[java] IDEA-216276 Add ShowDuplicateElementsFix that shows popup with a list of element duplicates and use it for duplicate-method error

#IDEA-216276 Fixed
Merge-request: IJ-MR-171237
Merged-by: Marcin Mikosik <marcin.mikosik@jetbrains.com>

GitOrigin-RevId: 5aab229888a361d3d4f653708b7f42d832c44645

java/java-analysis-api/src/com/intellij/codeInsight/intention/QuickFixFactory.java

@@ -127,6 +127,8 @@ public abstract class QuickFixFactory {
 
   public abstract @NotNull IntentionAction createNavigateToDuplicateElementFix(@NotNull NavigatablePsiElement element);
 
+  public abstract @NotNull IntentionAction createShowDuplicateElementsFix(@NotNull List<@NotNull ? extends NavigatablePsiElement> elements);
+
   public abstract @NotNull IntentionAction createConvertToStringLiteralAction();
 
   /**

java/java-analysis-impl/resources/messages/QuickFixBundle.properties

@@ -209,6 +209,11 @@ reuse.variable.declaration.text=Reuse previous variable ''{0}''
 navigate.variable.declaration.family=Navigate to variable declaration
 navigate.variable.declaration.text=Navigate to previous declared variable ''{0}''
 navigate.duplicate.element.text=Navigate to duplicate {0}
+show.duplicate.elements.family=Show duplicates
+show.duplicate.elements.text=Show ''{0}'' duplicates
+show.duplicate.elements.popup.title=Choose Duplicate
+show.duplicate.elements.navigate.family=Navigate to duplicate
+show.duplicate.elements.navigate.text=Line #{0}
 safe.delete.family=Safe delete
 safe.delete.text=Safe delete ''{0}''
 setup.jdk.location.family=Setup JDK Location

java/java-analysis-impl/src/com/intellij/codeInsight/daemon/impl/analysis/DefaultJavaErrorFixProvider.java

@@ -338,11 +338,11 @@ public final class DefaultJavaErrorFixProvider extends AbstractJavaErrorFixProvi
       error.context().method() instanceof SyntheticElement ?
       null : myFactory.createSameErasureButDifferentMethodsFix(error.context().method(), error.context().superMethod()));
     fixes(METHOD_DUPLICATE, (error, sink) -> {
-      error.context().methods().stream()
-        .filter(m -> !m.equals(error.psi()))
-        .filter(m -> !(m instanceof SyntheticElement)) // filters out synthetic methods, such as Enum#values()
-        .findFirst()
-        .ifPresent(m -> sink.accept(myFactory.createNavigateToDuplicateElementFix(m)));
+      // filters out synthetic methods, such as Enum#values()
+      var duplicates = ContainerUtil.filter(error.context().methods(), m -> !(m instanceof SyntheticElement));
+      if (duplicates.size() > 1) {
+        sink.accept(myFactory.createShowDuplicateElementsFix(duplicates));
+      }
     });
   }
 

java/java-impl/src/com/intellij/codeInsight/daemon/impl/quickfix/NavigateToDuplicateElementFix.java

@@ -27,6 +27,10 @@ public class NavigateToDuplicateElementFix extends PsiBasedModCommandAction<Navi
 
   @Override
   protected @NotNull ModCommand perform(@NotNull ActionContext context, @NotNull NavigatablePsiElement element) {
+    return createSelectCommand(element);
+  }
+
+  static @NotNull ModCommand createSelectCommand(@NotNull NavigatablePsiElement element) {
     if (element instanceof PsiNameIdentifierOwner owner) {
       PsiElement identifier = owner.getNameIdentifier();
       if (identifier != null) {

java/java-impl/src/com/intellij/codeInsight/daemon/impl/quickfix/ShowDuplicateElementsFix.java

