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

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

@@ -0,0 +1,44 @@
+// !DIAGNOSTICS: -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE
+
+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(): String = v
+}
+
+fun `should not work with local val for different type`() {
+    val property = StringProperty("OK")
+    <!VAL_REASSIGNMENT!>property<!> = <!TYPE_MISMATCH!>"Fail"<!>
+}
+
+fun `should not work with local val for same type`() {
+    val property = StringProperty("OK")
+    <!VAL_REASSIGNMENT!>property<!> = StringProperty("Fail")
+}
+
+fun `should not work with local var for different type`() {
+    var property = StringProperty("OK")
+    property = <!TYPE_MISMATCH!>"Fail"<!>
+}
+
+fun `should work with local var for same type`() {
+    var property = StringProperty("OK")
+    property = StringProperty("Fail")
+}
+
+fun `should not work with method parameters`() {
+    fun m1(property: StringProperty): Unit {
+        <!VAL_REASSIGNMENT!>property<!> = <!TYPE_MISMATCH!>"Fail"<!>
+    }
+
+    fun m2(property: StringProperty): Unit {
+        <!VAL_REASSIGNMENT!>property<!> = StringProperty("Fail")
+    }
+}