Allow non-copyable Return types in with-style methods (#81)

### Motivation

Fixes #77.

Users might create a value of a non-copyable type and return it from one
of the watch methods. Today, that's not possible, because the `Return`
type implicitly requires `Copyable`. But that's just because it's the
default, we don't do any copying in the watch methods themselves, so we
can relax it.

### Modifications

Allow returning non-copyable values from watch methods.

### Result

Easier usage of non-copyable types with Swift Configuration.

### Test Plan

Adapted tests, all pass.

Author: czechboy0

Sources/Configuration/ConfigProvider.swift

@@ -97,7 +97,7 @@ public protocol ConfigProvider: Sendable {
     ///   - updatesHandler: The closure that processes the async sequence of value updates.
     /// - Throws: Provider-specific errors or errors thrown by the handler closure.
     /// - Returns: The value returned by the closure.
-    func watchValue<Return>(
+    func watchValue<Return: ~Copyable>(
         forKey key: AbsoluteConfigKey,
         type: ConfigType,
         updatesHandler: (
@@ -128,7 +128,7 @@ public protocol ConfigProvider: Sendable {
     /// - Parameter updatesHandler: The closure that processes the asynchronous sequence of snapshot updates.
     /// - Throws: Provider-specific errors or errors thrown by the handler closure.
     /// - Returns: The value returned by the closure.
-    func watchSnapshot<Return>(
+    func watchSnapshot<Return: ~Copyable>(
         updatesHandler: (ConfigUpdatesAsyncSequence<any ConfigSnapshot, Never>) async throws -> Return
     ) async throws -> Return
 }

Sources/Configuration/ConfigProviderHelpers.swift

@@ -43,7 +43,7 @@ extension ConfigProvider {
     /// - Returns: The value returned by the handler closure.
     /// - Throws: Provider-specific errors or errors thrown by the handler.
     nonisolated(nonsending)
-        public func watchValueFromValue<Return>(
+        public func watchValueFromValue<Return: ~Copyable>(
             forKey key: AbsoluteConfigKey,
             type: ConfigType,
             updatesHandler: (
@@ -85,7 +85,7 @@ extension ConfigProvider {
     /// - Parameter updatesHandler: The closure that processes the async sequence of snapshot updates.
     /// - Returns: The value returned by the handler closure.
     /// - Throws: Provider-specific errors or errors thrown by the handler.
-    nonisolated(nonsending) public func watchSnapshotFromSnapshot<Return>(
+    nonisolated(nonsending) public func watchSnapshotFromSnapshot<Return: ~Copyable>(
         updatesHandler: (ConfigUpdatesAsyncSequence<any ConfigSnapshot, Never>) async throws -> Return
     ) async throws -> Return {
         let (stream, continuation) = AsyncStream<any ConfigSnapshot>

Sources/Configuration/ConfigReader+internalHelpers.swift

@@ -637,7 +637,7 @@ extension ConfigReader {
     ///   - updatesHandler: A closure that handles the stream of updates, emits `nil` when no value is found.
     /// - Throws: An error if the watch operation fails.
     /// - Returns: The value returned by the updates handler.
-    internal func watchValue<Value: Sendable, Return>(
+    internal func watchValue<Value: Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         type: ConfigType,
         isSecret: Bool,
@@ -713,7 +713,7 @@ extension ConfigReader {
     ///   - updatesHandler: A closure that handles the stream of updates, emits the default when no value is found.
     /// - Throws: An error if the watch operation fails.
     /// - Returns: The value returned by the updates handler.
-    internal func watchValue<Value: Sendable, Return>(
+    internal func watchValue<Value: Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         type: ConfigType,
         isSecret: Bool,
@@ -790,7 +790,7 @@ extension ConfigReader {
     ///     or the value cannot be unwrapped.
     /// - Throws: An error if the watch operation fails.
     /// - Returns: The value returned by the updates handler.
-    internal func watchRequiredValue<Value: Sendable, Return>(
+    internal func watchRequiredValue<Value: Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         type: ConfigType,
         isSecret: Bool,

