Codable support for enums with struct associated values is broken

[ben-p-commits]

I believe I may have found a possible bug and may also have a fix for it.

Automatic encoding / decoding of enums that use structs for one or more associated values always fails

@Test
func nonOptionalStructInEnum() throws {
    struct SimpleStruct: Codable, Equatable {
        let name: String
    }
    enum TestEnum: Codable, Equatable {
        case withStruct(value: Int, item: SimpleStruct)
    }
    let testValue = TestEnum.withStruct(value: 10, item: SimpleStruct(name: "test"))
    let doc = Document()
    try AutomergeEncoder(doc: doc).encode(testValue)
    let decoded = try AutomergeDecoder(doc: doc).decode(TestEnum.self)
    #expect(decoded == testValue)
}

That test will fail:

.SchemaMissing("Nothing in schema exists at [CodingKeys(stringValue: \"withStruct\", intValue: nil), WithStructCodingKeys(stringValue: \"item\", intValue: nil)] - look u returns nil")

After digging around a bit- I think I know why as well.

// AutomergeKeyedEncodingContainer.swift:400
mutating func nestedContainer<NestedKey>(keyedBy _: NestedKey.Type, forKey key: Self.Key) -> ... {
    // 🔥 I believe we shoud be using self.codingPath, not impl.codingPath here.
    // 🔥 impl.codingPath provides the wrong result here.
    let newPath = impl.codingPath + [key]  

    let nestedContainer = AutomergeKeyedEncodingContainer<NestedKey>(
        impl: impl,  // Reuses parent's impl
        codingPath: newPath,
        doc: document
    )
    return KeyedEncodingContainer(nestedContainer)
}

The decoder expected:

// what we would get from codingPath + [key]
path: ["value", "simple", "_0"]

But the data was at:

// what we get from impl.codingPath + [key]
path: ["value", "_0"]

I've got a fork ready to roll where I fix this issue in 6 places, and it adds new test coverage without regressing existing ones. Will link a PR for it soon.