Suggestion

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

@@ -314,4 +314,17 @@ ImmutableSet<T> after(Set<T> set, Multimap<K, V> multimap) {
       return Sets.intersection(set, multimap.keySet()).immutableCopy();
     }
   }
+
+  /** Prefer an immutable copy of {@link Sets#union(Set, Set)} over more contrived alternatives. */
+  static final class SetsUnion<S, T extends S, U extends S> {
+    @BeforeTemplate
+    ImmutableSet<S> before(Set<T> set1, Set<U> set2) {
+      return Stream.concat(set1.stream(), set2.stream()).collect(toImmutableSet());
+    }
+
+    @AfterTemplate
+    ImmutableSet<S> after(Set<T> set1, Set<U> set2) {
+      return Sets.union(set1, set2).immutableCopy();
+    }
+  }
 }

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

@@ -123,4 +123,9 @@ ImmutableSet<Integer> testSetsIntersectionMultimap() {
         .filter(ImmutableSetMultimap.of(2, 3)::containsKey)
         .collect(toImmutableSet());
   }
+
+  ImmutableSet<Integer> testSetsUnion() {
+    return Stream.concat(ImmutableSet.of(1).stream(), ImmutableSet.of(2).stream())
+        .collect(toImmutableSet());
+  }
 }

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

@@ -102,4 +102,8 @@ ImmutableSet<Integer> testSetsIntersectionMultimap() {
     return Sets.intersection(ImmutableSet.of(1), ImmutableSetMultimap.of(2, 3).keySet())
         .immutableCopy();
   }
+
+  ImmutableSet<Integer> testSetsUnion() {
+    return Sets.union(ImmutableSet.of(1), ImmutableSet.of(2)).immutableCopy();
+  }
 }