Sources/Configuration/ConfigReader+methods.swift

@@ -698,7 +698,7 @@ extension ConfigReader {
     ///     produces `nil` if the value is missing or can't be converted.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watch${primitive_type["name"]}<Return>(
+    public func watch${primitive_type["name"]}<Return: ~Copyable>(
         forKey key: ConfigKey,
         isSecret: Bool = false,
         fileID: String = #fileID,
@@ -741,7 +741,7 @@ extension ConfigReader {
     ///     produces the default value if the provider returned a `nil` value or conversion failed.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watch${primitive_type["name"]}<Return>(
+    public func watch${primitive_type["name"]}<Return: ~Copyable>(
         forKey key: ConfigKey,
         isSecret: Bool = false,
         default defaultValue: ${primitive_type["type"]},
@@ -785,7 +785,7 @@ extension ConfigReader {
     ///     produces an error if the provider returned a `nil` value or if the value was of an incorrect type.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchRequired${primitive_type["name"]}<Return>(
+    public func watchRequired${primitive_type["name"]}<Return: ~Copyable>(
         forKey key: ConfigKey,
         isSecret: Bool = false,
         fileID: String = #fileID,
@@ -834,7 +834,7 @@ extension ConfigReader {
     ///     produces `nil` if the value is missing or can't be converted.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchString<Value: ${string_convertible_type["protocol"]} & Sendable, Return>(
+    public func watchString<Value: ${string_convertible_type["protocol"]} & Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         as type: Value.Type = Value.self,
         isSecret: Bool = false,
@@ -879,7 +879,7 @@ extension ConfigReader {
     ///     produces the default value if the provider returned a `nil` value or conversion failed.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchString<Value: ${string_convertible_type["protocol"]} & Sendable, Return>(
+    public func watchString<Value: ${string_convertible_type["protocol"]} & Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         as type: Value.Type = Value.self,
         isSecret: Bool = false,
@@ -925,7 +925,7 @@ extension ConfigReader {
     ///     produces an error if the provider returned a `nil` value or if the value was of an incorrect type.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchRequiredString<Value: ${string_convertible_type["protocol"]} & Sendable, Return>(
+    public func watchRequiredString<Value: ${string_convertible_type["protocol"]} & Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         as type: Value.Type = Value.self,
         isSecret: Bool = false,
@@ -973,7 +973,7 @@ extension ConfigReader {
     ///     produces `nil` if the value is missing or can't be converted.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchStringArray<Value: ${string_convertible_type["protocol"]} & Sendable, Return>(
+    public func watchStringArray<Value: ${string_convertible_type["protocol"]} & Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         as type: Value.Type = Value.self,
         isSecret: Bool = false,
@@ -1018,7 +1018,7 @@ extension ConfigReader {
     ///     produces the default value if the provider returned a `nil` value or conversion failed.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchStringArray<Value: ${string_convertible_type["protocol"]} & Sendable, Return>(
+    public func watchStringArray<Value: ${string_convertible_type["protocol"]} & Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         as type: Value.Type = Value.self,
         isSecret: Bool = false,
@@ -1064,7 +1064,7 @@ extension ConfigReader {
     ///     produces an error if the provider returned a `nil` value or if the value was of an incorrect type.
     /// - Returns: The result produced by the handler.
     /// - Throws: Rethrows any error thrown by the handler or the underlying watch operation.
-    public func watchRequiredStringArray<Value: ${string_convertible_type["protocol"]} & Sendable, Return>(
+    public func watchRequiredStringArray<Value: ${string_convertible_type["protocol"]} & Sendable, Return: ~Copyable>(
         forKey key: ConfigKey,
         as type: Value.Type = Value.self,
         isSecret: Bool = false,

Sources/Configuration/ConfigReader+methods.swift.gyb

@@ -278,7 +278,7 @@ extension ConfigReader {
     ///   - updatesHandler: A closure that receives an async sequence of `ConfigSnapshotReader` instances.
     /// - Returns: The value returned by the handler.
     /// - Throws: Any error thrown by the handler.
-    public func watchSnapshot<Return>(
+    public func watchSnapshot<Return: ~Copyable>(
         fileID: String = #fileID,
         line: UInt = #line,
         updatesHandler: (ConfigUpdatesAsyncSequence<ConfigSnapshotReader, Never>) async throws -> Return

