Add IDEA data-flow analysis to guess nullability

Add "if return..." folding to "return if"

idea/src/org/jetbrains/kotlin/idea/intentions/branchedTransformations/IfThenUtils.kt

@@ -63,6 +63,11 @@ fun KtExpression.unwrapBlockOrParenthesis(): KtExpression {
     return innerExpression
 }
 
+fun KtExpression?.isTrivialStatementBody(): Boolean = when (this?.unwrapBlockOrParenthesis()) {
+    is KtIfExpression, is KtBlockExpression -> false
+    else -> true
+}
+
 fun KtExpression?.isNullExpression(): Boolean = this?.unwrapBlockOrParenthesis()?.node?.elementType == KtNodeTypes.NULL
 
 fun KtExpression?.isNullExpressionOrEmptyBlock(): Boolean = this.isNullExpression() || this is KtBlockExpression && this.statements.isEmpty()

idea/src/org/jetbrains/kotlin/idea/j2k/IdeaJavaToKotlinServices.kt

@@ -16,8 +16,13 @@
 
 package org.jetbrains.kotlin.idea.j2k
 
+import com.intellij.codeInspection.dataFlow.DfaUtil
+import com.intellij.codeInspection.dataFlow.Nullness
+import com.intellij.psi.PsiElement
 import com.intellij.psi.PsiMethod
+import com.intellij.psi.PsiVariable
 import org.jetbrains.kotlin.j2k.*
+import org.jetbrains.kotlin.j2k.ast.Nullability
 
 object IdeaJavaToKotlinServices : JavaToKotlinConverterServices {
     override val referenceSearcher: ReferenceSearcher
@@ -31,8 +36,25 @@ object IdeaJavaToKotlinServices : JavaToKotlinConverterServices {
 
     override val docCommentConverter: DocCommentConverter
         get() = IdeaDocCommentConverter
+
+    override val javaDataFlowAnalyzerFacade: JavaDataFlowAnalyzerFacade
+        get() = IdeaJavaDataFlowAnalyzerFacade
 }
 
 object IdeaSuperMethodSearcher : SuperMethodsSearcher {
     override fun findDeepestSuperMethods(method: PsiMethod) = method.findDeepestSuperMethods().asList()
+}
+
+private object IdeaJavaDataFlowAnalyzerFacade : JavaDataFlowAnalyzerFacade {
+    override fun variableNullability(variable: PsiVariable, context: PsiElement): Nullability =
+            DfaUtil.checkNullness(variable, context).toNullability()
+
+    override fun methodNullability(method: PsiMethod): Nullability =
+            DfaUtil.inferMethodNullity(method).toNullability()
+
+    private fun Nullness.toNullability() = when (this) {
+        Nullness.UNKNOWN -> Nullability.Default
+        Nullness.NOT_NULL -> Nullability.NotNull
+        Nullness.NULLABLE -> Nullability.Nullable
+    }
 }

idea/src/org/jetbrains/kotlin/idea/j2k/J2kPostProcessings.kt

@@ -27,8 +27,11 @@ import org.jetbrains.kotlin.idea.core.replaced
 import org.jetbrains.kotlin.idea.core.setVisibility
 import org.jetbrains.kotlin.idea.inspections.RedundantSamConstructorInspection
 import org.jetbrains.kotlin.idea.intentions.*
+import org.jetbrains.kotlin.idea.intentions.branchedTransformations.intentions.FoldIfToReturnAsymmetricallyIntention
+import org.jetbrains.kotlin.idea.intentions.branchedTransformations.intentions.FoldIfToReturnIntention
 import org.jetbrains.kotlin.idea.intentions.branchedTransformations.intentions.IfThenToElvisIntention
 import org.jetbrains.kotlin.idea.intentions.branchedTransformations.intentions.IfThenToSafeAccessIntention
+import org.jetbrains.kotlin.idea.intentions.branchedTransformations.isTrivialStatementBody
 import org.jetbrains.kotlin.idea.intentions.conventionNameCalls.ReplaceGetOrSetInspection
 import org.jetbrains.kotlin.idea.intentions.conventionNameCalls.ReplaceGetOrSetIntention
 import org.jetbrains.kotlin.idea.quickfix.RemoveModifierFix
@@ -76,9 +79,14 @@ object J2KPostProcessingRegistrar {
         _processings.add(RemoveRedundantCastToNullableProcessing())
 
         registerIntentionBasedProcessing(ConvertToExpressionBodyIntention(convertEmptyToUnit = false)) { it is KtPropertyAccessor }
+
+        registerIntentionBasedProcessing(FoldInitializerAndIfToElvisIntention())
+
+        registerIntentionBasedProcessing(FoldIfToReturnIntention()) { it.then.isTrivialStatementBody() && it.`else`.isTrivialStatementBody() }
+        registerIntentionBasedProcessing(FoldIfToReturnAsymmetricallyIntention()) { it.then.isTrivialStatementBody() && (KtPsiUtil.skipTrailingWhitespacesAndComments(it) as KtReturnExpression).returnedExpression.isTrivialStatementBody() }
+
         registerIntentionBasedProcessing(IfThenToSafeAccessIntention())
         registerIntentionBasedProcessing(IfThenToElvisIntention())
-        registerIntentionBasedProcessing(FoldInitializerAndIfToElvisIntention())
         registerIntentionBasedProcessing(SimplifyNegatedBinaryExpressionIntention())
         registerIntentionBasedProcessing(ReplaceGetOrSetIntention(), additionalChecker = ReplaceGetOrSetInspection.additionalChecker)
         registerIntentionBasedProcessing(AddOperatorModifierIntention())