Introduce FluxCollectToImmutableSet Refaster rule

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

@@ -1,6 +1,7 @@
 package tech.picnic.errorprone.refasterrules;
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 import static com.google.common.collect.MoreCollectors.toOptional;
 import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
 import static com.google.errorprone.refaster.ImportPolicy.STATIC_IMPORT_ALWAYS;
@@ -12,6 +13,7 @@
 import com.google.common.collect.ImmutableCollection;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
 import com.google.errorprone.refaster.Refaster;
 import com.google.errorprone.refaster.annotation.AfterTemplate;
 import com.google.errorprone.refaster.annotation.BeforeTemplate;
@@ -33,6 +35,7 @@
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
+import java.util.stream.Collector;
 import org.jspecify.annotations.Nullable;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -1166,6 +1169,23 @@ Flux<T> after(Flux<T> flux, Predicate<? super T> predicate, Comparator<? super T
     }
   }
 
+  /**
+   * Prefer {@link Flux#collect(Collector)} with {@link ImmutableSet#toImmutableSet()} over more
+   * contrived alternatives.
+   */
+  static final class FluxCollectToImmutableSet<T> {
+    @BeforeTemplate
+    Mono<ImmutableSet<T>> before(Flux<T> flux) {
+      return flux.collect(toImmutableList()).map(ImmutableSet::copyOf);
+    }
+
+    @AfterTemplate
+    @UseImportPolicy(STATIC_IMPORT_ALWAYS)
+    Mono<ImmutableSet<T>> after(Flux<T> flux) {
+      return flux.collect(toImmutableSet());
+    }
+  }
+
   /** Prefer {@link reactor.util.context.Context#empty()}} over more verbose alternatives. */
   // XXX: Consider introducing an `IsEmpty` matcher that identifies a wide range of guaranteed-empty
   // `Collection` and `Map` expressions.

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

@@ -377,6 +377,10 @@ Flux<Integer> testFluxFilterSortWithComparator() {
     return Flux.just(1, 4, 3, 2).sort(reverseOrder()).filter(i -> i % 2 == 0);
   }
 
+  Mono<ImmutableSet<Integer>> testFluxCollectToImmutableSet() {
+    return Flux.just(1).collect(toImmutableList()).map(ImmutableSet::copyOf);
+  }
+
   ImmutableSet<Context> testContextEmpty() {
     return ImmutableSet.of(Context.of(new HashMap<>()), Context.of(ImmutableMap.of()));
   }

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

@@ -1,6 +1,7 @@
 package tech.picnic.errorprone.refasterrules;
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 import static com.google.common.collect.MoreCollectors.toOptional;
 import static java.util.Comparator.reverseOrder;
 import static java.util.function.Function.identity;
@@ -365,6 +366,10 @@ Flux<Integer> testFluxFilterSortWithComparator() {
     return Flux.just(1, 4, 3, 2).filter(i -> i % 2 == 0).sort(reverseOrder());
   }
 
+  Mono<ImmutableSet<Integer>> testFluxCollectToImmutableSet() {
+    return Flux.just(1).collect(toImmutableSet());
+  }
+
   ImmutableSet<Context> testContextEmpty() {
     return ImmutableSet.of(Context.empty(), Context.empty());
   }