show folded closure type when it can't be inferred from the context (IDEA-87048)

java/java-impl/src/com/intellij/codeInsight/folding/impl/JavaFoldingBuilder.java

@@ -15,6 +15,8 @@
  */
 package com.intellij.codeInsight.folding.impl;
 
+import com.intellij.codeInsight.ExpectedTypeInfo;
+import com.intellij.codeInsight.ExpectedTypesProvider;
 import com.intellij.codeInsight.daemon.impl.CollectHighlightsUtil;
 import com.intellij.codeInsight.daemon.impl.analysis.HighlightUtil;
 import com.intellij.codeInsight.folding.JavaCodeFoldingSettings;
@@ -44,6 +46,7 @@ import com.intellij.psi.util.PropertyUtil;
 import com.intellij.psi.util.PsiTreeUtil;
 import com.intellij.psi.util.PsiUtil;
 import com.intellij.util.Function;
+import com.intellij.util.ObjectUtils;
 import com.intellij.util.text.CharArrayUtil;
 import org.jetbrains.annotations.NonNls;
 import org.jetbrains.annotations.NotNull;
@@ -646,13 +649,24 @@ public class JavaFoldingBuilder extends CustomFoldingBuilder implements DumbAwar
               if (lastLineEnd > 0 && seq.charAt(lastLineEnd) == '\n') lastLineEnd--;
               if (lastLineEnd < firstLineStart) return false;
 
+              String type = "";
+              if (!quick) {
+                ExpectedTypeInfo[] types = ExpectedTypesProvider.getExpectedTypes(expression, false);
+                if (types.length != 1 || !types[0].getType().equals(anonymousClass.getBaseClassType())) {
+                  final String baseClassName = ObjectUtils.assertNotNull(anonymousClass.getBaseClassType().resolve()).getName();
+                  if (baseClassName != null) {
+                    type = "(" + baseClassName + ") ";
+                  }
+                }
+              }
+
               final String params = StringUtil.join(method.getParameterList().getParameters(), new Function<PsiParameter, String>() {
                 @Override
                 public String fun(final PsiParameter psiParameter) {
                   return psiParameter.getName();
                 }
               }, ", ");
-              @NonNls final String lambdas = "(" + params + ") -> {";
+              @NonNls final String lambdas = type + "(" + params + ") -> {";
 
               final int closureStart = expression.getTextRange().getStartOffset();
               final int closureEnd = expression.getTextRange().getEndOffset();

java/java-tests/testSrc/com/intellij/codeInsight/folding/JavaFoldingTest.groovy

@@ -217,6 +217,48 @@ class Test {
     assert closureStartFold
   }
 
+  public void "test closure folding placeholder texts"() {
+    myFixture.addClass('interface Runnable2 { void run(); }')
+    myFixture.addClass('interface Runnable3 { void run(); }')
+    myFixture.addClass('interface Runnable4 { void run(); }')
+    def text = """\
+class Test {
+  void test() {
+    Runnable r = new Runnable() {
+      public void run() {
+        System.out.println();
+      }
+    };
+    new Runnable2() {
+      public void run() {
+        System.out.println();
+      }
+    }.run();
+    foo(new Runnable3() {
+      public void run() {
+        System.out.println();
+      }
+    });
+    bar(new Runnable4() {
+      public void run() {
+        System.out.println();
+      }
+    });
+  }
+
+  void foo(Object o) {}
+  void bar(Runnable4 o) {}
+}
+"""
+    configure text
+    def foldingModel = myFixture.editor.foldingModel as FoldingModelImpl
+
+    assert foldingModel.getCollapsedRegionAtOffset(text.indexOf("Runnable(")).placeholderText == '() -> { '
+    assert foldingModel.getCollapsedRegionAtOffset(text.indexOf("Runnable2(")).placeholderText == '(Runnable2) () -> { '
+    assert foldingModel.getCollapsedRegionAtOffset(text.indexOf("Runnable3(")).placeholderText == '(Runnable3) () -> { '
+    assert foldingModel.getCollapsedRegionAtOffset(text.indexOf("Runnable4(")).placeholderText == '() -> { '
+  }
+
   public void "test no closure folding when the method throws an unresolved exception"() {
     def text = """\
 class Test {