corelib: Remove the extern call in the vtable for wasm

In fact, remove it for non-C++

Because wasm doesn't support C-unwind. And actually warns about
incompatible ABI for the rest

Fixes #8449

helper_crates/vtable/macro/macro.rs

@@ -68,7 +68,7 @@ allowing to access methods from the trait directly from VRef.
 This macro does the following transformation:
 
 For function type fields:
- - The ABI of each functions is changed to `extern "C"`
+ - If the function has no ABI, it is changed to `extern "C"` (unless `no_extern` is passed as `#[vtable(no_extern)]`)
  - `unsafe` is added to the signature, since it is unsafe to call these functions directly from
    the vtable without having a valid pointer to the actual object. But if the original function was
    marked unsafe, the unsafety is forwarded to the trait.
@@ -165,7 +165,9 @@ assert_eq!(dog.is_hungry, true);
 
 */
 #[proc_macro_attribute]
-pub fn vtable(_attr: TokenStream, item: TokenStream) -> TokenStream {
+pub fn vtable(attr: TokenStream, item: TokenStream) -> TokenStream {
+    let no_extern = attr.to_string().trim() == "no_extern";
+
     let mut input = parse_macro_input!(item as ItemStruct);
 
     let fields = if let Fields::Named(fields) = &mut input.fields {
@@ -394,7 +396,7 @@ pub fn vtable(_attr: TokenStream, item: TokenStream) -> TokenStream {
                 {
                     return Error::new(a.span(), "invalid ABI").to_compile_error().into();
                 }
-            } else {
+            } else if !no_extern {
                 f.abi = Some(parse_str("extern \"C\"").unwrap());
             }
             sig_extern.abi.clone_from(&f.abi);
@@ -478,9 +480,10 @@ pub fn vtable(_attr: TokenStream, item: TokenStream) -> TokenStream {
                 continue;
             }
             if ident == "dealloc" {
+                let abi = &sig_extern.abi;
                 vtable_ctor.push(quote!(#ident: {
                     #[allow(unsafe_code)]
-                    unsafe extern "C" fn #ident(_: &#vtable_name, ptr: *mut u8, layout: vtable::Layout) {
+                    unsafe #abi fn #ident(_: &#vtable_name, ptr: *mut u8, layout: vtable::Layout) {
                         use ::core::convert::TryInto;
                         vtable::internal::dealloc(ptr, layout.try_into().unwrap())
                     }

helper_crates/vtable/src/lib.rs

@@ -26,6 +26,7 @@ struct AnimalVTable {
     /// the self object.
     ///
     /// Note: the #[vtable] macro will automatically add `extern "C"` if that is missing.
+    /// (unless `no_extern` is specified)
     make_noise: fn(VRef<AnimalVTable>, i32) -> i32,
 
     /// if there is a 'drop' member, it is considered as the destructor.

internal/core-macros/lib.rs

@@ -207,3 +207,30 @@ pub fn slint_doc_str(input: TokenStream) -> TokenStream {
     assert!(visitor.1, "No slint link found");
     quote!(#doc).into()
 }
+
+/// Attribute macro that removes `extern "..."` from the function signatures
+///
+/// This is useful because wasm does not support `extern "C-unwind"` and also
+/// warn about ABI incompatibilities we wouldn't care about.
+///
+/// (can be applied to a function or a vtable struct)
+#[proc_macro_attribute]
+pub fn remove_extern(_attr: TokenStream, item: TokenStream) -> TokenStream {
+    let mut input = syn::parse_macro_input!(item as syn::Item);
+
+    match &mut input {
+        syn::Item::Fn(item_fn) => {
+            item_fn.sig.abi.take();
+        }
+        syn::Item::Struct(item_struct) => {
+            for f in item_struct.fields.iter_mut() {
+                if let syn::Type::BareFn(f) = &mut f.ty {
+                    f.abi.take();
+                }
+            }
+        }
+        _ => (),
+    }
+
+    quote!(#input).into()
+}

internal/core/graphics/image.rs

@@ -20,15 +20,16 @@ mod htmlimage;
 mod svg;
 
 #[allow(missing_docs)]
-#[vtable::vtable]
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
+#[vtable::vtable(no_extern)]
 #[repr(C)]
 pub struct OpaqueImageVTable {
-    drop_in_place: fn(VRefMut<OpaqueImageVTable>) -> Layout,
-    dealloc: fn(&OpaqueImageVTable, ptr: *mut u8, layout: Layout),
+    drop_in_place: extern "C-unwind" fn(VRefMut<OpaqueImageVTable>) -> Layout,
+    dealloc: extern "C-unwind" fn(&OpaqueImageVTable, ptr: *mut u8, layout: Layout),
     /// Returns the image size
-    size: fn(VRef<OpaqueImageVTable>) -> IntSize,
+    size: extern "C-unwind" fn(VRef<OpaqueImageVTable>) -> IntSize,
     /// Returns a cache key
-    cache_key: fn(VRef<OpaqueImageVTable>) -> ImageCacheKey,
+    cache_key: extern "C-unwind" fn(VRef<OpaqueImageVTable>) -> ImageCacheKey,
 }
 