@@ -0,0 +1,119 @@
+// Copyright 2000-2024 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.intellij.codeInsight.daemon.impl.quickfix;
+
+import com.intellij.codeInsight.daemon.QuickFixBundle;
+import com.intellij.codeInsight.intention.preview.IntentionPreviewInfo;
+import com.intellij.codeInspection.util.IntentionFamilyName;
+import com.intellij.modcommand.*;
+import com.intellij.openapi.util.text.HtmlBuilder;
+import com.intellij.openapi.util.text.HtmlChunk;
+import com.intellij.psi.NavigatablePsiElement;
+import com.intellij.psi.PsiMethod;
+import com.intellij.psi.SmartPointerManager;
+import com.intellij.psi.SmartPsiElementPointer;
+import com.intellij.psi.util.JavaElementKind;
+import com.intellij.util.containers.ContainerUtil;
+import org.jetbrains.annotations.NotNullByDefault;
+
+import java.util.List;
+import java.util.Objects;
+
+/**
+ * Action that displays a popup with a list of duplicate elements (classes, methods, fields, etc.) passed to constructor.
+ * Code for the currently selected element in the list is displayed in the preview and also highlighted in the editor.
+ * Selecting an element from the list closes the popup and navigates to the selected element.
+ */
+@NotNullByDefault
+public class ShowDuplicateElementsFix extends PsiBasedModCommandAction<NavigatablePsiElement> {
+  private final List<SmartPsiElementPointer<NavigatablePsiElement>> myNavigatablePsiElements;
+
+  public ShowDuplicateElementsFix(List<? extends NavigatablePsiElement> duplicates) {
+    super(NavigatablePsiElement.class);
+    myNavigatablePsiElements = ContainerUtil.map(duplicates, dup -> SmartPointerManager.createPointer(dup));
+  }
+
+  @Override
+  public String getFamilyName() {
+    return QuickFixBundle.message("show.duplicate.elements.family");
+  }
+
+  @Override
+  protected Presentation getPresentation(ActionContext context, NavigatablePsiElement section) {
+    var name = QuickFixBundle.message("show.duplicate.elements.text", elementName(getDuplicatePsiElements()));
+    return Presentation.of(name);
+  }
+
+  @Override
+  protected ModCommand perform(ActionContext context, NavigatablePsiElement element) {
+    var title = QuickFixBundle.message("show.duplicate.elements.popup.title");
+    var navigateActions = ContainerUtil.map(getDuplicatePsiElements(), d -> navigateAction(d));
+    return ModCommand.chooseAction(title, navigateActions);
+  }
+
+  private static String elementName(List<NavigatablePsiElement> elements) {
+    if (!elements.isEmpty()) {
+      NavigatablePsiElement element = elements.get(0);
+      if (element instanceof PsiMethod method) {
+        return method.getName() + "()";
+      }
+      return Objects.requireNonNullElse(element.getName(), "");
+    }
+    return "";
+  }
+
+  @Override
+  protected IntentionPreviewInfo generatePreview(ActionContext context, NavigatablePsiElement element) {
+    var builder = new HtmlBuilder();
+    var elements = getDuplicatePsiElements();
+    for (int i = 0; i < elements.size(); i++) {
+      if (i != 0) {
+        builder.append(HtmlChunk.br());
+      }
+      var current = elements.get(i);
+      builder.append(IntentionPreviewInfo.navigatePreviewHtmlChunk(current.getContainingFile(), current.getTextOffset()));
+    }
+    return new IntentionPreviewInfo.Html(builder.toFragment());
+  }
+
+  private List<NavigatablePsiElement> getDuplicatePsiElements() {
+    return ContainerUtil.mapNotNull(myNavigatablePsiElements, SmartPsiElementPointer::getElement);
+  }
+
+  private static ModCommandAction navigateAction(NavigatablePsiElement navigatablePsiElement) {
+    return new NavigateToAction(navigatablePsiElement);
+  }
+
+  private static class NavigateToAction extends PsiBasedModCommandAction<NavigatablePsiElement> {
+    @IntentionFamilyName
+    private final String myFamilyName;
+
+    private NavigateToAction(NavigatablePsiElement navigatablePsiElement) {
+      super(navigatablePsiElement);
+      myFamilyName =
+        QuickFixBundle.message("show.duplicate.elements.navigate.family", JavaElementKind.fromElement(navigatablePsiElement).object());
+    }
+
+    @Override
+    public String getFamilyName() {
+      return myFamilyName;
+    }
+
+    @Override
+    protected Presentation getPresentation(ActionContext context, NavigatablePsiElement element) {
+      int lineNumber = element.getContainingFile().getFileDocument().getLineNumber(element.getTextOffset());
+      var title = QuickFixBundle.message("show.duplicate.elements.navigate.text", (lineNumber + 1));
+      return Presentation.of(title).withHighlighting(element.getTextRange());
+    }
+
+    @Override
+    protected IntentionPreviewInfo generatePreview(ActionContext context, NavigatablePsiElement element) {
+      return IntentionPreviewInfo.snippet(element);
+    }
+
+
+    @Override
+    protected ModCommand perform(ActionContext context, NavigatablePsiElement element) {
+      return NavigateToDuplicateElementFix.createSelectCommand(element);
+    }
+  }
+}

