[Assign plugin] Add a compiler plugin for overloading assign ('=') operator

plugins/assign-plugin/testData/codegen/varBehaviour.kt

@@ -0,0 +1,65 @@
+annotation class ValueContainer
+
+@ValueContainer
+data class StringProperty(var v: String) {
+    fun assign(v: String) {
+        this.v = v
+    }
+    fun assign(v: StringProperty) {
+        this.v = v.get()
+    }
+    fun get() = v
+}
+
+data class Task(var input: StringProperty)
+
+fun `test local var reference and value`(): String {
+    var property = StringProperty("OK")
+    var originalProperty = property
+    property = StringProperty("Fail")
+
+    return when {
+        originalProperty.get() != "OK" -> "Fail: ${originalProperty.get()}"
+        originalProperty == property -> "Fail: originalProperty == property"
+        else -> "OK"
+    }
+}
+
+fun `test class property var reference and value`(): String {
+    val task = Task(StringProperty("OK"))
+    val originalProperty = task.input
+    task.input = StringProperty("Fail")
+
+    return when {
+        originalProperty.get() != "OK" -> "Fail: ${originalProperty.get()}"
+        originalProperty == task.input -> "Fail: originalProperty == task.input"
+        else -> "OK"
+    }
+}
+
+fun `test class property var reference and value with apply`(): String {
+    val task = Task(StringProperty("OK"))
+    val originalProperty = task.input
+    task.apply {
+        input = StringProperty("Fail")
+    }
+
+    return when {
+        originalProperty.get() != "OK" -> "Fail: ${originalProperty.get()}"
+        originalProperty == task.input -> "Fail: originalProperty == task.input"
+        else -> "OK"
+    }
+}
+
+fun box(): String {
+    var result = `test local var reference and value`()
+    if (result != "OK") return result
+
+    result = `test class property var reference and value`()
+    if (result != "OK") return result
+
+    result = `test class property var reference and value with apply`()
+    if (result != "OK") return result
+
+    return "OK"
+}