 #[cfg(feature = "svg")]

internal/core/item_tree.rs

@@ -41,7 +41,8 @@ impl From<IndexRange> for core::ops::Range<usize> {
 }
 
 /// A ItemTree is representing an unit that is allocated together
-#[vtable]
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
+#[vtable(no_extern)]
 #[repr(C)]
 pub struct ItemTreeVTable {
     /// Visit the children of the item at index `index`.
@@ -150,10 +151,10 @@ pub struct ItemTreeVTable {
     ),
 
     /// in-place destructor (for VRc)
-    pub drop_in_place: unsafe fn(VRefMut<ItemTreeVTable>) -> vtable::Layout,
+    pub drop_in_place: unsafe extern "C-unwind" fn(VRefMut<ItemTreeVTable>) -> vtable::Layout,
 
     /// dealloc function (for VRc)
-    pub dealloc: unsafe fn(&ItemTreeVTable, ptr: *mut u8, layout: vtable::Layout),
+    pub dealloc: unsafe extern "C-unwind" fn(&ItemTreeVTable, ptr: *mut u8, layout: vtable::Layout),
 }
 
 #[cfg(test)]
@@ -1047,8 +1048,9 @@ impl<'a> From<&'a [ItemTreeNode]> for ItemTreeNodeArray<'a> {
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
+#[vtable(no_extern)]
 #[repr(C)]
-#[vtable]
 /// Object to be passed in visit_item_children method of the ItemTree.
 pub struct ItemVisitorVTable {
     /// Called for each child of the visited item
@@ -1057,14 +1059,14 @@ pub struct ItemVisitorVTable {
     /// as the parent's ItemTree.
     /// `index` is to be used again in the visit_item_children function of the ItemTree (the one passed as parameter)
     /// and `item` is a reference to the item itself
-    visit_item: fn(
+    visit_item: extern "C-unwind" fn(
         VRefMut<ItemVisitorVTable>,
         item_tree: &VRc<ItemTreeVTable, vtable::Dyn>,
         index: u32,
         item: Pin<VRef<ItemVTable>>,
     ) -> VisitChildrenResult,
     /// Destructor
-    drop: fn(VRefMut<ItemVisitorVTable>),
+    drop: extern "C-unwind" fn(VRefMut<ItemVisitorVTable>),
 }
 
 /// Type alias to `vtable::VRefMut<ItemVisitorVTable>`

internal/core/items.rs

@@ -112,7 +112,8 @@ pub enum RenderingResult {
 }
 
 /// Items are the nodes in the render tree.
-#[vtable]
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
+#[vtable(no_extern)]
 #[repr(C)]
 pub struct ItemVTable {
     /// This function is called by the run-time after the memory for the item

internal/interpreter/Cargo.toml

@@ -135,6 +135,7 @@ internal-json = ["dep:serde_json"]
 i-slint-compiler = { workspace = true }
 i-slint-common = { workspace = true }
 i-slint-core = { workspace = true, features = ["default", "rtti"] }
+i-slint-core-macros = { workspace = true }
 i-slint-backend-selector = { workspace = true, features = ["rtti"] }
 
