Remove the error when using @children on the root component

This complicates multi-component handling

internal/compiler/lib.rs

@@ -224,11 +224,6 @@ pub async fn compile_syntax_node(
         &type_registry,
     );
 
-    if let Some((_, _, node)) = &*doc.root_component.child_insertion_point.borrow() {
-        diagnostics
-            .push_error("@children placeholder not allowed in the final component".into(), node)
-    }
-
     if !diagnostics.has_error() {
         passes::run_passes(&doc, &mut loader, false, &mut diagnostics).await;
     } else {

internal/compiler/passes.rs

@@ -116,7 +116,7 @@ pub async fn run_passes(
 
     focus_handling::call_focus_on_init(root_component);
 
-    ensure_window::ensure_window(root_component, &doc.local_registry, &style_metrics);
+    ensure_window::ensure_window(root_component, &doc.local_registry, &style_metrics, diag);
 
     doc.visit_all_used_components(|component| {
         border_radius::handle_border_radius(component, diag);

internal/compiler/passes/ensure_window.rs

@@ -3,6 +3,7 @@
 
 //! Make sure that the top level element of the component is always a Window
 
+use crate::diagnostics::BuildDiagnostics;
 use crate::expression_tree::{BindingExpression, BuiltinFunction, Expression};
 use crate::langtype::Type;
 use crate::namedreference::NamedReference;
@@ -16,7 +17,15 @@ pub fn ensure_window(
     component: &Rc<Component>,
     type_register: &TypeRegister,
     style_metrics: &Rc<Component>,
+    diag: &mut BuildDiagnostics,
 ) {
+    if component.inherits_popup_window.get() {
+        diag.push_error(
+            "PopupWindow cannot be the top level".into(),
+            &*component.root_element.borrow(),
+        );
+    }
+
     if component.root_element.borrow().builtin_type().map_or(true, |b| {
         matches!(b.name.as_str(), "Window" | "Dialog" | "WindowItem" | "PopupWindow")
     }) {

internal/compiler/passes/lower_popups.rs

@@ -45,13 +45,6 @@ fn lower_popup_window(
     let parent_component = popup_window_element.borrow().enclosing_component.upgrade().unwrap();
     let parent_element = match parent_element {
         None => {
-            if parent_component.is_root_component.get() {
-                diag.push_error(
-                    "PopupWindow cannot be the top level".into(),
-                    &*popup_window_element.borrow(),
-                );
-                return;
-            }
             if matches!(popup_window_element.borrow().base_type, ElementType::Builtin(_)) {
                 popup_window_element.borrow_mut().base_type = window_type.clone();
             }

internal/compiler/tests/syntax/basic/children_placeholder.slint

@@ -36,6 +36,5 @@ export TestBox2 := Rectangle {
 export Final := TestBox {
     Rectangle {
         @children
-//      ^error{children placeholder not allowed in the final component}
     }
 }

internal/compiler/tests/syntax/basic/children_placeholder_2.slint

@@ -1,9 +0,0 @@
-// Copyright © SixtyFPS GmbH <info@slint.dev>
-// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
-
-export Final := Window {
-    VerticalLayout {
-        @children
-//      ^error{children placeholder not allowed in the final component}
-    }
-}

tests/cases/children/children_placeholder.slint

@@ -1,7 +1,7 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
 
-Container := Rectangle {
+component Container inherits Rectangle {
     GridLayout {
         padding: 0phx;
         spacing: 0phx;
@@ -19,15 +19,16 @@ Container := Rectangle {
     }
 }
 
-TestCase := Container {
+export component TestCase inherits Container {
     width: 300phx;
     height: 200phx;
 
     rect1 := Rectangle {
         background: black;
+        @children
     }
 
-    property <bool> rect1_pos_ok: rect1.x == 150phx;
+    out property <bool> rect1_pos_ok: rect1.x == 150phx;
 
     out property <bool> test: rect1_pos_ok;
 }