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

plugins/assign-plugin/testData/diagnostics/methodDeclaration.kt

@@ -0,0 +1,36 @@
+annotation class ValueContainer
+
+@ValueContainer
+data class StringProperty(var v: String) {
+    fun assign(v: String): <!DECLARATION_ERROR_ASSIGN_METHOD_SHOULD_RETURN_UNIT!>String<!> {
+        this.v = v
+        return ""
+    }
+    fun assign(v: StringProperty): <!DECLARATION_ERROR_ASSIGN_METHOD_SHOULD_RETURN_UNIT!>String<!> {
+        this.v = v.get()
+        return ""
+    }
+    fun get(): String = v
+}
+
+fun StringProperty.assign(v: Int): <!DECLARATION_ERROR_ASSIGN_METHOD_SHOULD_RETURN_UNIT!>String<!> {
+    this.v = "OK"
+    return ""
+}
+
+data class Task(val input: StringProperty)
+
+fun `should report an error for assign method return type on assignment for annotated class`() {
+    val task = Task(StringProperty("Fail"))
+    <!VAL_REASSIGNMENT!>task.input<!> <!CALL_ERROR_ASSIGN_METHOD_SHOULD_RETURN_UNIT!>=<!> "42"
+    <!VAL_REASSIGNMENT!>task.input<!> <!CALL_ERROR_ASSIGN_METHOD_SHOULD_RETURN_UNIT!>=<!> 42
+}
+
+fun `should not report an error for assign return type for unannotated class`() {
+    data class IntProperty(var v: Int)
+    fun IntProperty.assign(v: Int): String = "OK"
+    data class IntTask(val input: IntProperty)
+
+    val task = IntTask(IntProperty(42))
+    <!VAL_REASSIGNMENT!>task.input<!> = <!CONSTANT_EXPECTED_TYPE_MISMATCH!>42<!>
+}