[swift-evolution] Possible issue with SE-0166 Swift Archival & Serialization implementation

[James Froggatt]

I've just been trying out the new Coding protocol, and was rather surprised when trying to implement the `encode(to encoder: Encoder)` method.

The Swift evolution proposal provides the following example code:

    public func encode(to encoder: Encoder) throws {
        // Generic keyed encoder gives type-safe key access: cannot encode with keys of the wrong type.
        let container = encoder.container(keyedBy: CodingKeys.self)

        // The encoder is generic on the key -- free key autocompletion here.
        try container.encode(latitude, forKey: .latitude)
        try container.encode(longitude, forKey: .longitude)
    }


Here, container is stored as a `let` value, and uses reference semantics, while the proposal also clearly lists these `encode` methods as mutating. With the current implementation of the proposal, the container must be stored as a `var`, which leads to code like the following:

    var container = encoder.singleValueContainer()
    try container.encode(data)

This clearly wont work as expected if the container were to have value semantics, and writing code like this feels plain wrong. Is SE-0166 really intended to work with referrence-type encoders only?