java/java-impl/src/com/intellij/codeInsight/intention/impl/config/QuickFixFactoryImpl.java

@@ -246,6 +246,11 @@ public final class QuickFixFactoryImpl extends QuickFixFactory {
     return new NavigateToDuplicateElementFix(element).asIntention();
   }
 
+  @Override
+  public @NotNull IntentionAction createShowDuplicateElementsFix(@NotNull List<@NotNull ? extends NavigatablePsiElement> elements) {
+    return new ShowDuplicateElementsFix(elements).asIntention();
+  }
+
   @Override
   public @NotNull IntentionAction createConvertToStringLiteralAction() {
     return new ConvertToStringLiteralAction().asIntention();

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/afterNavigatesFromFirstToSecond.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'foo()' duplicates|->Line #5" "true"
 
 public class MyClass {
   public void foo() {}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/afterNavigatesFromFirstToSecondGenericMethod.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'foo()' duplicates|->Line #5" "true"
 
 public class MyClass {
   public <T> void foo(T bar) {}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/afterNavigatesFromThirdToFirst.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'foo()' duplicates|->Line #4" "true"
 
 public class MyClass {
   public void <selection><caret>foo</selection>() {}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/afterNavigatesToEnumValueOfWhenDeclaredTwice.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'valueOf()' duplicates|->Line #8" "true"
 
 enum MyEnum {
   FIRST;

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/afterNavigatesToEnumValuesWhenDeclaredTwice.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'values()' duplicates|->Line #8" "true"
 
 enum MyEnum {
   FIRST;

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeDoesntNavigateToEnumValueOf.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "false"
+// "Show 'foo()' duplicates" "false"
 
 enum MyEnum {
   FIRST;

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeDoesntNavigateToEnumValues.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "false"
+// "Show 'values()' duplicates" "false"
 
 enum MyEnum {
   FIRST;

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeNavigatesFromFirstToSecond.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'foo()' duplicates|->Line #5" "true"
 
 public class MyClass {
   public void foo<caret>() {}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeNavigatesFromFirstToSecondGenericMethod.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'foo()' duplicates|->Line #5" "true"
 
 public class MyClass {
   public <T> void foo<caret>(T bar) {}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeNavigatesFromThirdToFirst.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'foo()' duplicates|->Line #4" "true"
 
 public class MyClass {
   public void foo() {}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeNavigatesToEnumValueOfWhenDeclaredTwice.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'valueOf()' duplicates|->Line #8" "true"
 
 enum MyEnum {
   FIRST;

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix/beforeNavigatesToEnumValuesWhenDeclaredTwice.java

