diff --git a/Package.swift b/Package.swift
index 8ea8bdf..091950f 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.9
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
@@ -13,5 +13,6 @@ let package = Package(
     targets: [
         .target(name: "Mustache", dependencies: []),
         .testTarget(name: "MustacheTests", dependencies: ["Mustache"]),
-    ]
+    ],
+    swiftLanguageVersions: [.v5, .version("6")]
 )
diff --git a/Sources/Mustache/ContentType.swift b/Sources/Mustache/ContentType.swift
index a0fae50..615e920 100644
--- a/Sources/Mustache/ContentType.swift
+++ b/Sources/Mustache/ContentType.swift
@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Foundation
+
 /// Protocol for content types
 public protocol MustacheContentType: Sendable {
     /// escape text for this content type eg for HTML replace "<" with "&lt;"
@@ -38,8 +40,13 @@ struct HTMLContentType: MustacheContentType {
 /// the content type required. The default available types are `TEXT` and `HTML`. You can register your own
 /// with `MustacheContentTypes.register`.
 public enum MustacheContentTypes {
+
+    private static let lock = NSLock()
+
     static func get(_ name: String) -> MustacheContentType? {
-        self.types[name]
+        lock.withLock {
+            self.types[name]
+        }
     }
 
     /// Register new content type
@@ -47,11 +54,19 @@ public enum MustacheContentTypes {
     ///   - contentType: Content type
     ///   - name: String to identify it
     public static func register(_ contentType: MustacheContentType, named name: String) {
-        self.types[name] = contentType
+        lock.withLock {
+            self.types[name] = contentType
+        }
     }
 
-    static var types: [String: MustacheContentType] = [
+    private static let _types: [String: MustacheContentType] = [
         "HTML": HTMLContentType(),
         "TEXT": TextContentType(),
     ]
+
+    #if compiler(>=6.0)
+    nonisolated(unsafe) static var types: [String: MustacheContentType] = _types
+    #else
+    static var types: [String: MustacheContentType] = _types
+    #endif
 }
diff --git a/Sources/Mustache/Library+FileSystem.swift b/Sources/Mustache/Library+FileSystem.swift
index ccdf0c8..1769f1b 100644
--- a/Sources/Mustache/Library+FileSystem.swift
+++ b/Sources/Mustache/Library+FileSystem.swift
@@ -16,7 +16,7 @@ import Foundation
 
 extension MustacheLibrary {
     /// Load templates from a folder
-    static func loadTemplates(from directory: String, withExtension extension: String = "mustache") async throws -> [String: MustacheTemplate] {
+    static func loadTemplates(from directory: String, withExtension extension: String = "mustache") throws -> [String: MustacheTemplate] {
         var directory = directory
         if !directory.hasSuffix("/") {
             directory += "/"
diff --git a/Sources/Mustache/Library.swift b/Sources/Mustache/Library.swift
index dde685e..00ccefa 100644
--- a/Sources/Mustache/Library.swift
+++ b/Sources/Mustache/Library.swift
@@ -39,7 +39,7 @@ public struct MustacheLibrary: Sendable {
     /// - Parameter directory: Directory to look for mustache templates
     /// - Parameter extension: Extension of files to look for
     public init(directory: String, withExtension extension: String = "mustache") async throws {
-        self.templates = try await Self.loadTemplates(from: directory, withExtension: `extension`)
+        self.templates = try Self.loadTemplates(from: directory, withExtension: `extension`)
     }
 
     /// Register template under name
diff --git a/Sources/Mustache/NSLock+Backport.swift b/Sources/Mustache/NSLock+Backport.swift
new file mode 100644
index 0000000..d4650a0
--- /dev/null
+++ b/Sources/Mustache/NSLock+Backport.swift
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Hummingbird server framework project
+//
+// Copyright (c) 2021-2021 the Hummingbird authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See hummingbird/CONTRIBUTORS.txt for the list of Hummingbird authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+#if compiler(<6.0)
+import Foundation
+
+extension NSLock {
+    func withLock<Value>(_ operation: () throws -> Value) rethrows -> Value {
+        self.lock()
+        defer {
+            self.unlock()
+        }
+        return try operation()
+    }
+}
+#endif
diff --git a/Sources/Mustache/Parser.swift b/Sources/Mustache/Parser.swift
index 790e987..85b5d4a 100644
--- a/Sources/Mustache/Parser.swift
+++ b/Sources/Mustache/Parser.swift
@@ -303,7 +303,7 @@ extension Parser {
 }
 
 /// context used in parser error
-public struct MustacheParserContext {
+public struct MustacheParserContext: Sendable {
     public let line: String
     public let lineNumber: Int
     public let columnNumber: Int
diff --git a/Tests/MustacheTests/TemplateParserTests.swift b/Tests/MustacheTests/TemplateParserTests.swift
index 9e022c4..5baf2c0 100644
--- a/Tests/MustacheTests/TemplateParserTests.swift
+++ b/Tests/MustacheTests/TemplateParserTests.swift
@@ -65,11 +65,7 @@ extension MustacheTemplate {
     }
 }
 
-#if hasFeature(RetroactiveAttribute)
-extension MustacheTemplate: @retroactive Equatable {}
-#else
 extension MustacheTemplate: Equatable {}
-#endif
 
 extension MustacheTemplate.Token {
     public static func == (lhs: MustacheTemplate.Token, rhs: MustacheTemplate.Token) -> Bool {
@@ -92,8 +88,4 @@ extension MustacheTemplate.Token {
     }
 }
 
-#if hasFeature(RetroactiveAttribute)
-extension MustacheTemplate.Token: @retroactive Equatable {}
-#else
 extension MustacheTemplate.Token: Equatable {}
-#endif