Update compatibility-faq with more information about rxjava
Bug: b/280277628
Change-Id: Idba1d63634fd2c9091b5978acbb1f66ee6bc2997
diff --git a/compatibility-faq.md b/compatibility-faq.md
index f0d9c05..cb33077 100644
--- a/compatibility-faq.md
+++ b/compatibility-faq.md
@@ -175,6 +175,34 @@
See also https://github.com/square/retrofit/issues/3005 ("Insufficient keep
rules for R8 in full mode").
+### Return type must be parameterized
+
+Consider a service as the following:
+```
+interface Api {
+
+ @GET("<uri>")
+ fun getData(): Observable<Data>
+
+}
+```
+
+Retrofit instantiate a return type by inspecting the generic signature, here
+`Observable` from `io.reactivex.rxjava3.core`. Those classes are not guarded by
+keep rules prior to https://github.com/square/retrofit/pull/3886 so one has to
+manually add keep rules to prevent R8 in full mode stripping the generic
+signature. The proposed rule in https://github.com/square/retrofit/pull/3886 is:
+```
+-if interface * { @retrofit2.http.* public *** *(...); }
+-keep,allowoptimization,allowshrinking,allowobfuscation class <3>
+```
+After https://github.com/square/retrofit/pull/3886 is merged, the above rule
+is automatically included in your build. You can add the rule to your build
+until then.
+
+Note, the `Data` class probably also needs to be kept if used since this is
+also constructed reflectively.
+
### Kotlin suspend functions and generic signatures
For Kotlin suspend functions the generic signature is reflectively read.