@@ -1,4 +1,4 @@
-// "Navigate to duplicate method" "true"
+// "Show 'values()' duplicates|->Line #8" "true"
 
 enum MyEnum {
   FIRST;

java/java-tests/testSrc/com/intellij/codeInsight/daemon/impl/quickfix/NavigateToDuplicateMethodFixTest.java

@@ -1,11 +0,0 @@
-// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
-package com.intellij.codeInsight.daemon.impl.quickfix;
-
-import com.intellij.codeInsight.daemon.quickFix.LightQuickFixParameterizedTestCase;
-
-public class NavigateToDuplicateMethodFixTest extends LightQuickFixParameterizedTestCase {
-  @Override
-  protected String getBasePath() {
-    return "/codeInsight/daemonCodeAnalyzer/quickFix/navigateToDuplicateMethodFix";
-  }
-}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/impl/quickfix/ShowDuplicateMethodsFixTest.java

@@ -0,0 +1,28 @@
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.intellij.codeInsight.daemon.impl.quickfix;
+
+import com.intellij.codeInsight.daemon.quickFix.LightQuickFixParameterizedTestCase;
+import com.intellij.codeInsight.intention.IntentionAction;
+import com.intellij.openapi.util.text.StringUtil;
+import com.intellij.ui.ChooserInterceptor;
+import com.intellij.ui.UiInterceptors;
+import com.intellij.util.IncorrectOperationException;
+import org.jetbrains.annotations.NotNull;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+public class ShowDuplicateMethodsFixTest extends LightQuickFixParameterizedTestCase {
+  @Override
+  protected String getBasePath() {
+    return "/codeInsight/daemonCodeAnalyzer/quickFix/showDuplicateMethodsFix";
+  }
+
+  @Override
+  protected boolean shouldBeAvailableAfterExecution() {
+    return true;
+  }
+}

java/java-tests/testSrc/com/intellij/codeInsight/intention/impl/preview/IntentionPreviewDiffResultTest.kt

@@ -294,7 +294,22 @@ class IntentionPreviewDiffResultTest : LightPlatformCodeInsightFixture4TestCase(
     ).formatResult()
     assertEquals("", result)
   }
-  
+
+  @Test
+  fun testSnippet() {
+    val text = """
+      public void method() {
+        int a = 1;
+      }
+    """.trimIndent()
+    val result = IntentionPreviewDiffResult.fromSnippet(IntentionPreviewInfo.Snippet(JavaFileType.INSTANCE, text, 5)).formatResult()
+    assertEquals("""
+      5 : public void method() {
+      6 :   int a = 1;
+      7 : }
+    """.trimIndent(), result)
+  }
+
   /**
    * Returns a textual representation of diffs from created and modified text, how they will look
    * in the intention preview window. Each diff chunk is separated via a horizontal dashed line.

platform/analysis-api/api-dump-experimental.txt

@@ -82,6 +82,17 @@ a:com.intellij.codeInsight.completion.CompletionService
 - highlightMessage():java.lang.String
 - highlightReference(com.intellij.lang.annotation.AnnotationBuilder):com.intellij.lang.annotation.AnnotationBuilder
 - highlightSeverity():com.intellij.lang.annotation.HighlightSeverity
+com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
+- *s:snippet(com.intellij.psi.NavigatablePsiElement):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$Snippet
+*f:com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$Snippet
+- java.lang.Record
+- com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
+- <init>(com.intellij.openapi.fileTypes.FileType,java.lang.String,I):V
+- f:equals(java.lang.Object):Z
+- fileType():com.intellij.openapi.fileTypes.FileType
+- f:hashCode():I
+- startLine():I
+- text():java.lang.String
 f:com.intellij.codeInsight.intention.preview.IntentionPreviewUtils
 - *s:getModCommandPreview(com.intellij.modcommand.ModCommand,com.intellij.modcommand.ActionContext):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
 com.intellij.codeInsight.lookup.LookupEx

