How to update HashMap when iterating over Vec

Question

In my project, I want to iterate over a simple Vec and update a HashMap using another function, but I'm stuck with the borrow checker complaining that it cannot borrow *self as mutable more than once at a time.

Here is an example:

use std::collections::HashMap;

struct Test {
    entries: Vec<String>,
    objects: HashMap<String, String>,
}

impl Test {
    fn new(entries: Vec<String>) -> Test {
        Test {
            entries,
            objects: HashMap::new(),
        }
    }

    fn process(&mut self) {
        for entry in self.entries.iter_mut() {
            self.update(entry);
        }
    }

    fn update(&mut self, entry: &String) {
        self.objects
            .entry(entry.to_owned())
            .or_insert(entry.to_owned());
    }
}

fn main() {
    let mut test = Test::new(vec!["abc".to_owned(), "def".to_owned()]);
    test.process();
}

And the resulting error:

error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src\main.rs:18:13
   |
17 |         for entry in self.entries.iter_mut() {
   |                      -----------------------
   |                      |
   |                      first mutable borrow occurs here
   |                      first borrow later used here
18 |             self.update(entry);
   |             ^^^^ second mutable borrow occurs here

I do understand why the error occurs but I don't know how to fix it. Any idea?