Add test for unused class to shrinker-test (#2455)

shrinker-test/common.pro

@@ -38,3 +38,7 @@
 -keepclassmembernames class com.example.ClassWithVersionAnnotations {
   <fields>;
 }
+
+# Keep the name of the class to allow using reflection to check if this class still exists
+# after shrinking
+-keepnames class com.example.UnusedClass

shrinker-test/src/main/java/com/example/UnusedClass.java

@@ -0,0 +1,17 @@
+package com.example;
+
+import com.google.gson.annotations.SerializedName;
+
+/**
+ * Class with no-args constructor and field annotated with {@code @SerializedName},
+ * but which is not actually used anywhere in the code.
+ *
+ * <p>The default ProGuard rules should not keep this class.
+ */
+public class UnusedClass {
+  public UnusedClass() {
+  }
+
+  @SerializedName("i")
+  public int i;
+}

shrinker-test/src/test/java/com/google/gson/it/ShrinkingIT.java

@@ -19,7 +19,9 @@ package com.google.gson.it;
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
+import static org.junit.Assume.assumeTrue;
 
+import com.example.UnusedClass;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.URL;
@@ -248,4 +250,18 @@ public class ShrinkingIT {
       }
     });
   }
+
+  @Test
+  public void testUnusedClassRemoved() throws Exception {
+    // For some reason this test only works for R8 but not for ProGuard; ProGuard keeps the unused class
+    assumeTrue(jarToTest.equals(R8_RESULT_PATH));
+
+    String className = UnusedClass.class.getName();
+    ClassNotFoundException e = assertThrows(ClassNotFoundException.class, () -> {
+      runTest(className, c -> {
+        fail("Class should have been removed during shrinking: " + c);
+      });
+    });
+    assertThat(e).hasMessageThat().contains(className);
+  }
 }