Refactor DocumentManager threading for Swift 6

Replaces DispatchQueue with ThreadSafeBox, relaxes T: Sendable constraints, and removes @unchecked Sendable from Document.

Sources/SourceKitD/SourceKitD.swift

@@ -23,8 +23,8 @@ extension sourcekitd_api_values: @unchecked Sendable {}
 fileprivate extension ThreadSafeBox {
   /// If the wrapped value is `nil`, run `compute` and store the computed value. If it is not `nil`, return the stored
   /// value.
-  func computeIfNil<WrappedValue>(compute: () -> WrappedValue) -> WrappedValue where T == WrappedValue? {
-    return withLock { value in
+  func computeIfNil<WrappedValue: Sendable>(compute: () -> WrappedValue) -> WrappedValue where T == WrappedValue? {
+    return withLock { (value: inout WrappedValue?) -> WrappedValue in
       if let value {
         return value
       }

Sources/SourceKitLSP/DocumentManager.swift

@@ -61,7 +61,7 @@ package struct DocumentSnapshot: Identifiable, Sendable {
   }
 }
 
-package final class Document: @unchecked Sendable {
+package final class Document {
   package let uri: DocumentURI
   package let language: Language
   var latestVersion: Int

Sources/SwiftExtensions/ThreadSafeBox.swift

@@ -15,12 +15,26 @@ import Foundation
 /// A thread safe container that contains a value of type `T`.
 ///
 /// - Note: Unchecked sendable conformance because value is guarded by a lock.
-package class ThreadSafeBox<T: Sendable>: @unchecked Sendable {
+package final class ThreadSafeBox<T>: Sendable {
   /// Lock guarding `_value`.
   private let lock = NSLock()
 
-  private var _value: T
+  private nonisolated(unsafe) var _value: T
 
+  package init(initialValue: T) {
+    _value = initialValue
+  }
+
+  /// Restrict the result of the body to `Sendable` so callers can safely use
+  /// the returned value outside the lock without requiring `T: Sendable`.
+  package func withLock<Result: Sendable>(_ body: (inout T) throws -> Result) rethrows -> Result {
+    return try lock.withLock {
+      return try body(&_value)
+    }
+  }
+}
+
+extension ThreadSafeBox where T: Sendable {
   package var value: T {
     get {
       return lock.withLock {
@@ -39,16 +53,6 @@ package class ThreadSafeBox<T: Sendable>: @unchecked Sendable {
     }
   }
 
-  package init(initialValue: T) {
-    _value = initialValue
-  }
-
-  package func withLock<Result>(_ body: (inout T) throws -> Result) rethrows -> Result {
-    return try lock.withLock {
-      return try body(&_value)
-    }
-  }
-
   /// If the value in the box is an optional, return it and reset it to `nil`
   /// in an atomic operation.
   package func takeValue<U>() -> T where U? == T {