Support forcing usage of ultra-light classes in tests

compiler/light-classes/src/org/jetbrains/kotlin/asJava/classes/KtLightClassForSourceDeclaration.kt

@@ -341,7 +341,7 @@ abstract class KtLightClassForSourceDeclaration(protected val classOrObject: KtC
                 return null
             }
 
-            if (Registry.`is`("kotlin.use.ultra.light.classes", false)) {
+            if (KtUltraLightClass.forceUsingUltraLightClasses || Registry.`is`("kotlin.use.ultra.light.classes", false)) {
                 LightClassGenerationSupport.getInstance(classOrObject.project).createUltraLightClass(classOrObject)?.let { return it }
             }
 

compiler/light-classes/src/org/jetbrains/kotlin/asJava/classes/ultraLightPsi.kt

@@ -14,6 +14,7 @@ import com.intellij.psi.impl.PsiSuperMethodImplUtil
 import com.intellij.psi.impl.light.*
 import com.intellij.psi.util.TypeConversionUtil
 import org.jetbrains.annotations.NonNls
+import org.jetbrains.annotations.TestOnly
 import org.jetbrains.kotlin.asJava.LightClassGenerationSupport
 import org.jetbrains.kotlin.asJava.builder.LightClassData
 import org.jetbrains.kotlin.asJava.builder.LightMemberOriginForDeclaration
@@ -47,6 +48,15 @@ import org.jetbrains.kotlin.types.KotlinType
 
 class KtUltraLightClass(classOrObject: KtClassOrObject, private val support: UltraLightSupport) :
     KtLightClassImpl(classOrObject) {
+    companion object {
+
+        // This property may be removed once IntelliJ versions earlier than 2018.3 become unsupported
+        // And usages of that property may be replaced with relevant registry change
+        @Volatile
+        @TestOnly
+        var forceUsingUltraLightClasses = false
+    }
+
     private val tooComplex: Boolean by lazyPub { support.isTooComplexForUltraLightGeneration(classOrObject) }
 
     override fun isFinal(isFinalByPsi: Boolean) = if (tooComplex) super.isFinal(isFinalByPsi) else isFinalByPsi
@@ -658,4 +668,4 @@ interface UltraLightSupport {
     val moduleName: String
     fun findAnnotation(owner: KtAnnotated, fqName: FqName): Pair<KtAnnotationEntry, AnnotationDescriptor>?
     fun isTooComplexForUltraLightGeneration(element: KtClassOrObject): Boolean
-}
+}

idea/idea-core/src/org/jetbrains/kotlin/idea/perf/UltraLightChecker.kt

@@ -5,7 +5,10 @@
 
 package org.jetbrains.kotlin.idea.perf
 
+import com.intellij.openapi.Disposable
+import com.intellij.openapi.util.Disposer
 import com.intellij.psi.*
+import com.intellij.testFramework.UsefulTestCase
 import org.jetbrains.annotations.TestOnly
 import org.jetbrains.kotlin.asJava.LightClassGenerationSupport
 import org.jetbrains.kotlin.asJava.classes.KtLightClassForSourceDeclaration
@@ -14,6 +17,13 @@ import org.jetbrains.kotlin.psi.KtClassOrObject
 import org.jetbrains.kotlin.psi.KtFile
 import org.junit.Assert
 
+fun UsefulTestCase.forceUsingUltraLightClassesForTest() {
+    KtUltraLightClass.forceUsingUltraLightClasses = true
+    Disposer.register(testRootDisposable, Disposable {
+        KtUltraLightClass.forceUsingUltraLightClasses = false
+    })
+}
+
 @TestOnly
 object UltraLightChecker {
     fun checkClassEquivalence(file: KtFile) {
@@ -122,4 +132,4 @@ object UltraLightChecker {
                 "}"
     }
 
-}
+}