The Rust programming language has strong type checking and does not allow null values as you would have in other languages like Java or JavaScript. The idiomatic way to represent the absence of a value in Rust is by using the `Option` enum, which has two variants: `Some` and `None`.

If `Custom` is a `HashMap`, and you want to initialize it such that it represents the absence of a value, you should use `Option>`. This allows you to use `None` to represent the absence of a map.

Here's an example:

let custom: Option> = None;

This statement initializes `custom` to `None`, representing the absence of a value. Later, when you want to use the map, you can initialize it with `Some(HashMap::new())`.

let custom = Some(HashMap::new());

When accessing the map, you will need to handle the `Option` value properly, typically using `match` or `if let` to check if it's `Some` or `None`.