule`. ([#115969](https://github.com/kubernetes/kubernetes/pull/115969), [@DangerOnTheRanger](https://github.com/DangerOnTheRanger))