Function gtk_sys::gtk_menu_popup_at_rect

pub unsafe extern "C" fn gtk_menu_popup_at_rect(
    menu: *mut GtkMenu, 
    rect_window: *mut GdkWindow, 
    rect: *const GdkRectangle, 
    rect_anchor: GdkGravity, 
    menu_anchor: GdkGravity, 
    trigger_event: *const GdkEvent
)

This is supported on crate feature v3_22 only.