show functional type instead of not helpful <lambda expression> type in type information popup

java/java-impl/src/com/intellij/codeInsight/hint/JavaTypeProvider.java

@@ -37,6 +37,12 @@ public class JavaTypeProvider extends ExpressionTypeProvider<PsiExpression> {
   @Override
   public String getInformationHint(@NotNull PsiExpression element) {
     PsiType type = element.getType();
+    if (type instanceof PsiLambdaExpressionType) {
+      type = ((PsiLambdaExpressionType)type).getExpression().getFunctionalInterfaceType();
+    }
+    else if (type instanceof PsiMethodReferenceType) {
+      type = ((PsiMethodReferenceType)type).getExpression().getFunctionalInterfaceType();
+    }
     String text = type == null ? "<unknown>" : type.getPresentableText();
     return StringUtil.escapeXml(text);
   }

java/java-tests/testSrc/com/intellij/codeInsight/hint/JavaTypeProviderTest.java

@@ -36,6 +36,16 @@ public class JavaTypeProviderTest extends LightCodeInsightTestCase {
            "</table>");
   }
 
+  public void testFunctionalType() {
+    doTest("  void test() {\n" +
+           "      Runnable r = <selection>() -> {}</selection>;\n" +
+           "  }", "Runnable", 
+           "<table>" +
+           "<tr><td align='left' valign='top' style='color:#909090'>Type:</td><td>Runnable</td></tr>" +
+           "<tr><td align='left' valign='top' style='color:#909090'>Nullability:</td><td>non-null</td></tr>" +
+           "</table>");
+  }
+
   public void testTypeConstraint() {
     doTest("  void x(Object a) {\n" +
            "    if(a instanceof String || a instanceof Number) {\n" +