Apply user-defined attributes in single-platform plugins

The attributes applied by the user to the target and its compilations
were ignored in single-platform plugins while the target was not a part
of the API.

Since the user will be able to access and configure the target, we need
to apply the attributes to the relevant configurations in the same way
as in MPP.

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/KotlinGradlePluginIT.kt

@@ -921,4 +921,46 @@ class KotlinGradleIT : BaseGradleIT() {
             assertFileExists(moduleDir + "new-model-1.0-sources.jar")
         }
     }
+
+    @Test
+    fun testUserDefinedAttributesInSinglePlatformProject() =
+        with(Project("multiprojectWithDependency", GradleVersionRequired.AtLeast("4.7"))) {
+            setupWorkingDir()
+            gradleBuildScript("projA").appendText(
+                "\n" + """
+                def targetAttribute = Attribute.of("com.example.target", String)
+                def compilationAttribute = Attribute.of("com.example.compilation", String)
+                kotlin.target.attributes.attribute(targetAttribute, "foo")
+                kotlin.target.compilations["main"].attributes.attribute(compilationAttribute, "foo")
+                """.trimIndent()
+            )
+            gradleBuildScript("projB").appendText(
+                "\n" + """
+                def targetAttribute = Attribute.of("com.example.target", String)
+                def compilationAttribute = Attribute.of("com.example.compilation", String)
+                kotlin.target.attributes.attribute(targetAttribute, "foo")
+                kotlin.target.compilations["main"].attributes.attribute(compilationAttribute, "foo")
+                """.trimIndent()
+            )
+            build(":projB:compileKotlin") {
+                assertSuccessful()
+            }
+            // Break dependency resolution by providing incompatible custom attributes in the target:
+            gradleBuildScript("projB").appendText("\nkotlin.target.attributes.attribute(targetAttribute, \"bar\")")
+            build(":projB:compileKotlin") {
+                assertFailed()
+                assertContains("Required com.example.target 'bar'")
+            }
+            // And using the compilation attributes (fix the target attributes first):
+            gradleBuildScript("projB").appendText(
+                "\n" + """
+                kotlin.target.attributes.attribute(targetAttribute, "foo")
+                kotlin.target.compilations["main"].attributes.attribute(compilationAttribute, "bar")
+                """.trimIndent()
+            )
+            build(":projB:compileKotlin") {
+                assertFailed()
+                assertContains("Required com.example.compilation 'bar'")
+            }
+        }
 }