chore: import upstream snapshot with attribution

Sources/ContainerPersistence/ConfigSnapshotDecoder.swift

@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Configuration
+
+/// A decoder that decodes `Decodable` types from a ``ConfigSnapshotReader``.
+///
+/// `ConfigSnapshotDecoder` bridges Swift's `Decodable` protocol with the configuration
+/// provider system, allowing you to decode structured configuration into typed
+/// Swift values.
+///
+/// ```swift
+/// struct AppConfig: Decodable {
+///     var host: String
+///     var port: Int
+///     var database: DatabaseConfig
+/// }
+///
+/// struct DatabaseConfig: Decodable {
+///     var connectionString: String
+///     var maxConnections: Int
+/// }
+///
+/// let reader = ConfigReader(providers: [envProvider, jsonProvider])
+/// let snapshot = reader.snapshot()
+/// let config = try ConfigSnapshotDecoder().decode(AppConfig.self, from: snapshot)
+/// ```
+///
+/// Nested structs map to dot-separated key paths. In the example above,
+/// `database.connectionString` and `database.maxConnections` are looked up
+/// from the snapshot.
+public struct ConfigSnapshotDecoder: Sendable {
+
+    public var userInfo: [CodingUserInfoKey: any Sendable]
+    private let typeDecodingStrategies: [ObjectIdentifier: AnyConfigDecodingStrategy]
+
+    public init(decodingStrategies: [any ConfigDecodingStrategy] = [URLConfigDecodingStrategy()]) {
+        self.userInfo = [:]
+        var strategies: [ObjectIdentifier: AnyConfigDecodingStrategy] = [:]
+        for strategy in decodingStrategies {
+            let erased = AnyConfigDecodingStrategy(strategy)
+            strategies[erased.valueTypeID] = erased
+        }
+        self.typeDecodingStrategies = strategies
+    }
+
+    public func decode<T: Decodable>(
+        _ type: T.Type,
+        from snapshot: ConfigSnapshotReader
+    ) throws -> T {
+        if type is any UnsupportedDictionaryDecoding.Type {
+            throw DecodingError.typeMismatch(
+                T.self,
+                DecodingError.Context(
+                    codingPath: [],
+                    debugDescription:
+                        "ConfigSnapshotDecoder does not support decoding dictionaries (got \(T.self)). Represent dynamic keys as nested structs with known property names."
+                )
+            )
+        }
+        let decoder = ConfigSnapshotDecoderImpl(
+            snapshot: snapshot,
+            codingPath: [],
+            userInfo: userInfo.mapValues { $0 as Any },
+            typeDecodingStrategies: typeDecodingStrategies
+        )
+        return try T(from: decoder)
+    }
+}
+
+struct ConfigSnapshotDecoderImpl: Decoder {
+    let snapshot: ConfigSnapshotReader
+    let codingPath: [any CodingKey]
+    let userInfo: [CodingUserInfoKey: Any]
+    let typeDecodingStrategies: [ObjectIdentifier: AnyConfigDecodingStrategy]
+
+    func container<Key: CodingKey>(
+        keyedBy type: Key.Type
+    ) throws -> KeyedDecodingContainer<Key> {
+        KeyedDecodingContainer(
+            KeyedContainer<Key>(
+                snapshot: snapshot,
+                codingPath: codingPath,
+                userInfo: userInfo,
+                typeDecodingStrategies: typeDecodingStrategies
+            )
+        )
+    }
+
+    func unkeyedContainer() throws -> any UnkeyedDecodingContainer {
+        UnkeyedContainer(
+            snapshot: snapshot,
+            codingPath: codingPath,
+            userInfo: userInfo,
+            typeDecodingStrategies: typeDecodingStrategies
+        )
+    }
+
+    func singleValueContainer() throws -> any SingleValueDecodingContainer {
+        SingleValueContainer(
+            snapshot: snapshot,
+            codingPath: codingPath,
+            userInfo: userInfo,
+            typeDecodingStrategies: typeDecodingStrategies
+        )
+    }
+}