title: Mapstruct @Mapping(expression="...") in kotlin
body: <p>Is it possible to express a Kotlin expression in the <code>@Mapping</code> annotation?</p>
  <p>The (simplified) problem is the following:</p>
  <p>I have this data class:</p>
  <pre><code>data class Data (
      val myClass: MyClass
  )
  </code></pre>
  <p>and:</p>
  <pre><code>data class DataDto (
      myString: String
  )
  
  fun String.toMyClass() = ...
  </code></pre>
  <p>Currently I would like to do something like that:</p>
  <pre><code>@Mapper
  interface RefDataMapper {
  
      @Mapping(expression = &quot;kotlin(myString?.toMyClass())&quot;, target = &quot;myClass&quot;)
      fun toDomain(dataDto: DataDto) : Data
  
  }
  </code></pre>
  <p>Currently it seems that expressions can written only in Java so the above solution doesn't work. It is possible to do what I'm trying to do?</p>
  <p>N.B. I cannot use the decorator because in the <code>Data</code> class this property is set as <code>val</code> so, I can set this property only during instantiation.</p>