 vtable = { workspace = true }

internal/interpreter/dynamic_item_tree.rs

@@ -705,6 +705,7 @@ impl ItemTreeDescription<'_> {
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn visit_children_item(
     component: ItemTreeRefPin,
     index: isize,
@@ -1845,6 +1846,7 @@ pub fn get_repeater_by_name<'a, 'id>(
     (rep_in_comp.offset.apply_pin(instance_ref.instance), rep_in_comp.item_tree_to_repeat.clone())
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn layout_info(
     component: ItemTreeRefPin,
     orientation: Orientation,
@@ -1878,6 +1880,7 @@ extern "C-unwind" fn layout_info(
     result
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 unsafe extern "C-unwind" fn get_item_ref(component: ItemTreeRefPin, index: u32) -> Pin<ItemRef> {
     let tree = get_item_tree(component);
     match &tree[index as usize] {
@@ -1893,6 +1896,7 @@ unsafe extern "C-unwind" fn get_item_ref(component: ItemTreeRefPin, index: u32)
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn get_subtree_range(component: ItemTreeRefPin, index: u32) -> IndexRange {
     generativity::make_guard!(guard);
     let instance_ref = unsafe { InstanceRef::from_pin_ref(component, guard) };
@@ -1922,6 +1926,7 @@ extern "C-unwind" fn get_subtree_range(component: ItemTreeRefPin, index: u32) ->
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn get_subtree(
     component: ItemTreeRefPin,
     index: u32,
@@ -1960,6 +1965,7 @@ extern "C-unwind" fn get_subtree(
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn get_item_tree(component: ItemTreeRefPin) -> Slice<ItemTreeNode> {
     generativity::make_guard!(guard);
     let instance_ref = unsafe { InstanceRef::from_pin_ref(component, guard) };
@@ -1967,6 +1973,7 @@ extern "C-unwind" fn get_item_tree(component: ItemTreeRefPin) -> Slice<ItemTreeN
     unsafe { core::mem::transmute::<&[ItemTreeNode], &[ItemTreeNode]>(tree) }.into()
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn subtree_index(component: ItemTreeRefPin) -> usize {
     generativity::make_guard!(guard);
     let instance_ref = unsafe { InstanceRef::from_pin_ref(component, guard) };
@@ -1977,6 +1984,7 @@ extern "C-unwind" fn subtree_index(component: ItemTreeRefPin) -> usize {
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 unsafe extern "C-unwind" fn parent_node(component: ItemTreeRefPin, result: &mut ItemWeak) {
     generativity::make_guard!(guard);
     let instance_ref = InstanceRef::from_pin_ref(component, guard);
@@ -2016,6 +2024,7 @@ unsafe extern "C-unwind" fn parent_node(component: ItemTreeRefPin, result: &mut
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 unsafe extern "C-unwind" fn embed_component(
     component: ItemTreeRefPin,
     parent_component: &ItemTreeWeak,
@@ -2046,6 +2055,7 @@ unsafe extern "C-unwind" fn embed_component(
     extra_data.embedding_position.set((parent_component.clone(), parent_item_tree_index)).is_ok()
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn item_geometry(component: ItemTreeRefPin, item_index: u32) -> LogicalRect {
     generativity::make_guard!(guard);
     let instance_ref = unsafe { InstanceRef::from_pin_ref(component, guard) };
@@ -2068,6 +2078,7 @@ extern "C-unwind" fn item_geometry(component: ItemTreeRefPin, item_index: u32) -
 
 // silence the warning despite `AccessibleRole` is a `#[non_exhaustive]` enum from another crate.
 #[allow(improper_ctypes_definitions)]
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn accessible_role(component: ItemTreeRefPin, item_index: u32) -> AccessibleRole {
     generativity::make_guard!(guard);
     let instance_ref = unsafe { InstanceRef::from_pin_ref(component, guard) };
@@ -2086,6 +2097,7 @@ extern "C-unwind" fn accessible_role(component: ItemTreeRefPin, item_index: u32)
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn accessible_string_property(
     component: ItemTreeRefPin,
     item_index: u32,
@@ -2115,6 +2127,7 @@ extern "C-unwind" fn accessible_string_property(
     }
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn accessibility_action(
     component: ItemTreeRefPin,
     item_index: u32,
@@ -2150,6 +2163,7 @@ extern "C-unwind" fn accessibility_action(
     };
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn supported_accessibility_actions(
     component: ItemTreeRefPin,
     item_index: u32,
@@ -2172,6 +2186,7 @@ extern "C-unwind" fn supported_accessibility_actions(
     val
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn item_element_infos(
     component: ItemTreeRefPin,
     item_index: u32,
@@ -2186,6 +2201,7 @@ extern "C-unwind" fn item_element_infos(
     true
 }
 
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
 extern "C-unwind" fn window_adapter(
     component: ItemTreeRefPin,
     do_create: bool,
@@ -2200,14 +2216,22 @@ extern "C-unwind" fn window_adapter(
     }
 }
 
-unsafe extern "C" fn drop_in_place(component: vtable::VRefMut<ItemTreeVTable>) -> vtable::Layout {
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
+unsafe extern "C-unwind" fn drop_in_place(
+    component: vtable::VRefMut<ItemTreeVTable>,
+) -> vtable::Layout {
     let instance_ptr = component.as_ptr() as *mut Instance<'static>;
     let layout = (*instance_ptr).type_info().layout();
     dynamic_type::TypeInfo::drop_in_place(instance_ptr);
     layout.into()
 }
 
-unsafe extern "C" fn dealloc(_vtable: &ItemTreeVTable, ptr: *mut u8, layout: vtable::Layout) {
+#[cfg_attr(not(feature = "ffi"), i_slint_core_macros::remove_extern)]
+unsafe extern "C-unwind" fn dealloc(
+    _vtable: &ItemTreeVTable,
+    ptr: *mut u8,
+    layout: vtable::Layout,
+) {
     std::alloc::dealloc(ptr, layout.try_into().unwrap());
 }