Provide default icon for native distributions

Resolves #274

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/application/internal/DefaultIcons.kt

@@ -0,0 +1,38 @@
+package org.jetbrains.compose.desktop.application.internal
+
+import org.gradle.api.Project
+import org.gradle.api.file.RegularFile
+import org.gradle.api.provider.Provider
+import org.jetbrains.compose.ComposeBuildConfig
+
+internal object DefaultIcons {
+    fun forLinux(project: Project): Provider<RegularFile> =
+        unpackIconIfNeeded(project, platformName = "linux", iconExt = "png")
+
+    fun forWindows(project: Project): Provider<RegularFile> =
+        unpackIconIfNeeded(project, platformName = "windows", iconExt = "ico")
+
+    fun forMac(project: Project): Provider<RegularFile> =
+        unpackIconIfNeeded(project, platformName = "mac", iconExt = "icns")
+
+    private fun unpackIconIfNeeded(project: Project, platformName: String, iconExt: String): Provider<RegularFile> {
+        val iconsDir = project.layout.buildDirectory.dir("compose/default-icons/${ComposeBuildConfig.composeVersion}")
+        val targetFile = iconsDir.map { it.file("icon-$platformName.$iconExt") }
+        val targetIoFile = targetFile.ioFile
+        val sourceIconName = "default-compose-desktop-icon-$platformName.$iconExt"
+
+        if (targetIoFile.exists()) return targetFile
+
+        val iconResourceStream = DefaultIcons.javaClass.classLoader.getResourceAsStream(sourceIconName)
+            ?: error("Could not find default icon resource: $sourceIconName")
+        iconResourceStream.use { input ->
+            targetIoFile.parentFile.mkdirs()
+            targetIoFile.createNewFile()
+            targetIoFile.outputStream().buffered().use { output ->
+                input.copyTo(output)
+            }
+        }
+
+        return targetFile
+    }
+}

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/application/internal/configureApplication.kt

@@ -213,7 +213,7 @@ internal fun AbstractJPackageTask.configurePlatformSettings(app: Application) {
                 linuxMenuGroup.set(provider { linux.menuGroup })
                 linuxPackageName.set(provider { linux.packageName })
                 linuxRpmLicenseType.set(provider { linux.rpmLicenseType })
-                iconFile.set(linux.iconFile)
+                iconFile.set(linux.iconFile.orElse(DefaultIcons.forLinux(project)))
                 installationPath.set(linux.installationPath)
             }
         }
@@ -226,7 +226,7 @@ internal fun AbstractJPackageTask.configurePlatformSettings(app: Application) {
                 winMenu.set(provider { win.menu })
                 winMenuGroup.set(provider { win.menuGroup })
                 winUpgradeUuid.set(provider { win.upgradeUuid })
-                iconFile.set(win.iconFile)
+                iconFile.set(win.iconFile.orElse(DefaultIcons.forWindows(project)))
                 installationPath.set(win.installationPath)
             }
         }
@@ -241,7 +241,7 @@ internal fun AbstractJPackageTask.configurePlatformSettings(app: Application) {
                 )
                 nonValidatedMacBundleID.set(provider { mac.bundleID })
                 nonValidatedMacSigningSettings = app.nativeDistributions.macOS.signing
-                iconFile.set(mac.iconFile)
+                iconFile.set(mac.iconFile.orElse(DefaultIcons.forMac(project)))
                 installationPath.set(mac.installationPath)
             }
         }