How can I update a value in a mutable HashMap?

Question

Here is what I am trying to do:

use std::collections::HashMap;

fn main() {
    let mut my_map = HashMap::new();
    my_map.insert("a", 1);
    my_map.insert("b", 3);

    my_map["a"] += 10;
    // I expect my_map becomes {"b": 3, "a": 11}
}

But this raises an error:

Rust 2015

error[E0594]: cannot assign to immutable indexed content
 --> src/main.rs:8:5
  |
8 |     my_map["a"] += 10;
  |     ^^^^^^^^^^^^^^^^^ cannot borrow as mutable
  |
  = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, i32>`

Rust 2018

error[E0594]: cannot assign to data in a `&` reference
 --> src/main.rs:8:5
  |
8 |     my_map["a"] += 10;
  |     ^^^^^^^^^^^^^^^^^ cannot assign

I don't really understand what that means, since I made the HashMap mutable. When I try to update an element in a vector, I get the expected result:

let mut my_vec = vec![1, 2, 3];

my_vec[0] += 10;
println! {"{:?}", my_vec};
// [11, 2, 3]

What is different about HashMap that I am getting the above error? Is there a way to update a value?

Answer 1

Indexing immutably and indexing mutably are provided by two different traits: Index and IndexMut, respectively.

Currently, HashMap does not implement IndexMut, while Vec does.

The commit that removed HashMap's IndexMut implementation states:

This commit removes the IndexMut impls on HashMap and BTreeMap, in order to future-proof the API against the eventual inclusion of an IndexSet trait.

It's my understanding that a hypothetical IndexSet trait would allow you to assign brand-new values to a HashMap, and not just read or mutate existing entries:

let mut map = HashMap::new();
map["key"] = "value";

For now, you can use get_mut:

*my_map.get_mut("a").unwrap() += 10;

Or the entry API:

*my_map.entry("a").or_insert(42) += 10;