TrailingCommaPostFormatProcessor: add language feature check

#KT-34744

idea/resources/META-INF/extensions/ide.xml

@@ -120,6 +120,10 @@
                      description="Use IDEA project import mechanics to load Kotlin Gradle scripts configuration"
                      defaultValue="false"
                      restartRequired="false"/>
+        <registryKey key="kotlin.formatter.allowTrailingCommaInAnyProject"
+                     description="Allow a trailing comma regardless of plugin version"
+                     defaultValue="false"
+                     restartRequired="false"/>
         <fileEditorProvider implementation="org.jetbrains.kotlin.idea.scratch.ui.KtScratchFileEditorProvider"/>
 
         <registryKey key="kotlin.experimental.project.wizard"

idea/src/org/jetbrains/kotlin/idea/formatter/TrailingCommaPostFormatProcessor.kt

@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.idea.formatter
 import com.intellij.lang.ASTNode
 import com.intellij.openapi.diagnostic.Logger
 import com.intellij.openapi.util.TextRange
+import com.intellij.openapi.util.registry.Registry
 import com.intellij.psi.PsiComment
 import com.intellij.psi.PsiElement
 import com.intellij.psi.PsiFile
@@ -18,7 +19,10 @@ import com.intellij.psi.impl.source.PostprocessReformattingAspect
 import com.intellij.psi.impl.source.codeStyle.PostFormatProcessor
 import com.intellij.psi.impl.source.codeStyle.PostFormatProcessorHelper
 import com.intellij.psi.tree.TokenSet
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.idea.project.languageVersionSettings
 import org.jetbrains.kotlin.idea.util.isMultiline
+import org.jetbrains.kotlin.idea.util.module
 import org.jetbrains.kotlin.idea.util.needTrailingComma
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
@@ -32,12 +36,22 @@ import org.jetbrains.kotlin.utils.ifEmpty
 
 class TrailingCommaPostFormatProcessor : PostFormatProcessor {
     override fun processElement(source: PsiElement, settings: CodeStyleSettings): PsiElement =
-        TrailingCommaVisitor(settings).process(source)
+        if (trailingCommaIsAllowedInModule(source))
+            TrailingCommaVisitor(settings).process(source)
+        else
+            source
 
     override fun processText(source: PsiFile, rangeToReformat: TextRange, settings: CodeStyleSettings): TextRange =
-        TrailingCommaVisitor(settings).processText(source, rangeToReformat)
+        if (trailingCommaIsAllowedInModule(source))
+            TrailingCommaVisitor(settings).processText(source, rangeToReformat)
+        else
+            rangeToReformat
 }
 
+private fun trailingCommaIsAllowedInModule(source: PsiElement): Boolean =
+    Registry.`is`("kotlin.formatter.allowTrailingCommaInAnyProject", false) ||
+            source.module?.languageVersionSettings?.supportsFeature(LanguageFeature.TrailingCommas) == true
+
 private fun postFormatIsEnable(source: PsiElement): Boolean = !PostprocessReformattingAspect.getInstance(source.project).isDisabled
 
 private class TrailingCommaVisitor(val settings: CodeStyleSettings) : KtTreeVisitorVoid() {