Introduce FIR_IDENTICAL for FIR vs old frontend tests #KT-36879 Fixed

compiler/tests-common/tests/org/jetbrains/kotlin/fir/AbstractFirOldFrontendDiagnosticsTest.kt

@@ -12,7 +12,7 @@ import java.io.File
 
 abstract class AbstractFirOldFrontendDiagnosticsTest : AbstractFirDiagnosticsTest() {
     override fun createTestFileFromPath(filePath: String): File {
-        val newPath = filePath.replace(".kt", ".fir.kt")
+        val newPath = if (File(filePath).readText().contains("// FIR_IDENTICAL")) filePath else filePath.replace(".kt", ".fir.kt")
         return File(newPath).also {
             prepareTestDataFile(filePath, it)
         }
@@ -40,6 +40,11 @@ abstract class AbstractFirOldFrontendDiagnosticsTest : AbstractFirDiagnosticsTes
             checkResultingFirFiles(allFirFiles, testDataFile)
             assertFalse("Test is good but there is expected exception", failureFile.exists())
             checkDiagnostics(testDataFile, testFiles, allFirFiles)
+            if (testDataFile.absolutePath.endsWith(".fir.kt")) {
+                val oldFrontendTestDataFile = File(testDataFile.absolutePath.replace(".fir.kt", ".kt"))
+                compareAndMergeFirFileAndOldFrontendFile(oldFrontendTestDataFile, testDataFile)
+            }
+
             val needDump = testFiles.any { it.directives.containsKey("FIR_DUMP") }
             if (needDump) {
                 checkFir(testDataFile, allFirFiles)

compiler/tests-common/tests/org/jetbrains/kotlin/fir/FirTestUtils.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir
+
+import junit.framework.TestCase
+import java.io.File
+
+fun compareAndMergeFirFileAndOldFrontendFile(oldFrontendTestDataFile: File, frontendIRTestDataFile: File) {
+    if (oldFrontendTestDataFile.exists() && frontendIRTestDataFile.exists()) {
+        val originalLines = oldFrontendTestDataFile.readLines()
+        val firLines = frontendIRTestDataFile.readLines()
+        val sameDumps = firLines.withIndex().all { (index, line) ->
+            val trimmed = line.trim()
+            val originalTrimmed = originalLines.getOrNull(index)?.trim()
+            trimmed.isEmpty() && originalTrimmed?.isEmpty() != false || trimmed == originalTrimmed
+        } && originalLines.withIndex().all { (index, line) ->
+            index < firLines.size || line.trim().isEmpty()
+        }
+        if (sameDumps) {
+            frontendIRTestDataFile.delete()
+            oldFrontendTestDataFile.writeText("// FIR_IDENTICAL\n" + oldFrontendTestDataFile.readText())
+        }
+        TestCase.assertFalse(
+            "Dumps via FIR & via old FE are the same. Deleted .fir.txt dump, added // FIR_IDENTICAL to test source",
+            sameDumps
+        )
+    }
+}