Unhelpful error message when trying to add elements to hash map

[est31]

The following code works fine in cpp:

    std::map<int, int> m;
    m[0] = 0;

In Rust however, you are getting an error:

    let mut f :HashMap<u8, u8> = HashMap::new();
    f[&0] = 0;

error[E0594]: cannot assign to immutable indexed content
 --> src/main.rs:4:2
  |
4 | 	f[&0] = 0;
  | 	^^^^^^^^^ cannot borrow as mutable

The error doesn't really tell you what is indexed immutably. IMO it should explain that hashmaps have the insert function instead and maybe even should suggest it.

[leonardo-m]

That functionality was removed:
#23559

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.

Do you know if there's a enhancement request for IndexSet trait? Or a RFC.

[est31]

@leonardo-m there are multiple discussions about IndexAssign, but I don't want to change the language, I just want the compiler output to be nicer.

[estebank]

CC #52788