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

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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

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