Allow empty body blocks with comments (#10969)

close #10849

Changelog:
- update: empty body blocks return an `Empty` node resulting in `Nothing`

(cherry picked from commit bc3ab2c)

engine/runtime-integration-tests/src/test/java/org/enso/compiler/ErrorCompilerTest.java

@@ -4,6 +4,7 @@
 import static org.junit.Assert.assertTrue;
 
 import org.enso.compiler.core.ir.Empty;
+import org.enso.compiler.core.ir.Expression;
 import org.enso.compiler.core.ir.Location;
 import org.enso.compiler.core.ir.Module;
 import org.enso.compiler.core.ir.expression.errors.Syntax;
@@ -580,6 +581,19 @@ public void testEmptyBody() throws Exception {
     assertTrue(method.body() instanceof Empty);
   }
 
+  @Test
+  public void testBodyWithComment() throws Exception {
+    var ir = parse("""
+    main =
+        # comment
+    """);
+
+    var method = (Method) ir.bindings().apply(0);
+    var body = (Expression.Block) method.body();
+    assertTrue(body.expressions().isEmpty());
+    assertTrue(body.returnValue() instanceof Empty);
+  }
+
   @Test
   public void exportAllIsNotAllowed() {
     var ir = parse("""

engine/runtime-integration-tests/src/test/scala/org/enso/interpreter/test/instrument/RuntimeServerTest.scala

@@ -7463,6 +7463,54 @@ class RuntimeServerTest
     )
   }
 
+  it should "run methods with empty body ending with comment" in {
+    val contextId  = UUID.randomUUID()
+    val requestId  = UUID.randomUUID()
+    val moduleName = "Enso_Test.Test.Main"
+    val code =
+      """foo =
+        |    # TODO
+        |
+        |main =
+        |    foo
+        |""".stripMargin.linesIterator.mkString("\n")
+
+    val mainFile = context.writeMain(code)
+
+    // create context
+    context.send(Api.Request(requestId, Api.CreateContextRequest(contextId)))
+    context.receive shouldEqual Some(
+      Api.Response(requestId, Api.CreateContextResponse(contextId))
+    )
+
+    // Set sources for the module
+    context.send(
+      Api.Request(requestId, Api.OpenFileRequest(mainFile, code))
+    )
+    context.receive shouldEqual Some(
+      Api.Response(Some(requestId), Api.OpenFileResponse)
+    )
+
+    // push main
+    context.send(
+      Api.Request(
+        requestId,
+        Api.PushContextRequest(
+          contextId,
+          Api.StackItem.ExplicitCall(
+            Api.MethodPointer(moduleName, moduleName, "main"),
+            None,
+            Vector()
+          )
+        )
+      )
+    )
+    context.receiveNIgnoreStdLib(2) should contain theSameElementsAs Seq(
+      Api.Response(requestId, Api.PushContextResponse(contextId)),
+      context.executionComplete(contextId)
+    )
+  }
+
   it should "support file edit notification with IdMap" in {
     val contextId  = UUID.randomUUID()
     val requestId  = UUID.randomUUID()

engine/runtime-parser/src/main/java/org/enso/compiler/core/TreeToIr.java

@@ -1019,20 +1019,21 @@ yield translateSyntaxError(l.getExpression().getExpression(),
           }
           last = translateExpression(expr, false);
         }
-        // If the block ended in a documentation node without an expression, last may be null;
-        // the return value of the block is a doc comment.
-        // (This is to match the behavior of AstToIr; after the parser transition, we should probably
-        // ignore the orphaned documentation and return the last actual expression in the block.)
-        if (last == null && expressions.size() > 0) {
-          last = expressions.get(expressions.size() - 1);
-          expressions.remove(expressions.size() - 1);
+        var locationWithANewLine = getIdentifiedLocation(body, 0, 0, null);
+        if (last == null) {
+          if (expressions.isEmpty()) {
+            last = new Empty(locationWithANewLine, meta());
+          } else {
+            last = expressions.get(expressions.size() - 1);
+            expressions.remove(expressions.size() - 1);
+          }
         }
         var list = CollectionConverters.asScala(expressions.iterator()).toList();
-        var locationWithANewLine = getIdentifiedLocation(body, 0, 0, null);
-        if (last != null && last.location().isDefined()
+        if (last != null
+            && last.location().isDefined()
             && last.location().get().end() != locationWithANewLine.get().end()) {
-          var patched = new Location(last.location().get().start(),
-              locationWithANewLine.get().end() - 1);
+          var patched =
+              new Location(last.location().get().start(), locationWithANewLine.get().end() - 1);
           var id = IdentifiedLocation.create(patched, last.location().get().id());
           last = last.setLocation(Option.apply(id));
         }