Introduce MapIsEmpty Refaster rule

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/MapRules.java

@@ -41,6 +41,19 @@ static final class MapGetOrNull<K, V, T> {
     }
   }
 
+  /** Prefer {@link Map#isEmpty()} over more contrived alternatives. */
+  static final class MapIsEmpty<K, V> {
+    @BeforeTemplate
+    boolean before(Map<K, V> map) {
+      return Refaster.anyOf(map.keySet(), map.values(), map.entrySet()).isEmpty();
+    }
+
+    @AfterTemplate
+    boolean after(Map<K, V> map) {
+      return map.isEmpty();
+    }
+  }
+
   /** Prefer {@link Map#size()} over more contrived alternatives. */
   static final class MapSize<K, V> {
     @BeforeTemplate

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/MapRulesTestInput.java

@@ -22,6 +22,13 @@ String testMapGetOrNull() {
     return ImmutableMap.of(1, "foo").getOrDefault("bar", null);
   }
 
+  ImmutableSet<Boolean> testMapIsEmpty() {
+    return ImmutableSet.of(
+        ImmutableMap.of("foo", 1).keySet().isEmpty(),
+        ImmutableMap.of("bar", 2).values().isEmpty(),
+        ImmutableMap.of("baz", 3).entrySet().isEmpty());
+  }
+
   ImmutableSet<Integer> testMapSize() {
     return ImmutableSet.of(
         ImmutableMap.of("foo", 1).keySet().size(),

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/MapRulesTestOutput.java

@@ -23,6 +23,13 @@ String testMapGetOrNull() {
     return ImmutableMap.of(1, "foo").get("bar");
   }
 
+  ImmutableSet<Boolean> testMapIsEmpty() {
+    return ImmutableSet.of(
+        ImmutableMap.of("foo", 1).isEmpty(),
+        ImmutableMap.of("bar", 2).isEmpty(),
+        ImmutableMap.of("baz", 3).isEmpty());
+  }
+
   ImmutableSet<Integer> testMapSize() {
     return ImmutableSet.of(
         ImmutableMap.of("foo", 1).size(),