Get HashMap entry or add it if there isn't one - rust

Question

I want to do something like this:
fn some_fn() {
let mut my_map = HashMap::from([
(1, "1.0".to_string()),
(2, "2.0".to_string()),
]);
let key = 3;
let res = match my_map.get(&key) {
Some(child) => child,
None => {
let value = "3.0".to_string();
my_map.insert(key, value);
&value // HERE IT FAILS
}
};
println!("{}", res);
}
but it compiles with errors:
error[E0597]: `value` does not live long enough
--> src/lib.rs:16:13
|
16 | &value // HERE IT FAILS
| ^^^^^^
| |
| borrowed value does not live long enough
| borrow later used here
17 | }
| - `value` dropped here while still borrowed
error[E0382]: borrow of moved value: `value`
--> src/lib.rs:16:13
|
14 | let value = "3.0".to_string();
| ----- move occurs because `value` has type `String`, which does not implement the `Copy` trait
15 | my_map.insert(key, value);
| ----- value moved here
16 | &value // HERE IT FAILS
| ^^^^^^ value borrowed here after move
Playground
How can I fix it elegantly? Make a copy of string seems to me non-optimal.

Answer 1

The issue here is that you're inserting the value string then trying to return a reference to it, despite it already being moved into the HashMap. What you really want is to insert the value then get a reference to the inserted value, which could look something like this:
let res = match my_map.get(&key) {
Some(child) => child,
None => {
let value = "3.0".to_string();
my_map.insert(key, value);
my_map.get(&key).unwrap() // unwrap is guaranteed to work
}
};
BUT DON'T DO THIS. It's ugly and slow, as it has to look up the key in the map twice. Rust has a convenient Entry type that lets you get an entry of a HashMap then perform operations on that:
// or_insert returns the value if it exists, otherwise it inserts a default and returns it
let res = my_map.entry(key).or_insert("3.0".to_string());
Alternatively, if the operation to generate the default value is expensive, you can use or_insert_with to pass a closure:
// this won't call "3.0".to_string() unless it needs to
let res = my_map.entry(key).or_insert_with(|| "3.0".to_string());