MessageExpression: `self + " `,
								},
							},
						},
					},
				},
			},
			expectedErrors: []validationMatch{
				invalid("spec.validation.openAPIV3Schema.properties[f].x-kubernetes-validations[0].messageExpression"),
			},
		},
		{
			name: "x-kubernetes-validations rule invalidated by messageExpression not returning a string",

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

### Feature

- A ValidatingAdmissionPolicy now has its `messageExpression` field checked against resolved types. ([#119209](https://github.com/kubernetes/kubernetes/pull/119209), [@jiahuif](https://github.com/jiahuif)) [SIG API Machinery]