Sources/Configuration/ConfigSnapshotReader.swift

@@ -72,7 +72,7 @@ extension ConfigReader {
     /// - Returns: The value returned by the closure.
     /// - Throws: Rethrows any errors thrown by the provided closure.
     @available(*, deprecated, message: "Renamed to snapshot().")
-    public func withSnapshot<Failure: Error, Return>(
+    public func withSnapshot<Failure: Error, Return: ~Copyable>(
         _ body: (ConfigSnapshotReader) throws(Failure) -> Return
     ) throws(Failure) -> Return {
         let multiSnapshot = provider.snapshot()

Sources/Configuration/Deprecations.swift

@@ -4,29 +4,29 @@
 
 ### Watching string values
 - ``ConfigReader/watchString(forKey:isSecret:fileID:line:updatesHandler:)``
-- ``ConfigReader/watchString(forKey:as:isSecret:fileID:line:updatesHandler:)-7mxw1``
-- ``ConfigReader/watchString(forKey:as:isSecret:fileID:line:updatesHandler:)-818sy``
+- ``ConfigReader/watchString(forKey:as:isSecret:fileID:line:updatesHandler:)-4zs5y``
+- ``ConfigReader/watchString(forKey:as:isSecret:fileID:line:updatesHandler:)-8hegv``
 - ``ConfigReader/watchString(forKey:isSecret:default:fileID:line:updatesHandler:)``
-- ``ConfigReader/watchString(forKey:as:isSecret:default:fileID:line:updatesHandler:)-6m0yu``
-- ``ConfigReader/watchString(forKey:as:isSecret:default:fileID:line:updatesHandler:)-6dpc3``
+- ``ConfigReader/watchString(forKey:as:isSecret:default:fileID:line:updatesHandler:)-hi1l``
+- ``ConfigReader/watchString(forKey:as:isSecret:default:fileID:line:updatesHandler:)-9qsww``
 
 ### Watching required string values
 - ``ConfigReader/watchRequiredString(forKey:isSecret:fileID:line:updatesHandler:)``
-- ``ConfigReader/watchRequiredString(forKey:as:isSecret:fileID:line:updatesHandler:)-86ot1``
-- ``ConfigReader/watchRequiredString(forKey:as:isSecret:fileID:line:updatesHandler:)-3lrs7``
+- ``ConfigReader/watchRequiredString(forKey:as:isSecret:fileID:line:updatesHandler:)-581rg``
+- ``ConfigReader/watchRequiredString(forKey:as:isSecret:fileID:line:updatesHandler:)-13z9o``
 
 ### Watching lists of string values
 - ``ConfigReader/watchStringArray(forKey:isSecret:fileID:line:updatesHandler:)``
-- ``ConfigReader/watchStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-8t4nb``
-- ``ConfigReader/watchStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-9cmju``
+- ``ConfigReader/watchStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-15oye``
+- ``ConfigReader/watchStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-7snta``
 - ``ConfigReader/watchStringArray(forKey:isSecret:default:fileID:line:updatesHandler:)``
-- ``ConfigReader/watchStringArray(forKey:as:isSecret:default:fileID:line:updatesHandler:)-59de``
-- ``ConfigReader/watchStringArray(forKey:as:isSecret:default:fileID:line:updatesHandler:)-8nsil``
+- ``ConfigReader/watchStringArray(forKey:as:isSecret:default:fileID:line:updatesHandler:)-9t8bb``
+- ``ConfigReader/watchStringArray(forKey:as:isSecret:default:fileID:line:updatesHandler:)-256k2``
 
 ### Watching required lists of string values
 - ``ConfigReader/watchRequiredStringArray(forKey:isSecret:fileID:line:updatesHandler:)``
-- ``ConfigReader/watchRequiredStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-3whiy``
-- ``ConfigReader/watchRequiredStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-4zyyq``
+- ``ConfigReader/watchRequiredStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-4acy``
+- ``ConfigReader/watchRequiredStringArray(forKey:as:isSecret:fileID:line:updatesHandler:)-8wvyc``
 
