StringConcatenationInLoopsInspection: add null-check for unknown variables

IDEA-183139 "String to StringBuilder" quick fix may cause NPE

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/CommonDataflow.java

@@ -40,6 +40,9 @@ public class CommonDataflow {
       if(existing != DfaFactMap.EMPTY) {
         DfaValue value = memState.peek();
         DfaFactMap newMap = memState.getFactMap(value);
+        if (!Boolean.FALSE.equals(newMap.get(DfaFactType.CAN_BE_NULL)) && memState.isNotNull(value)) {
+          newMap = newMap.with(DfaFactType.CAN_BE_NULL, false);
+        }
         myFacts.put(expression, existing == null ? newMap : existing.union(newMap));
       }
     }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/stringConcatInLoop/afterConcatNull.java

@@ -5,7 +5,7 @@ public class Main {
         StringBuilder res = null;
         for (String s : strings) {
             if(res == null) {
-                res = new StringBuilder(s);
+                res = s == null ? null : new StringBuilder(s);
             } else {
                 res.append(s);
             }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/stringConcatInLoop/afterConcatNull2.java

@@ -0,0 +1,16 @@
+// "Convert variable 'res' from String to StringBuilder" "true"
+
+public class Main {
+    String test(String[] strings) {
+        StringBuilder res = null;
+        for (String s : strings) {
+            if (s == null) continue;
+            if(res == null) {
+                res = new StringBuilder(s);
+            } else {
+                res.append(s);
+            }
+        }
+        return res.toString();
+    }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/stringConcatInLoop/beforeConcatNull2.java

@@ -0,0 +1,16 @@
+// "Convert variable 'res' from String to StringBuilder" "true"
+
+public class Main {
+    String test(String[] strings) {
+        String res = null;
+        for (String s : strings) {
+            if (s == null) continue;
+            if(res == null) {
+                res = s;
+            } else {
+                res<caret>+=s;
+            }
+        }
+        return res;
+    }
+}

plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/performance/StringConcatenationInLoopsInspection.java

@@ -17,6 +17,8 @@ package com.siyeh.ig.performance;
 
 import com.intellij.codeInsight.PsiEquivalenceUtil;
 import com.intellij.codeInspection.ProblemDescriptor;
+import com.intellij.codeInspection.dataFlow.CommonDataflow;
+import com.intellij.codeInspection.dataFlow.DfaFactType;
 import com.intellij.codeInspection.util.ChangeToAppendUtil;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.util.text.StringUtil;
@@ -329,7 +331,13 @@ public class StringConcatenationInLoopsInspection extends BaseInspection {
         return ct.text(initializer);
       }
       String text = initializer == null || ExpressionUtils.isLiteral(initializer, "") ? "" : ct.text(initializer);
-      return "new " + myTargetType + "(" + text + ")";
+      String stringBuilderText = "new " + myTargetType + "(" + text + ")";
+      PsiReferenceExpression ref = ObjectUtils.tryCast(PsiUtil.skipParenthesizedExprDown(initializer), PsiReferenceExpression.class);
+      if (ref != null && ref.getQualifierExpression() == null &&
+          !Boolean.FALSE.equals(CommonDataflow.getExpressionFact(ref, DfaFactType.CAN_BE_NULL))) {
+        return ref.getText() + "==null?null:" + stringBuilderText;
+      }
+      return stringBuilderText;
     }
 
     void replaceAll(PsiVariable variable,