Enable explicit sendable checks

Author: gjcairo

Package.swift

@@ -10,7 +10,8 @@ let extraSettings: [SwiftSetting] = [
     .enableUpcomingFeature("InferIsolatedConformances"),
     .enableUpcomingFeature("ExistentialAny"),
     .enableUpcomingFeature("MemberImportVisibility"),
-    .enableUpcomingFeature("InternalImportsByDefault")
+    .enableUpcomingFeature("InternalImportsByDefault"),
+    .unsafeFlags(["-Xfrontend", "-require-explicit-sendable", "-warnings-as-errors"])
 ]
 
 let package = Package(

Sources/Middleware/MiddlewareBuilder.swift

@@ -21,7 +21,7 @@
 /// }
 /// ```
 @resultBuilder
-public struct MiddlewareChainBuilder {
+public struct MiddlewareChainBuilder: Sendable {
     /// Builds a middleware chain from a single middleware component.
     ///
     /// This is the base case for the result builder pattern, handling a single middleware.