platform/analysis-api/api-dump.txt

@@ -128,6 +128,7 @@ com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
 - s:moveToDirectory(com.intellij.openapi.vfs.VirtualFile,com.intellij.openapi.vfs.VirtualFile):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
 - s:navigate(com.intellij.psi.NavigatablePsiElement):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
 - s:navigate(com.intellij.psi.PsiFile,I):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$Html
+- s:navigatePreviewHtmlChunk(com.intellij.psi.PsiFile,I):com.intellij.openapi.util.text.HtmlChunk
 - s:rename(com.intellij.openapi.vfs.VirtualFile,java.lang.String):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
 - s:rename(com.intellij.psi.PsiFile,java.lang.String):com.intellij.codeInsight.intention.preview.IntentionPreviewInfo
 com.intellij.codeInsight.lookup.Lookup

platform/analysis-api/src/com/intellij/codeInsight/intention/preview/IntentionPreviewInfo.java

@@ -8,6 +8,7 @@ import com.intellij.openapi.editor.Editor;
 import com.intellij.openapi.fileTypes.FileType;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.util.NlsSafe;
+import com.intellij.openapi.util.TextRange;
 import com.intellij.openapi.util.text.HtmlBuilder;
 import com.intellij.openapi.util.text.HtmlChunk;
 import com.intellij.openapi.vfs.VfsUtilCore;
@@ -450,6 +451,10 @@ public interface IntentionPreviewInfo {
    * @return a presentation describing that the action will navigate to the specified target element
    */
   static @NotNull Html navigate(@NotNull PsiFile file, int offset) {
+    return new Html(navigatePreviewHtmlChunk(file, offset).wrapWith("p"));
+  }
+
+  static @NotNull HtmlChunk navigatePreviewHtmlChunk(@NotNull PsiFile file, int offset) {
     Icon icon = file.getIcon(0);
     Document document = file.getFileDocument();
     HtmlBuilder builder = new HtmlBuilder();
@@ -461,7 +466,7 @@ public interface IntentionPreviewInfo {
       builder.append(AnalysisBundle.message("html.preview.navigate.line"))
         .append(String.valueOf(lineNumber+1));
     }
-    return new Html(builder.wrapWith("p"));
+    return builder.toFragment();
   }
 
   /**
@@ -497,4 +502,35 @@ public interface IntentionPreviewInfo {
       .toFragment();
     return new Html(content);
   }
+
+  /**
+   * Preview of a code snippet. It will be rendered using the color scheme configured for specified {@code fileType}
+   * with line numbers.
+   *
+   * @param fileType  file type of code snippet
+   * @param text      snippet text
+   * @param startLine line number of the first line of the snippet
+   */
+  @ApiStatus.Experimental
+  record Snippet(@NotNull FileType fileType, @NotNull String text, int startLine) implements IntentionPreviewInfo {
+  }
+
+  /**
+   * Creates snippet preview containing text of {@code element}.
+   *
+   * @param element element which text will be used as a snippet
+   * @return snippet preview for given element
+   */
+  @ApiStatus.Experimental
+  static @NotNull Snippet snippet(@NotNull NavigatablePsiElement element) {
+    var textRange = element.getTextRange();
+    var document = element.getContainingFile().getFileDocument();
+    int startOffset = textRange.getStartOffset();
+    int startLine = document.getLineNumber(startOffset);
+    int startOffsetOfFirstLine = document.getLineStartOffset(startLine);
+    int endOffset = textRange.getEndOffset();
+    var fileType = element.getContainingFile().getFileType();
+    var text = document.getText(TextRange.create(startOffsetOfFirstLine, endOffset)).stripIndent();
+    return new Snippet(fileType, text, startLine);
+  }
 }

