From 006665ee6d56a3a58bce0b5eb7c6b351d956315d Mon Sep 17 00:00:00 2001
From: Rick Ossendrijver <rick.ossendrijver@gmail.com>
Date: Wed, 5 Jul 2023 13:54:49 +0200
Subject: [PATCH] Introduce `MonoSingle` Refaster rule (#703)

---
 .../errorprone/refasterrules/ReactorRules.java      | 13 +++++++++++++
 .../refasterrules/ReactorRulesTestInput.java        |  4 ++++
 .../refasterrules/ReactorRulesTestOutput.java       |  4 ++++
 3 files changed, 21 insertions(+)

diff --git a/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ReactorRules.java b/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ReactorRules.java
index f7c9d15eaa..c30bbd11df 100644
--- a/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ReactorRules.java
+++ b/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ReactorRules.java
@@ -418,6 +418,19 @@ Mono<T> after(Mono<T> mono) {
     }
   }
 
+  /** Don't unnecessarily transform a {@link Mono} to a {@link Flux} to expect exactly one item. */
+  static final class MonoSingle<T> {
+    @BeforeTemplate
+    Mono<T> before(Mono<T> mono) {
+      return mono.flux().single();
+    }
+
+    @AfterTemplate
+    Mono<T> after(Mono<T> mono) {
+      return mono.single();
+    }
+  }
+
   /** Don't unnecessarily pass an empty publisher to {@link Flux#switchIfEmpty(Publisher)}. */
   static final class FluxSwitchIfEmptyOfEmptyPublisher<T> {
     @BeforeTemplate
diff --git a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestInput.java b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestInput.java
index 3a15f612b9..18ce1d1dab 100644
--- a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestInput.java
+++ b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestInput.java
@@ -145,6 +145,10 @@ ImmutableSet<Mono<?>> testMonoIdentity() {
         Mono.<ImmutableList<String>>empty().map(ImmutableList::copyOf));
   }
 
+  Mono<Integer> testMonoSingle() {
+    return Mono.just(1).flux().single();
+  }
+
   ImmutableSet<Flux<Integer>> testFluxSwitchIfEmptyOfEmptyPublisher() {
     return ImmutableSet.of(
         Flux.just(1).switchIfEmpty(Mono.empty()), Flux.just(2).switchIfEmpty(Flux.empty()));
diff --git a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestOutput.java b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestOutput.java
index 67812db80f..c44d57e0ff 100644
--- a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestOutput.java
+++ b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ReactorRulesTestOutput.java
@@ -150,6 +150,10 @@ ImmutableSet<Mono<?>> testMonoIdentity() {
         Mono.<ImmutableList<String>>empty());
   }
 
+  Mono<Integer> testMonoSingle() {
+    return Mono.just(1).single();
+  }
+
   ImmutableSet<Flux<Integer>> testFluxSwitchIfEmptyOfEmptyPublisher() {
     return ImmutableSet.of(Flux.just(1), Flux.just(2));
   }