What needs to be done for a user structure to become a key in a dictionary?

It is necessary to implement the Hashable protocol.

The Hashable protocol inherits from Equatable.

• Equatable: Requires the implementation of the equality operator == to compare two instances of a structure.
• Hashable: Requires the implementation of the hash(into:) property, which computes a hash value for an instance of the structure.

Example implementation:

struct Coordinate: Hashable {
    let x: Int
    let y: Int

    // MARK: - Equatable
    static func == (lhs: Coordinate, rhs: Coordinate) -> Bool {
        return lhs.x == rhs.x && lhs.y == rhs.y
    }

    // MARK: - Hashable
    func hash(into hasher: inout Hasher) {
        hasher.combine(x)
        hasher.combine(y)
    }
}

// Usage in a dictionary
let coordinatesDictionary: [Coordinate: String] = [
    Coordinate(x: 0, y: 0): "Origin",
    Coordinate(x: 1, y: 1): "Point (1,1)"
]

print(coordinatesDictionary[Coordinate(x: 0, y: 0)]!)

Hasher is used to generate a hash value. The combine method takes various hashable values and combines them to create the final hash.