Cannot borrow `*self.element` as mutable more than once

help
1

I've used rust for a while and i've seen this error many times and still can't figure out the issue. here is where it occurred: self.element.update(self, key::read_key());

this would be perfectly safe imo idk why the borrow checker is buggin.

error[E0499]: cannot borrow `*self.element` as mutable more than once at a time
  --> src/lib.rs:84:13
   |
84 |             self.element.update(self, key::read_key());
   |             ^^^^^^^^^^^^^------^----^^^^^^^^^^^^^^^^^^
   |             |            |      |
   |             |            |      first mutable borrow occurs here
   |             |            first borrow later used by call
   |             second mutable borrow occurs here

error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src/lib.rs:84:33
   |
84 |             self.element.update(self, key::read_key());
   |             ------------ ------ ^^^^ second mutable borrow occurs here
   |             |            |
   |             |            first borrow later used by call
   |             first mutable borrow occurs here

For more information about this error, try `rustc --explain E0499`.
2

In the update method, the &mut self and first_param.element would alias each other, which is UB.

(Based purely on the error since you have shared no code.)

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3

here is the whole function:

pub fn run(&mut self) {
        // Initialize
        utils::hide_cursor();
        utils::clear();
        let mut stdout = std::io::stdout();
        stdout
            .execute(crossterm::terminal::EnterAlternateScreen)
            .unwrap();
        crossterm::terminal::enable_raw_mode().unwrap();

        loop {
            self.element.update(self, key::read_key());
        }
    }

here is the app struct

pub struct App {
    element: Box<dyn Element>,
}

and Element:

pub trait Element: std::fmt::Debug {
    fn update(&mut self, _app: &mut App, _k: crate::key::Key) -> UpdateResponse {
        UpdateResponse::None
    }
}

App has some functions that update needs to access

4

Yep, that confirms it. You'll have to factor things out so that you don't need to pass both self.element (as the receiver) and self to Element::update in run.

See this article for some potential workarounds.

https://smallcultfollowing.com/babysteps/blog/2018/11/01/after-nll-interprocedural-conflicts/

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