Decode an object nested under a key

_Usage.swift

let json = """
{
    "results": [
        {"value": "foo"},
        {"value": "bar"},
        {"value": "baz"}
    ]
}
"""

struct Item: Decodable {
    let value: String
}

let items = try JSONDecoder().decode([Item].self, from: Data(json.utf8), under: "results")
print(items) // [Item(value: "foo"), Item(value: "bar"), Item(value: "baz")]

TopLevelDecoder+Nested.swift

extension TopLevelDecoder {
    func decode<T: Decodable>(_ type: T.Type, from input: Input, under: String) throws -> T {
        let decoder = try decode(_Decoder.self, from: input).decoder
        let container = try decoder.container(keyedBy: AnyCodingKey.self)
        return try container.decode(T.self, forKey: .init(under))
    }
}
private struct _Decoder: Decodable {
    var decoder: Decoder

    init(from decoder: Decoder) throws {
        self.decoder = decoder
    }
}

AnyCodingKey: https://gist.github.com/IanKeen/3d226854c8c59a17e151a0022b71f6bb