 ### Watching Boolean values
 - ``ConfigReader/watchBool(forKey:isSecret:fileID:line:updatesHandler:)``

Sources/Configuration/Documentation.docc/Reference/ConfigReader-Watch.md

@@ -196,7 +196,7 @@ extension MultiProvider {
     /// - Returns: The value returned by the body closure.
     /// - Throws: Any error thrown by the nested providers or the body closure.
     nonisolated(nonsending)
-        func watchSnapshot<Return>(
+        func watchSnapshot<Return: ~Copyable>(
             _ body: (ConfigUpdatesAsyncSequence<MultiSnapshot, Never>) async throws -> Return
         ) async throws -> Return
     {
@@ -287,7 +287,7 @@ extension MultiProvider {
     /// - Throws: Any error thrown by the nested providers or the handler closure.
     /// - Returns: The value returned by the handler.
     nonisolated(nonsending)
-        func watchValue<Return>(
+        func watchValue<Return: ~Copyable>(
             forKey key: AbsoluteConfigKey,
             type: ConfigType,
             updatesHandler: (
@@ -338,13 +338,13 @@ extension MultiProvider {
 }
 
 @available(Configuration 1.0, *)
-nonisolated(nonsending) private func withProvidersWatchingValue<ReturnInner>(
+nonisolated(nonsending) private func withProvidersWatchingValue<Return: ~Copyable>(
     providers: ArraySlice<any ConfigProvider>,
     updateSequences: inout [any (AsyncSequence<Result<LookupResult, any Error>, Never> & Sendable)],
     key: AbsoluteConfigKey,
     configType: ConfigType,
-    body: ([any (AsyncSequence<Result<LookupResult, any Error>, Never> & Sendable)]) async throws -> ReturnInner
-) async throws -> ReturnInner {
+    body: ([any (AsyncSequence<Result<LookupResult, any Error>, Never> & Sendable)]) async throws -> Return
+) async throws -> Return {
     guard let provider = providers.first else {
         // Recursion termination, once we've collected all update sequences, execute the body.
         return try await body(updateSequences)
@@ -362,11 +362,11 @@ nonisolated(nonsending) private func withProvidersWatchingValue<ReturnInner>(
 }
 
 @available(Configuration 1.0, *)
-nonisolated(nonsending) private func withProvidersWatchingSnapshot<ReturnInner>(
+nonisolated(nonsending) private func withProvidersWatchingSnapshot<Return: ~Copyable>(
     providers: ArraySlice<any ConfigProvider>,
     updateSequences: inout [any (AsyncSequence<any ConfigSnapshot, Never> & Sendable)],
-    body: ([any (AsyncSequence<any ConfigSnapshot, Never> & Sendable)]) async throws -> ReturnInner
-) async throws -> ReturnInner {
+    body: ([any (AsyncSequence<any ConfigSnapshot, Never> & Sendable)]) async throws -> Return
+) async throws -> Return {
     guard let provider = providers.first else {
         // Recursion termination, once we've collected all update sequences, execute the body.
         return try await body(updateSequences)

Sources/Configuration/MultiProvider.swift

@@ -150,7 +150,7 @@ extension CommandLineArgumentsProvider: ConfigProvider {
     }
 
     // swift-format-ignore: AllPublicDeclarationsHaveDocumentation
-    public func watchValue<Return>(
+    public func watchValue<Return: ~Copyable>(
         forKey key: AbsoluteConfigKey,
         type: ConfigType,
         updatesHandler: (ConfigUpdatesAsyncSequence<Result<LookupResult, any Error>, Never>) async throws -> Return
@@ -164,7 +164,7 @@ extension CommandLineArgumentsProvider: ConfigProvider {
     }
 
     // swift-format-ignore: AllPublicDeclarationsHaveDocumentation
-    public func watchSnapshot<Return>(
+    public func watchSnapshot<Return: ~Copyable>(
         updatesHandler: (ConfigUpdatesAsyncSequence<any ConfigSnapshot, Never>) async throws -> Return
     ) async throws -> Return {
         try await watchSnapshotFromSnapshot(updatesHandler: updatesHandler)