field can be local: extend "immutable" types (IDEA-63561)

java/java-impl/src/com/intellij/codeInspection/varScopeCanBeNarrowed/FieldCanBeLocalInspection.java

@@ -31,6 +31,7 @@ import com.intellij.openapi.editor.ScrollType;
 import com.intellij.openapi.extensions.Extensions;
 import com.intellij.openapi.fileEditor.FileEditorManager;
 import com.intellij.openapi.project.Project;
+import com.intellij.openapi.util.Comparing;
 import com.intellij.openapi.util.JDOMExternalizableStringList;
 import com.intellij.openapi.util.Ref;
 import com.intellij.openapi.util.WriteExternalException;
@@ -190,7 +191,7 @@ public class FieldCanBeLocalInspection extends BaseLocalInspectionTool {
         final PsiElement resolved = readBeforeWrite.resolve();
         if (resolved instanceof PsiField) {
           final PsiField field = (PsiField)resolved;
-          if (!(field.getType() instanceof PsiPrimitiveType) || !PsiUtil.isConstantExpression(field.getInitializer()) || getWrittenVariables(controlFlow, writtenVariables).contains(field)){
+          if (!isImmutableState(field.getType()) || !PsiUtil.isConstantExpression(field.getInitializer()) || getWrittenVariables(controlFlow, writtenVariables).contains(field)){
             PsiElement parent = body.getParent();
             if (!(parent instanceof PsiMethod) ||
                 !((PsiMethod)parent).isConstructor() ||
@@ -208,6 +209,16 @@ public class FieldCanBeLocalInspection extends BaseLocalInspectionTool {
     }
   }
 
+  private static boolean isImmutableState(PsiType type) {
+    if (type instanceof PsiPrimitiveType) {
+      return true;
+    }
+    if (PsiPrimitiveType.getUnboxedType(type) != null) {
+      return true;
+    }
+    return Comparing.strEqual(CommonClassNames.JAVA_LANG_STRING, type.getCanonicalText());
+  }
+
   private static Collection<PsiVariable> getWrittenVariables(ControlFlow controlFlow, Ref<Collection<PsiVariable>> writtenVariables) {
     if (writtenVariables.get() == null) {
       writtenVariables.set(ControlFlowUtil.getWrittenVariables(controlFlow, 0, controlFlow.getSize(), false));

java/java-tests/testData/inspection/fieldCanBeLocal/fieldWithImmutableType/expected.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<problems>
+  <problem>
+    <file>Test.java</file>
+    <line>2</line>
+    <description>Field can be converted to a local variable</description>
+  </problem>
+</problems>

java/java-tests/testData/inspection/fieldCanBeLocal/fieldWithImmutableType/src/Test.java

@@ -0,0 +1,8 @@
+class Test {
+    private String f = "";
+
+    void foo () {
+      f.substring(0);
+    }
+
+}

java/java-tests/testSrc/com/intellij/codeInspection/FieldCanBeLocalTest.java

@@ -30,6 +30,7 @@ public class FieldCanBeLocalTest extends InspectionTestCase {
   public void testLocalStateVar2InnerClass() throws Exception { doTest(); }
   public void testNotConstantInitializer() throws Exception {doTest();}
   public void testInnerClassFieldInitializer() throws Exception {doTest();}
+  public void testFieldWithImmutableType() throws Exception {doTest();}
   public void testIgnoreAnnotated() throws Exception {
     final FieldCanBeLocalInspection inspection = new FieldCanBeLocalInspection();
     inspection.EXCLUDE_ANNOS.add(Deprecated.class.getName());