Add argument to allow running compiler without sources

`-Xbuild-file` argument allows the compiler to run without
passing any Kotlin source file in arguments.
We have been using this property in
Kotlin Gradle plugin for a few important cases:
1. incremental compilation (to update caches when there are only removed files);
2. for KAPT (Kotlin sources don't make sense in context
of running APs).

We want to stop using `-Xbuild-file` in Kotlin Gradle plugin,
and avoid breaking the Gradle plugin or IC in other build-systems.

This change adds an argument to explicitly run
the compiler without specifying any Kotlin source file.

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -275,6 +275,12 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     )
     var friendPaths: Array<String>? by FreezableVar(null)
 
+    @Argument(
+        value = "-Xallow-no-source-files",
+        description = "Allow no source files"
+    )
+    var allowNoSourceFiles: Boolean by FreezableVar(false)
+
     override fun configureAnalysisFlags(collector: MessageCollector): MutableMap<AnalysisFlag<*>, Any> {
         val result = super.configureAnalysisFlags(collector)
         result[JvmAnalysisFlags.strictMetadataVersionSemantics] = strictMetadataVersionSemantics

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/K2JVMCompiler.kt

@@ -80,10 +80,7 @@ class K2JVMCompiler : CLICompiler<K2JVMCompilerArguments>() {
         configuration.configureExplicitContentRoots(arguments)
         configuration.configureStandardLibs(paths, arguments)
 
-        // in case of IC we need to run compiler even
+        if (arguments.buildFile == null && arguments.freeArgs.isEmpty() && !arguments.version && !arguments.allowNoSourceFiles) {
-        // when there are no files to compile (to update caches & metadata when a file is removed)
-        val isICEnabled = configuration[JVMConfigurationKeys.INCREMENTAL_COMPILATION_COMPONENTS] != null
-        if (arguments.buildFile == null && arguments.freeArgs.isEmpty() && !(arguments.version || isICEnabled)) {
             if (arguments.script) {
                 messageCollector.report(ERROR, "Specify script source path to evaluate")
                 return COMPILATION_ERROR
@@ -154,7 +151,7 @@ class K2JVMCompiler : CLICompiler<K2JVMCompilerArguments>() {
                 if (!it) return COMPILATION_ERROR
             }
 
-            if (environment.getSourceFiles().isEmpty() && !isICEnabled && buildFile == null) {
+            if (environment.getSourceFiles().isEmpty() && !arguments.allowNoSourceFiles && buildFile == null) {
                 if (arguments.version) return OK
 
                 messageCollector.report(ERROR, "No source files")

compiler/incremental-compilation-impl/src/org/jetbrains/kotlin/incremental/IncrementalJvmCompilerRunner.kt

@@ -368,6 +368,7 @@ class IncrementalJvmCompilerRunner(
         val compiler = K2JVMCompiler()
         val freeArgsBackup = args.freeArgs.toList()
         args.freeArgs += sourcesToCompile.map { it.absolutePath }
+        args.allowNoSourceFiles = true
         val exitCode = compiler.exec(messageCollector, services, args)
         args.freeArgs = freeArgsBackup
         return exitCode

compiler/testData/cli/jvm/extraHelp.out

@@ -2,6 +2,7 @@ Usage: kotlinc-jvm <options> <source files>
 where advanced options include:
   -Xadd-modules=<module[,]>  Root modules to resolve in addition to the initial modules,
                              or all modules on the module path if <module> is ALL-MODULE-PATH
+  -Xallow-no-source-files    Allow no source files
   -Xassertions={always-enable|always-disable|jvm|legacy}
                              Assert calls behaviour
                              -Xassertions=always-enable:  enable, ignore jvm assertion settings;

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -405,6 +405,7 @@ open class KotlinCompile : AbstractKotlinCompile<K2JVMCompilerArguments>(), Kotl
 
         if (defaultsOnly) return
 
+        args.allowNoSourceFiles = true
         args.classpathAsList = compileClasspath.toList()
         args.destinationAsFile = destinationDir
         parentKotlinOptionsImpl?.updateArguments(args)