Why isn't Rust able to infer the type here?

I have a struct Widgetfor which I have a constructor Widget::stateless(...). It simply sets the state to be (). However, even though IDE autocomplete can see that the generic parameter State = (), Rust is unable to infer the type of State:

// widget.rs

impl<State> Widget<State>
where
    State: 'static + PartialEq,
{
    pub fn stateless(
        builder: impl Fn() -> Rc<RefCell<dyn Component>> + 'static,
    ) -> Rc<RefCell<dyn Component>> {
        Rc::new(RefCell::new(Widget {
            state: (),
            prev: None,
            needs_rebuild: true,
            builder: Box::new(move |_| builder()),
            on_keypress: Box::new(|_, _| ()),
            create_element: Rc::new(|this| this._build().borrow_mut().create_element()),
        }))
    }

    // ...
}

// number.rs

pub fn number(i: i32) -> Rc<RefCell<dyn Component>> {
    Widget::stateless(move || text(i.to_string()))
}

The above code yields the following error:

error[E0283]: type annotations needed
  --> react/src/widgets/number.rs:7:5
   |
 7 |     Widget::stateless(|| text(i.to_string()))
   |     ^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `State` declared on the struct `Widget`
   |
   = note: cannot satisfy `_: PartialEq`

Apparently I have to do this instead:

pub fn number(i: i32) -> Rc<RefCell<dyn Component>> {
    Widget::<()>::stateless(move || text(i.to_string()))
}

which works but isn't as pretty. Why can't Rust infer that State is () despite it being very clear in Widget::stateless? Is there anything I can do to avoid writing out ::<()> at the call site? Thanks