Consider annotating `ItemChange` with `#[derive(Hash)]` but it doesn't work

I am trying to create a struct that has a hashmap of changed values.

Example:

#[derive(Debug, Clone, Eq, PartialEq)]
struct ItemChange {
    time: i64,
    metadata: HashMap<String, String>,
    changed_metadata: HashMap<String, String>,
    user_defined_description: String,
    description: String,
}

#[derive(Debug, Clone)]
pub struct ProductItem {
    id: String,
    /// Ordered change_history by date
    change_history: HashMap<ItemChange, i64>,
}

impl ProductItem {
    pub fn add_change(&self, change: ItemChange, date: i64) {
        self.change_history.insert(); // <-- change_history doesn't have insert :( 
    }
}

The code doesn't compile with an error saying

58 | struct ItemChange {
   | ----------------- doesn't satisfy `ItemChange: Hash`
...
97 |         self.change_history.insert();
   |                             ^^^^^^
   |
   = note: the following trait bounds were not satisfied:
           `ItemChange: Hash`
help: consider annotating `ItemChange` with `#[derive(Hash)]`

What am I doing wrong?

For a type to be used as a key for a HashMap, it must implement Hash. The easiest way to do this for your own struct is to derive the trait, however in this case you run into other issues.

As kmdreko mentioned in their comment, the problem is that HashMap doesn't implement the Hash trait, which means it cannot be used as a key. Therefore, you can't derive Hash on a struct that has a HashMap field because #[derive(Hash)] requires every field to be implement Hash.

A minimal example like this shows a more useful compiler error:

use std::collections::HashMap;

#[derive(Hash)]
struct TestStruct {
    hashmap: HashMap<u32, i32>,
}

error[E0277]: the trait bound `HashMap<u32, i32>: Hash` is not satisfied
 --> src/lib.rs:5:5
  |
3 | #[derive(Hash)]
  |          ---- in this derive macro expansion
4 | struct TestStruct {
5 |     hashmap: HashMap<u32, i32>,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Hash` is not implemented for `HashMap<u32, i32>`
  |
  = note: this error originates in the derive macro `Hash` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

I don't see an easy way around this without changing your struct. One way would be to replace the HashMaps with another data structure which represents the same data but also implements Hash, such as Vec<(String, String)>. This will likely have worse performance depending on how you use the hashmaps, and it will need changes to all the code which interacts with them.

Edit: If you decide to use Vec<(String, String)>, you will also need to ensure that the vec is sorted so that two vecs containing the same key-value pairs have the same hash, which is not the case if the elements are not ordered the same.