Introduce VerifyOnlyElementInFlux Refaster rule (#617)

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

@@ -1332,6 +1332,23 @@ StepVerifier.Step<T> after(StepVerifier.Step<T> step, T object) {
     }
   }
 
+  /** Avoid list collection when verifying that a {@link Flux} emits exactly one value. */
+  // XXX: This rule assumes that the matched collector does not drop elements. Consider introducing
+  // a `@Matches(DoesNotDropElements.class)` or `@NotMatches(MayDropElements.class)` guard.
+  static final class FluxAsStepVerifierExpectNext<T, L extends List<T>> {
+    @BeforeTemplate
+    StepVerifier.Step<L> before(Flux<T> flux, Collector<? super T, ?, L> listCollector, T object) {
+      return flux.collect(listCollector)
+          .as(StepVerifier::create)
+          .assertNext(list -> assertThat(list).containsExactly(object));
+    }
+
+    @AfterTemplate
+    StepVerifier.Step<T> after(Flux<T> flux, T object) {
+      return flux.as(StepVerifier::create).expectNext(object);
+    }
+  }
+
   /** Prefer {@link StepVerifier.LastStep#verifyComplete()} over more verbose alternatives. */
   static final class StepVerifierLastStepVerifyComplete {
     @BeforeTemplate

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

@@ -422,6 +422,13 @@ ImmutableSet<StepVerifier.Step<String>> testStepVerifierStepExpectNext() {
         StepVerifier.create(Mono.just("baz")).expectNextMatches("qux"::equals));
   }
 
+  StepVerifier.Step<?> testFluxAsStepVerifierExpectNext() {
+    return Flux.just(1)
+        .collect(toImmutableList())
+        .as(StepVerifier::create)
+        .assertNext(list -> assertThat(list).containsExactly(2));
+  }
+
   Duration testStepVerifierLastStepVerifyComplete() {
     return StepVerifier.create(Mono.empty()).expectComplete().verify();
   }

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

@@ -413,6 +413,10 @@ ImmutableSet<StepVerifier.Step<String>> testStepVerifierStepExpectNext() {
         StepVerifier.create(Mono.just("baz")).expectNext("qux"));
   }
 
+  StepVerifier.Step<?> testFluxAsStepVerifierExpectNext() {
+    return Flux.just(1).as(StepVerifier::create).expectNext(2);
+  }
+
   Duration testStepVerifierLastStepVerifyComplete() {
     return StepVerifier.create(Mono.empty()).verifyComplete();
   }