How to fix "cannot borrow `*self` as mutable because it is also borrowed as immutable"

help
1

Hi folks !

I have a trait to apply for UI component created with tui-rs:

pub trait UIModalComponent {
    fn new(options: UIComponentOptions) -> Self where Self: Sized;

    fn get_title() -> String;

    fn get_items(&self) -> Vec<ListItem>;

    fn get_state(&mut self) -> &mut ListState;

    fn prev(&mut self) {
        let total_items = { self.get_items().len() };
        let state = self.get_state();

        let i = match state.selected() {
            Some(i) => if i == 0 { total_items - 1 } else { i - 1 },
            None => 0,
        };

        state.select(Some(i));
    }

    fn next(&mut self) {
        let total_items = { self.get_items().len() };
        let state = self.get_state();

        let i = match state.selected() {
            Some(i) => if i >= total_items - 1 { 0 } else { i + 1 },
            None => 0,
        };

        state.select(Some(i));
    }

    fn render<B: Backend>(&mut self, frame: &mut Frame<B>, rect: Rect) {
        let block = Block::default()
            .title(Self::get_title())
            .title_alignment(Alignment::Center)
            .style(Style::default().bg(Color::Black))
            .borders(Borders::ALL)
            .border_type(BorderType::Plain);

        let list = List::new(self.get_items())
            .block(block)
            .style(Style::default().fg(Color::White))
            .highlight_style(Style::default().add_modifier(Modifier::ITALIC))
            .highlight_symbol(">> ");

        frame.render_widget(Clear, rect);
        frame.render_stateful_widget(list, rect, &mut self.get_state());
    }
}

And my render method has an error on compile:

error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
  --> src/primary/traits/ui_component.rs:62:55
   |
55 |         let list = List::new(self.get_items())
   |                              ---- immutable borrow occurs here
...
62 |         frame.render_stateful_widget(list, rect, &mut self.get_state());
   |               ----------------------                  ^^^^^^^^^^^^^^^^ mutable borrow occurs here
   |               |
   |               immutable borrow later used by call

Could somebody explain what could be the best way to fix this issue ?

2

You can't use getters. The code may compile if you use fields directly, like &self.items and &mut self.state.

This is because getters have &self or &mut self arguments, and that really means borrowing all of self, with all fields, and you're not allowed to borrow &mut self and separately anything else from self at the same time.

When you use fields directly, the borrow checker borrows the individual fields, so their loans don't overlap.

3

But trait do not allow me to define properties and when I trying to use self.items or self.state directly in trait method:

let list = List::new(self.items)
            .block(block)
            .style(Style::default().fg(Color::White))
            .highlight_style(Style::default().add_modifier(Modifier::ITALIC))
            .highlight_symbol(">> ");

frame.render_widget(Clear, rect);
frame.render_stateful_widget(list, rect, &mut self.state);

I got an error:

error[E0609]: no field `state` on type `&mut Self`
  --> src/primary/traits/ui_component.rs:62:60
   |
14 | pub trait UIModalComponent {
   | -------------------------- type parameter 'Self' declared here
...
62 |         frame.render_stateful_widget(list, rect, &mut self.state);
   |                                                            ^^^^^
4

Add a method to the trait that gives you both at the same time:

fn get_items_and_state(&mut self) -> (Vec<ListItem>, &mut ListState);

Then use this in render.

1 Like
5

Unfortunately, this not helped. I still have an error:

error[E0502]: cannot borrow `self.state` as mutable because it is also borrowed as immutable
   --> src/features/ui/realm_modal.rs:102:17
    |
100 |     fn get_items_and_state(&mut self) -> (Vec<ListItem>, &mut ListState) {
    |                            - let's call the lifetime of this reference `'1`
101 |         let items = self.get_items();
    |                     ---- immutable borrow occurs here
102 |         (items, &mut self.state)
    |         --------^^^^^^^^^^^^^^^-
    |         |       |
    |         |       mutable borrow occurs here
    |         returning this value requires that `*self` is borrowed for `'1`

Let me share the component example:

pub struct RealmModal {
    state: ListState,
    realms: Vec<Realm>,
    _sender: Sender<HandlerOutput>,
}

impl UIModalComponent for RealmModal {
    fn new(options: UIComponentOptions) -> Self {
        Self {
            state: ListState::default(),
            realms: vec![],
            _sender: options.sender,
        }
    }

    fn get_title() -> String {
        "SELECT REALM".to_string()
    }

    fn get_items(&self) -> Vec<ListItem> {
        self.realms
            .iter()
            .map(|realm| ListItem::new(vec![
                Spans::from(vec![
                    Span::raw("Connect to ["),
                    Span::styled(
                        realm.name.to_string(),
                        Style::default()
                            .fg(Color::LightGreen)
                            .add_modifier(Modifier::BOLD)
                    ),
                    Span::raw("]: "),
                    Span::styled(
                        realm.address.to_string(),
                        Style::default()
                            .fg(Color::LightYellow)
                    ),
                ])
            ]))
            .collect()
    }

    fn get_state(&mut self) -> &mut ListState {
        &mut self.state
    }

    fn get_items_and_state(&mut self) -> (Vec<ListItem>, &mut ListState) {
        let items = self.get_items();
        (items, &mut self.state)
    }
}
6

First, I'd recommend you use #![deny(elided_lifetimes_in_paths)], which willmake the issue clearer. get_items borrows from self as long as its return value is used, because ListItem has a lifetime parameter. With your implementation, you just moved code around. One possible solution is sadly to copy the implementation of get_items into get_items_and_state. The keyword for further research is "partial borrowing".

2 Likes
7

Oh, it was my fall. Your idea with get_items_and_state works perfectly. My fall was that I didn't remove the get_items and get_state methods from the code. Thank you very much !

8

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