Add Refaster rule for Flux#concat for a single Mono or Flux

error-prone-contrib/src/main/java/tech/picnic/errorprone/refastertemplates/ReactorTemplates.java

@@ -161,6 +161,32 @@ Flux<S> after(Mono<T> mono) {
     }
   }
 
+  /** Don't use {@link Flux#concat(Publisher)} with only a single {@link Mono}. */
+  static final class FluxConcatMono<T> {
+    @BeforeTemplate
+    Flux<T> before(Mono<T> mono) {
+      return Flux.concat(mono);
+    }
+
+    @AfterTemplate
+    Flux<T> after(Mono<T> mono) {
+      return mono.flux();
+    }
+  }
+
+  /** Don't use {@link Flux#concat(Publisher)} with only a single {@link Flux}. */
+  static final class FluxConcatFlux<T> {
+    @BeforeTemplate
+    Flux<T> before(Flux<T> flux) {
+      return Flux.concat(flux);
+    }
+
+    @AfterTemplate
+    Flux<T> after(Flux<T> flux) {
+      return flux;
+    }
+  }
+
   /**
    * Prefer a collection using {@link MoreCollectors#toOptional()} over more contrived alternatives.
    */

error-prone-contrib/src/test/resources/tech/picnic/errorprone/bugpatterns/ReactorTemplatesTestInput.java

@@ -49,6 +49,14 @@ Flux<String> testMonoFlatMapToFlux() {
     return Mono.just("foo").flatMapMany(s -> Mono.just(s + s));
   }
 
+  Flux<String> testFluxConcatMono() {
+    return Flux.concat(Mono.just("foo"));
+  }
+
+  Flux<String> testFluxConcatFlux() {
+    return Flux.concat(Flux.just("foo", "bar"));
+  }
+
   ImmutableSet<Mono<Optional<String>>> testMonoCollectToOptional() {
     return ImmutableSet.of(
         Mono.just("foo").map(Optional::of).defaultIfEmpty(Optional.empty()),

error-prone-contrib/src/test/resources/tech/picnic/errorprone/bugpatterns/ReactorTemplatesTestOutput.java

@@ -50,6 +50,14 @@ Flux<String> testMonoFlatMapToFlux() {
     return Mono.just("foo").flatMap(s -> Mono.just(s + s)).flux();
   }
 
+  Flux<String> testFluxConcatMono() {
+    return Mono.just("foo").flux();
+  }
+
+  Flux<String> testFluxConcatFlux() {
+    return Flux.just("foo", "bar");
+  }
+
   ImmutableSet<Mono<Optional<String>>> testMonoCollectToOptional() {
     return ImmutableSet.of(
         Mono.just("foo").flux().collect(toOptional()),