Unable to Use `return` in Delegate Method: Type Mismatch with `RetainedReturnValue`

[giantpand2000]

After resolving the previous issue #730 , I encountered a new problem. Based on the guidance, I modified my code and the previous issue was fixed. However, I now find that using an early return in a delegate method causes a type mismatch error.

For example, consider the following code:

unsafe impl NSTableViewDelegate for Delegate {
    #[unsafe(method_id(tableView:viewForTableColumn:row:))]
    unsafe fn table_view_view_for_table_column_row(
        &self,
        table_view: &NSTableView,
        table_column: Option<&NSTableColumn>,
        row: NSInteger,
    ) -> Option<Retained<NSTableCellView>> {
        let identifier = NSUserInterfaceItemIdentifier::from_str("cell");

        let Some(path) = self.ivars().playlist.files.get(row as usize) else {
            return None;
        };

        if let Some(cell_view) = table_view
            .makeViewWithIdentifier_owner(&identifier, Some(self))
            .and_then(|cell| cell.downcast::<NSTableCellView>().ok())
        {
            if let Some(text_field) = cell_view.textField() {
                text_field.setStringValue(&NSString::from_str(path));
            }
            return Some(cell_view);
        }

        None
    }
}

When compiling, I receive an error like:

mismatched types
expected struct `RetainedReturnValue`
     found enum `Option<_>`
rustcClick for full compiler diagnostic
define_class.rs(1292, 14): expected `RetainedReturnValue` because of return type

And upon macro expansion, the generated code looks similar to:

impl Delegate {
    #[allow(clippy::diverging_sub_expression)]
    unsafe extern "C-unwind" fn table_view_view_for_table_column_row(
        &self,
        _: objc2::runtime::Sel,
        table_view: <&NSTableView as objc2::__macro_helpers::ConvertArgument>::__Inner,
        table_column: <Option<&NSTableColumn> as objc2::__macro_helpers::ConvertArgument>::__Inner,
        row: <NSInteger as objc2::__macro_helpers::ConvertArgument>::__Inner,
    ) -> objc2::__macro_helpers::RetainedReturnValue {
        let table_view =
            <&NSTableView as objc2::__macro_helpers::ConvertArgument>::__from_defined_param(
                table_view,
            );
        let table_column =  <Option< &NSTableColumn>as objc2::__macro_helpers::ConvertArgument> ::__from_defined_param(table_column);
        let row = <NSInteger as objc2::__macro_helpers::ConvertArgument>::__from_defined_param(row);
        let __objc2_result = {
            let identifier = NSUserInterfaceItemIdentifier::from_str("cell");
            let Some(path) = self.ivars().playlist.files.get(row as usize) else {
                return None;
            };
            if let Some(cell_view) = table_view
                .makeViewWithIdentifier_owner(&identifier, Some(self))
                .and_then(|cell| cell.downcast::<NSTableCellView>().ok())
            {
                if let Some(text_field) = cell_view.textField() {
                    text_field.setStringValue(&NSString::from_str(path));
                }
                return Some(cell_view);
            }
            None
        };
        #[allow(unreachable_code)]
                        <objc2::__macro_helpers::MethodFamily<{
                            objc2::__macro_helpers::method_family("tableView")
                        }>as objc2::__macro_helpers::MessageReceiveRetained< &Self,Option<Retained<NSTableCellView>> , >>::into_return(__objc2_result)
    }
}

Using an early return (e.g., return Some(cell_view);) within the inner block causes the macro to expect a value of type RetainedReturnValue while it instead encounters an Option<_>. This forces me to rewrite the method without any early return statements. Although I can work around it using alternative coding styles, it is not immediately obvious that this restriction exists.

[madsmtm]

Yeah, the current mapping that define_class! does is very hacky. Ideally, we shouldn't modify the functions at all, but instead generate a helper function that has the desired conversions. I think I have a draft branch somewhere where I've worked on this, but it's difficult because we want Self types to continue working.

Thanks for the issue, I'll use it to track improving the situation.

[madsmtm]

Since rust-analyzer also works more poorly in define_class!, for now, I'd suggest to move your implementation to a helper method created outside define_class!.

[giantpand2000]

I'm currently using a closure that executes in place. However, moving it out of define_class! is indeed a good approach.