platform/lang-impl/api-dump-unreviewed.txt

@@ -4175,6 +4175,7 @@ f:com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
 - equals(java.lang.Object):Z
 - sf:fromCustomDiff(com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$CustomDiff):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
 - sf:fromMultiDiff(com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$MultiFileDiff):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
+- sf:fromSnippet(com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$Snippet):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
 - f:getDiffs():java.util.List
 - f:getNewText():java.lang.String
 - hashCode():I
@@ -4184,6 +4185,7 @@ f:com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult$Com
 - bs:create$default(com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult$Companion,com.intellij.openapi.fileTypes.FileType,java.lang.String,java.lang.String,com.intellij.diff.comparison.ComparisonPolicy,Z,java.lang.String,I,java.lang.Object):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
 - f:fromCustomDiff(com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$CustomDiff):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
 - f:fromMultiDiff(com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$MultiFileDiff):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
+- f:fromSnippet(com.intellij.codeInsight.intention.preview.IntentionPreviewInfo$Snippet):com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult
 f:com.intellij.codeInsight.intention.impl.preview.IntentionPreviewDiffResult$DiffInfo
 - <init>(com.intellij.openapi.fileTypes.FileType,java.lang.String,I,I,java.util.List):V
 - f:component1():com.intellij.openapi.fileTypes.FileType

platform/lang-impl/src/com/intellij/codeInsight/completion/command/CommandCompletionDocumentationProvider.kt

@@ -277,6 +277,7 @@ private class CommandCompletionDocumentationTarget(
   private fun postprocess(info: IntentionPreviewInfo) = when (info) {
     is IntentionPreviewInfo.CustomDiff -> IntentionPreviewDiffResult.fromCustomDiff(info)
     is IntentionPreviewInfo.MultiFileDiff -> IntentionPreviewDiffResult.fromMultiDiff(info)
+    is IntentionPreviewInfo.Snippet -> IntentionPreviewDiffResult.fromSnippet(info)
     else -> info
   }
 }

platform/lang-impl/src/com/intellij/codeInsight/intention/impl/preview/IntentionPreviewDiffResult.kt

@@ -21,6 +21,15 @@ data class IntentionPreviewDiffResult(val diffs: List<DiffInfo>, @TestOnly val n
 
   data class Fragment(val type: HighlightingType, val start: Int, val end: Int)
 
+  /**
+   * Preview texts with specified fragment highlighting.
+   *
+   * @param fileType type of file from which fileText is taken
+   * @param fileText preview text
+   * @param startLine line in the file where fileText starts
+   * @param length fileText's line count
+   * @param fragments list fragments to be highlighted in preview
+   */
   data class DiffInfo(
     val fileType: FileType,
     val fileText: String,
@@ -178,5 +187,11 @@ data class IntentionPreviewDiffResult(val diffs: List<DiffInfo>, @TestOnly val n
       val text = diffs.joinToString("\n----------\n") { diff -> diff.newText }
       return IntentionPreviewDiffResult(diffInfos, text)
     }
+
+    @JvmStatic
+    fun fromSnippet(info: IntentionPreviewInfo.Snippet): IntentionPreviewDiffResult {
+      val diffInfo = DiffInfo(info.fileType, info.text, info.startLine, info.text.lines().size, listOf())
+      return IntentionPreviewDiffResult(listOf(diffInfo), info.text)
+    }
   }
 }

platform/lang-impl/src/com/intellij/codeInsight/intention/impl/preview/IntentionPreviewPopupUpdateProcessor.kt

@@ -305,6 +305,7 @@ class IntentionPreviewPopupUpdateProcessor internal constructor(
     private fun postprocess(info: IntentionPreviewInfo) = when (info) {
       is IntentionPreviewInfo.CustomDiff -> IntentionPreviewDiffResult.fromCustomDiff(info)
       is IntentionPreviewInfo.MultiFileDiff -> IntentionPreviewDiffResult.fromMultiDiff(info)
+      is IntentionPreviewInfo.Snippet -> IntentionPreviewDiffResult.fromSnippet(info)
       else -> info
     }