error[E0283]: type annotations needed for &O

[Wanpaku]

I'm trying to create an ActionEntry:

        let dict_group_dropdown = self.dictgroup_dropdown.clone();
        let stringlist_model = self.stringlist_model.clone();
        let dropdown_up = gio::ActionEntry::builder("dropdown_up")
            .activate(move |_, _, _| {
                let group_index = dict_group_dropdown.selected();
                let group_amount = stringlist_model.n_items();
                if group_index == 0{
                    dict_group_dropdown.set_selected(group_amount);
                }
                else{
                    dict_group_dropdown.set_selected(group_index - 1);
                }
            })
            .build();

But I got the error:

error[E0283]: type annotations needed for &O
--> src/main_window.rs:1120:29
|
1119 | let dropdown_up = gio::ActionEntry::builder("dropdown_up")
| ---------------------------------------- type must be known at this point
1120 | .activate(move |_, _, _| {
| ^
|
= note: multiple impls satisfying _: gtk4::prelude::IsA<ActionMap> found in the following crates: gio, gtk4:
- impl gtk4::prelude::IsA for ActionMap;
- impl gtk4::prelude::IsA for ApplicationWindow;
- impl gtk4::prelude::IsA for SimpleActionGroup;
- impl gtk4::prelude::IsA for gtk4::Application;
- impl gtk4::prelude::IsA for gtk4::gio::Application;
note: required by a bound in ActionEntry::<O>::builder

I've already created a few instances of ActionEntry and didn't have this kind of errors earlier.
I don't understand why it appears now and how to fix it.

[Wanpaku]

I found a solution:

.activate(move |_:& gtk::ApplicationWindow, _, _| {

But I still doesn't understand why it appears.

[sdroege]

The error will also disappear once you actually use the action entry somewhere. The compiler needs to know what type the first parameter to the closure is going to be, and without context (actual usage!) you have to provide it.