Easier to configure ProGuard rules (#1721)

These rules only require modification when serializable classed with named companion objects are used, and even then, less than the previous rules.

README.md

@@ -175,12 +175,62 @@ dependencies {
 
 ### Android
 
-Library should work on Android "as is". If you're using proguard, you need
-to add this to your `proguard-rules.pro`:
+The library works on Android, but, if you're using ProGuard,
+you need to add rules to your `proguard-rules.pro` configuration to cover all classes that are serialized at runtime.
 
+The following configuration keeps serializers for _all_ serializable classes that are retained after shrinking.
+Uncomment and modify the last section in case you're serializing classes with named companion objects.
+
+```proguard
+# Keep `Companion` object fields of serializable classes.
+# This avoids serializer lookup through `getDeclaredClasses` as done for named companion objects.
+-if @kotlinx.serialization.Serializable class **
+-keepclassmembers class <1> {
+    static <1>$Companion Companion;
+}
+
+# Keep `serializer()` on companion objects (both default and named) of serializable classes.
+-if @kotlinx.serialization.Serializable class ** {
+    static **$* *;
+}
+-keepclassmembers class <1>$<3> {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+
+# Keep `INSTANCE.serializer()` of serializable objects.
+-if @kotlinx.serialization.Serializable class ** {
+    public static ** INSTANCE;
+}
+-keepclassmembers class <1> {
+    public static <1> INSTANCE;
+    kotlinx.serialization.KSerializer serializer(...);
+}
+
+# @Serializable and @Polymorphic are used at runtime for polymorphic serialization.
+-keepattributes RuntimeVisibleAnnotations,AnnotationDefault
+
+# Serializer for classes with named companion objects are retrieved using `getDeclaredClasses`.
+# If you have any, uncomment and replace classes with those containing named companion objects.
+#-keepattributes InnerClasses # Needed for `getDeclaredClasses`.
+#-if @kotlinx.serialization.Serializable class
+#com.example.myapplication.HasNamedCompanion, # <-- List serializable classes with named companions.
+#com.example.myapplication.HasNamedCompanion2
+#{
+#    static **$* *;
+#}
+#-keepnames class <1>$$serializer { # -keepnames suffices; class is kept when serializer() is kept.
+#    static <1>$$serializer INSTANCE;
+#}
+```
+
+In case you want to exclude serializable classes that are used, but never serialized at runtime,
+you will need to write custom rules with narrower [class specifications](https://www.guardsquare.com/manual/configuration/usage).
+
+<details>
+<summary>Example of custom rules</summary>
+
 ```proguard
--keepattributes *Annotation*, InnerClasses
--dontnote kotlinx.serialization.AnnotationsKt # core serialization annotations
+-keepattributes RuntimeVisibleAnnotations,AnnotationDefault
 
 # kotlinx-serialization-json specific. Add this if you have java.lang.NoClassDefFoundError kotlinx.serialization.json.JsonObjectSerializer
 -keepclassmembers class kotlinx.serialization.json.** {
@@ -208,12 +258,12 @@ to add this to your `proguard-rules.pro`:
 
 # Serialization supports named companions but for such classes it is necessary to add an additional rule.
 # This rule keeps serializer and serializable class from obfuscation. Therefore, it is recommended not to use wildcards in it, but to write rules for each such class.
+-keepattributes InnerClasses # Needed for `getDeclaredClasses`.
 -keep class com.yourcompany.yourpackage.SerializableClassWithNamedCompanion$$serializer {
     *** INSTANCE;
 }
 ```
-
-You may also want to keep all custom serializers you've defined.
+</details>
 
 ### Multiplatform (Common, JS, Native)