ClassCastException thrown from unused Retrofit services with R8

[grandstaish]

Bit of an edge case here—but if all of the functions in a Retrofit interface are unused, R8 will strip them. This causes the following Proguard rule to miss:

retrofit/retrofit/src/main/resources/META-INF/proguard/retrofit2.pro

Line 31 in dfdad42

-if interface * { @retrofit2.http.* <methods>; }

Then once you try to instantiate the stripped service class, you get a fairly nondescript ClassCastException from Retrofit. E.g.:

Caused by: java.lang.ClassCastException
  at androidx.appcompat.app.ToolbarActionBar$$ExternalSyntheticThrowCCEIfNotNull0.m(:0)

Replacing the referenced rule with the following one fixes the issue for us:

-if interface *
-keepclasseswithmembers,allowobfuscation interface <1> {
  @retrofit2.http.* <methods>;
}

(Source: here)

Note: I barely know how to proguard, so this definitely requires some scrutiny. Raising issue here for visibility.

[aroyarexs]

I think we have a problem which relates to yours: https://stackoverflow.com/questions/78518260/r8-shrinker-removes-retrofit-interfaces
The difference is that we are using the methods of the interface. The methods are not unused.

But instead of using our instance that we created using Retrofit.create(Class<T> service), it somehow uses/references our demo implementation which implements our retrofit interface ... 🤷‍♂️
Adding @Keep rules to the interfaces seems to fix the issue like suggested in your source.

[Soltus]

Debug mode works fine, but once released mode causes very stubborn java.lang. ClassCastException

[glovebx]

Debug mode works fine, but once released mode causes very stubborn java.lang. ClassCastException

Add @keep annotation to the interface solved my problem

[Soltus]

我的问题得到了解决，方法是与服务端交互的JSON对象类不混淆，例如：
My problem is solved by not confusing JSON object classes that interact with the server level, for example:

-keep class sc.windom.sofill.dataClass.** { *; }

[ZacSweers]

FYI if you add the suggested rule

-if interface *
-keepclasseswithmembers,allowobfuscation interface <1> {
  @retrofit2.http.* <methods>;
}

It somehow ends up causing suspend functions in our interfaces to lose generic type information on the trailing Continuation parameters. We ended up needing to remove it. Possibly because it conflicts with this higher up rule.

[raquezha]

-if interface *
-keepclasseswithmembers,allowobfuscation interface <1> {
  @retrofit2.http.* <methods>;
}

this fixes my problem.

As for @ZacSweers concern:
#4134 (comment)

I'l observe it, So far I don't use generic type in interfaces