How can I call a mutating method while holding a reference to self?

Question

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)?

Answer 1

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