match .get() for None: how to avoid useless borrow?

Question

How can I mutate a hash_map in the None clause of match hash_map.get(...) (and similarly for vec.get)?

match hash_map.get(...) {
    None => {
        // How can I mutate hash_map here?
    }
}

As a full example, the following does not compile:

use std::collections::HashMap;

fn map_question(map: &mut HashMap<String, String>) {
    match map.get(&"foo".to_string()) {
        Some(s) => {
            // Do something with s
        },
        None => {
            // I'd like to mutate `map` here, but I'm getting "cannot borrow map"
            map.insert("bar".to_string(), "x".to_string());
        }
    }
}

The reason is that the Option<&String> returned by map.get() is still in scope, so map is borrowed. Here's the full error message:

error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
  --> test.rs:10:13
   |
4  |     match map.get(&"foo".to_string()) {
   |           --- immutable borrow occurs here
...
10 |             map.insert("bar".to_string(), "x".to_string());
   |             ^^^ mutable borrow occurs here
11 |         }
12 |     }
   |     - immutable borrow ends here

My solution has been to work around this problem by using if let:

fn map_alternative(map: &mut HashMap<String, String>) {
    if let Some(s) = map.get(&"foo".to_string()) {
        // Do something with s
    }
    // Now we can mutate `map` because the `Option` return by `.get`
    // is out of scope.
    map.insert("bar".to_string(), "x".to_string());
}

(Note that we're not using an else clause here, because the Option<&String> returned by .get would still be in scope in the else clause for some reason.)

But this seems somewhat unsatisfactory. Can I tell Rust that when I'm matching None, I'm done with the Option object I got from .get?