Add a regression test for a crash in MissingCasesInEnumSwitch when using an expression switch with RULE cases.

I had no idea this was allowed. I think I internalised expression switches and statement cases as being inextricably coupled. I am now worried I have leaked a bad assumption into other places!

PiperOrigin-RevId: 779180644

Author: graememorgan

core/src/test/java/com/google/errorprone/bugpatterns/MissingCasesInEnumSwitchTest.java

@@ -17,6 +17,7 @@
 package com.google.errorprone.bugpatterns;
 
 import static com.google.common.truth.TruthJUnit.assume;
+import static org.junit.Assert.assertThrows;
 
 import com.google.errorprone.CompilationTestHelper;
 import org.junit.Test;
@@ -482,4 +483,28 @@ void m(Case c) {
             """)
         .doTest();
   }
+
+  @Test
+  public void defaultInRuleCase_crash() {
+    compilationHelper.addSourceLines(
+        "Test.java",
+        """
+        public class Test {
+          public enum E {
+            A,
+            B
+          }
+
+          public static Object test(E e) {
+            return switch (e) {
+              case A:
+                yield new Object();
+              default:
+                yield null;
+            };
+          }
+        }
+        """);
+    assertThrows(AssertionError.class, () -> compilationHelper.doTest());
+  }
 }