More clever failing related to target and source presence

wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/WirePlugin.kt

@@ -120,9 +120,6 @@ class WirePlugin : Plugin<Project> {
     val projectDependenciesJvmConfiguration = project.configurations.getByName("protoProjectDependenciesJvm")
 
     val outputs = extension.outputs
-    check(outputs.isNotEmpty()) {
-      "At least one target must be provided for project '${project.path}\n" + "See our documentation for details: https://square.github.io/wire/wire_compiler/#customizing-output"
-    }
     val hasJavaOutput = outputs.any { it is JavaOutput }
     val hasKotlinOutput = outputs.any { it is KotlinOutput }
     check(!hasKotlinOutput || kotlin.get()) {

wire-gradle-plugin/src/test/kotlin/com/squareup/wire/gradle/WirePluginTest.kt

@@ -96,6 +96,20 @@ class WirePluginTest {
     assertThat(result.output).contains("Wire Gradle plugin applied in project ':' but unable to find either the Java, Kotlin, or Android plugin")
   }
 
+  @Test
+  fun noSourceNoTargetNoError() {
+    val fixtureRoot = File("src/test/projects/no-source-no-target-project")
+
+    gradleRunner.runFixture(fixtureRoot) { build() }
+  }
+
+  @Test
+  fun sourceWithNoTargetDoesError() {
+    val fixtureRoot = File("src/test/projects/source-but-no-target-project")
+
+    gradleRunner.runFixture(fixtureRoot) { build() }
+  }
+
   @Test
   fun sourcePathDirDoesNotExistButProtoPathDoes() {
     val fixtureRoot = File("src/test/projects/sourcepath-nonexistent-srcdir-with-protopath")

wire-gradle-plugin/src/test/projects/no-source-no-target-project/build.gradle

@@ -0,0 +1,4 @@
+plugins {
+  id 'kotlin'
+  id 'com.squareup.wire'
+}

wire-gradle-plugin/src/test/projects/source-but-no-target-project/build.gradle

@@ -0,0 +1,6 @@
+plugins {
+  id 'kotlin'
+  id 'com.squareup.wire'
+}
+
+wire {}

wire-gradle-plugin/src/test/projects/source-but-no-target-project/src/main/proto/squareup/dinosaurs/dinosaur.proto

@@ -0,0 +1,19 @@
+syntax = "proto2";
+
+package squareup.dinosaurs;
+
+option java_package = "com.squareup.dinosaurs";
+
+import "squareup/geology/period.proto";
+
+message Dinosaur {
+  /** Common name of this dinosaur, like "Stegosaurus". */
+  optional string name = 1;
+
+  /** URLs with images of this dinosaur. */
+  repeated string picture_urls = 2;
+
+  optional double length_meters = 3;
+  optional double mass_kilograms = 4;
+  optional squareup.geology.Period period = 5;
+}

wire-gradle-plugin/src/test/projects/source-but-no-target-project/src/main/proto/squareup/geology/period.proto

@@ -0,0 +1,16 @@
+syntax = "proto2";
+
+package squareup.geology;
+
+option java_package = "com.squareup.geology";
+
+enum Period {
+  /** 145.5 million years ago — 66.0 million years ago. */
+  CRETACEOUS = 1;
+
+  /** 201.3 million years ago — 145.0 million years ago. */
+  JURASSIC = 2;
+
+  /** 252.17 million years ago — 201.3 million years ago. */
+  TRIASSIC = 3;
+}

wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/WireRun.kt

@@ -255,6 +255,13 @@ class WireRun(
     val fullSchema = schemaLoader.loadSchema()
     eventListeners.forEach { it.loadSchemaSuccess(fullSchema) }
 
+    if (targets.isEmpty() && fullSchema.protoFiles.all { isWireRuntimeProto(it.location) }) {
+      // We have sources but no target to handle them.
+      IllegalStateException("Sources have been found but no target are set. " +
+        "At least one target must be provided for this project\n" +
+        "See our documentation for details: https://square.github.io/wire/wire_compiler/#customizing-output")
+    }
+
     // Refactor the schema.
     val schema = refactorSchema(
       schema = fullSchema,