Allow unit return type for suspend functions

slack-lint-annotations/src/main/kotlin/slack/lint/annotations/AllowUnitResponse.kt

@@ -0,0 +1,11 @@
+package slack.lint.annotations
+
+/**
+ * Annotation to allow suspend Retrofit functions to return Unit.
+ * 
+ * When applied to a suspend Retrofit function, this annotation permits the function 
+ * to have a return type of Unit, which otherwise will be flagged as an issue.
+ */
+@Target(AnnotationTarget.FUNCTION)
+@Retention(AnnotationRetention.BINARY)
+annotation class AllowUnitResponse

slack-lint-checks/src/main/java/slack/lint/retrofit/RetrofitUsageDetector.kt

@@ -39,11 +39,15 @@ class RetrofitUsageDetector : Detector(), SourceCodeScanner {
           HTTP_ANNOTATIONS.firstNotNullOfOrNull { node.findAnnotation(it) } ?: return
 
         val returnType = node.safeReturnType(context)
-        if (
-          returnType == null ||
-            returnType == PsiTypes.voidType() ||
-            returnType.canonicalText == "kotlin.Unit"
-        ) {
+        val isSuspend = context.evaluator.isSuspend(node)
+        val hasAllowUnitResponseAnnotation = node.findAnnotation("slack.lint.annotations.AllowUnitResponse") != null
+        val isVoidOrUnitReturnType = returnType == PsiTypes.voidType() ||
+                                     returnType?.canonicalText == "kotlin.Unit"
+        val shouldDisallowUnitReturnType = !isSuspend || !hasAllowUnitResponseAnnotation
+        val hasInvalidUnitReturnTypeUsage = returnType == null ||
+                                            (isVoidOrUnitReturnType && shouldDisallowUnitReturnType)
+
+        if (hasInvalidUnitReturnTypeUsage) {
           node.report(
             "Retrofit endpoints should return something other than Unit/void.",
             context.getNameLocation(node),

slack-lint-checks/src/test/java/slack/lint/retrofit/RetrofitUsageDetectorTest.kt

@@ -4,10 +4,31 @@ package slack.lint.retrofit
 
 import com.android.tools.lint.detector.api.Detector
 import org.junit.Test
+import retrofit2.http.GET
 import slack.lint.BaseSlackLintTest
 
 class RetrofitUsageDetectorTest : BaseSlackLintTest() {
 
+  private companion object {
+    private val allowUnitResponse =
+      kotlin(
+        """
+        package slack.lint.annotations
+
+        /**
+         * Annotation to allow suspend Retrofit functions to return Unit.
+         *
+         * When applied to a suspend Retrofit function, this annotation permits the function
+         * to have a return type of Unit, which otherwise will be flagged as an issue.
+         */
+        @Target(AnnotationTarget.FUNCTION)
+        @Retention(AnnotationRetention.BINARY)
+        annotation class AllowUnitResponse
+      """
+        )
+        .indented()
+  }
+
   private val retrofit2Jar = retrofit2Jar()
 
   override fun getDetector(): Detector = RetrofitUsageDetector()
@@ -132,21 +153,34 @@ class RetrofitUsageDetectorTest : BaseSlackLintTest() {
     lint()
       .files(
         retrofit2Jar,
+        allowUnitResponse,
         kotlin(
             """
             package test
 
             import retrofit2.http.GET
+            import slack.lint.annotations.AllowUnitResponse
 
             interface Example {
               @GET("/")
               fun unitMethod()
 
+              @AllowUnitResponse
+              @PUT("/")
+              suspend fun suspendUnitMethodAllowUnitResponse()
+
+              @GET("/")
+              fun unitMethodExplicit(): Unit
+
               @GET("/")
               suspend fun suspendUnitMethod()
 
+              @AllowUnitResponse
+              @DELETE("/")
+              suspend fun suspendUnitMethodExplicitAllowUnitResponse(): Unit
+
               @GET("/")
-              fun unitMethodExplicit(): Unit
+              suspend fun suspendUnitMethodExplicit(): Unit
             }
           """
           )
@@ -155,16 +189,19 @@ class RetrofitUsageDetectorTest : BaseSlackLintTest() {
       .run()
       .expect(
         """
-        src/test/Example.kt:7: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
+        src/test/Example.kt:8: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
           fun unitMethod()
               ~~~~~~~~~~
-        src/test/Example.kt:10: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
-          suspend fun suspendUnitMethod()
-                      ~~~~~~~~~~~~~~~~~
-        src/test/Example.kt:13: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
+        src/test/Example.kt:15: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
           fun unitMethodExplicit(): Unit
               ~~~~~~~~~~~~~~~~~~
-        3 errors, 0 warnings
+        src/test/Example.kt:18: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
+          suspend fun suspendUnitMethod()
+                      ~~~~~~~~~~~~~~~~~
+        src/test/Example.kt:25: Error: Retrofit endpoints should return something other than Unit/void. [RetrofitUsage]
+          suspend fun suspendUnitMethodExplicit(): Unit
+                      ~~~~~~~~~~~~~~~~~~~~~~~~~
+        4 errors, 0 warnings
         """
           .trimIndent()
       )