12

I'm having a hard time with the borrow checker.

for item in self.xxx.iter() {
    self.modify_self_but_not_xxx(item);
}

The above code worked before I refactored some code into modify_self_but_not_xxx():

error: cannot borrow `*self` as mutable because `self.xxx` is also borrowed as immutable

How can I call a mutating method while holding a reference to self (e.g. from within a for-loop)?

Shepmaster
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MarkOfDarkness
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1 Answers1

13

How can I call a mutating method while holding a reference to self (e.g. from within a for-loop)?

You can't, that's exactly what the borrowing rules prevent.

The main idea is that in your code, the borrow checker cannot possibly know that self.modify_self_but_not_xxx(..) will not modify xxx.

However, you can mutate self.yyy or any other parameters, so either you can:

  • do the computations of modify_self_but_not_xxx(..) directly in your loop body

  • define a helper function taking mutable references to update them:

    fn do_computations(item: Foo, a: &mut Bar, b: &mut Baz) { /* ... */ }

/* ... */

for item in self.xxx.iter() {
    do_computations(item, &mut self.bar, &mut self.baz);
}
  • define a helper struct that has helper methods
Shepmaster
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Levans
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  • Thanks, I understand now. **Lesson learned:** when designing functions avoid references to `self` if possible -- don't borrow more than what is minimally necessary. – MarkOfDarkness Dec 08 '14 at 07:56
  • There is distinction between method and function in Rust. [The Book ch](https://doc.rust-lang.org/book/ch05-03-method-syntax.html). In short method has reference to Self, while function is simply associated with a struct. – dobhareach Apr 06 '23 at 07:49