in Sources/_CollectionsTestSupport/MinimalTypes/MinimalEncoder.swift [566:577]
func nestedContainer<NestedKey: CodingKey>(
keyedBy keyType: NestedKey.Type
) -> KeyedEncodingContainer<NestedKey> {
expectTrue(isValid, "Container isn't valid", trapping: true)
commitPendingContainer()
let keyed = MinimalEncoder.KeyedContainer<NestedKey>(
base: base,
parent: self,
key: MinimalEncoder._Key(count))
pendingContainer = keyed
return .init(keyed)
}