diff --git a/Sources/CMakeLists.txt b/Sources/CMakeLists.txt
index 16088765c7d..a9172ce9150 100644
--- a/Sources/CMakeLists.txt
+++ b/Sources/CMakeLists.txt
@@ -39,4 +39,5 @@ add_subdirectory(SwiftParserDiagnostics)
 add_subdirectory(SwiftOperators)
 add_subdirectory(SwiftSyntaxBuilder)
 add_subdirectory(SwiftSyntaxMacros)
+add_subdirectory(SwiftCompilerPluginMessageHandling)
 add_subdirectory(IDEUtils)
diff --git a/Sources/SwiftCompilerPluginMessageHandling/CMakeLists.txt b/Sources/SwiftCompilerPluginMessageHandling/CMakeLists.txt
new file mode 100644
index 00000000000..5a838463580
--- /dev/null
+++ b/Sources/SwiftCompilerPluginMessageHandling/CMakeLists.txt
@@ -0,0 +1,22 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+add_swift_host_library(SwiftCompilerPluginMessageHandling
+  CompilerPluginMessageHandler.swift
+  Diagnostics.swift
+  Macros.swift
+  PluginMacroExpansionContext.swift
+  PluginMessages.swift
+)
+
+target_link_libraries(SwiftCompilerPluginMessageHandling PUBLIC
+  SwiftSyntax
+  SwiftDiagnostics
+  SwiftParser
+  SwiftSyntaxMacros
+  SwiftOperators)
diff --git a/Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift b/Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift
index 9d7f04d2a44..c01ad205b27 100644
--- a/Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift
+++ b/Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift
@@ -12,9 +12,23 @@
 
 import SwiftSyntaxMacros
 
+/// Optional features.
+public enum PluginFeature: String {
+  case loadPluginLibrary = "load-plugin-library"
+}
+
 /// A type that provides the actual plugin functions.
 public protocol PluginProvider {
+  /// Resolve macro type by the module name and the type name.
   func resolveMacro(moduleName: String, typeName: String) -> Macro.Type?
+
+  /// Load dynamic link library at `libraryPath`. Implementations can use
+  /// `moduleName` to associate the loaded library with it.
+  func loadPluginLibrary(libraryPath: String, moduleName: String) throws
+
+  /// Optional plugin features. This is sent to the host so the it can decide
+  /// the behavior depending on these.
+  var features: [PluginFeature] { get }
 }
 
 /// Low level message connection to the plugin host.
@@ -67,9 +81,11 @@ extension CompilerPluginMessageHandler {
   fileprivate func handleMessage(_ message: HostToPluginMessage) throws {
     switch message {
     case .getCapability:
-      try self.sendMessage(
-        .getCapabilityResult(capability: PluginMessage.capability)
+      let capability = PluginMessage.PluginCapability(
+        protocolVersion: PluginMessage.PROTOCOL_VERSION_NUMBER,
+        features: provider.features.map({ $0.rawValue })
       )
+      try self.sendMessage(.getCapabilityResult(capability: capability))
 
     case .expandFreestandingMacro(let macro, let discriminator, let expandingSyntax):
       try expandFreestandingMacro(
@@ -87,6 +103,42 @@ extension CompilerPluginMessageHandler {
         declSyntax: declSyntax,
         parentDeclSyntax: parentDeclSyntax
       )
+
+    case .loadPluginLibrary(let libraryPath, let moduleName):
+      var diags: [PluginMessage.Diagnostic] = []
+      do {
+        try provider.loadPluginLibrary(libraryPath: libraryPath, moduleName: moduleName)
+      } catch {
+        diags.append(
+          PluginMessage.Diagnostic(
+            message: String(describing: error),
+            severity: .error,
+            position: .invalid,
+            highlights: [],
+            notes: [],
+            fixIts: []
+          )
+        )
+      }
+      try self.sendMessage(.loadPluginLibraryResult(loaded: diags.isEmpty, diagnostics: diags));
     }
   }
 }
+
+struct UnimplementedError: Error, CustomStringConvertible {
+  var description: String { "unimplemented" }
+}
+
+/// Default implementation of 'PluginProvider' requirements.
+public extension PluginProvider {
+  var features: [PluginFeature] {
+    // No optional features by default.
+    return []
+  }
+
+  func loadPluginLibrary(libraryPath: String, moduleName: String) throws {
+    // This should be unreachable. The host should not call 'loadPluginLibrary'
+    // unless the feature is not declared.
+    throw UnimplementedError()
+  }
+}
diff --git a/Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift b/Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift
index de5e44f0cfa..df67a178f43 100644
--- a/Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift
+++ b/Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift
@@ -14,14 +14,17 @@
 // NOTE: Types in this file should be self-contained and should not depend on any non-stdlib types.
 
 internal enum HostToPluginMessage: Codable {
+  /// Get capability of this plugin.
   case getCapability
 
+  /// Expand a '@freestanding' macro.
   case expandFreestandingMacro(
     macro: PluginMessage.MacroReference,
     discriminator: String,
     syntax: PluginMessage.Syntax
   )
 
+  /// Expand an '@attached' macro.
   case expandAttachedMacro(
     macro: PluginMessage.MacroReference,
     macroRole: PluginMessage.MacroRole,
@@ -30,9 +33,21 @@ internal enum HostToPluginMessage: Codable {
     declSyntax: PluginMessage.Syntax,
     parentDeclSyntax: PluginMessage.Syntax?
   )
+
+  /// Optionally implemented message to load a dynamic link library.
+  /// 'moduleName' can be used as a hint indicating that the library
+  /// provides the specified module.
+  case loadPluginLibrary(
+    libraryPath: String,
+    moduleName: String
+  )
 }
 
 internal enum PluginToHostMessage: Codable {
+  case getCapabilityResult(
+    capability: PluginMessage.PluginCapability
+  )
+
   case expandFreestandingMacroResult(
     expandedSource: String?,
     diagnostics: [PluginMessage.Diagnostic]
@@ -43,18 +58,21 @@ internal enum PluginToHostMessage: Codable {
     diagnostics: [PluginMessage.Diagnostic]
   )
 
-  case getCapabilityResult(capability: PluginMessage.PluginCapability)
+  case loadPluginLibraryResult(
+    loaded: Bool,
+    diagnostics: [PluginMessage.Diagnostic]
+  )
 }
 
 /*namespace*/ internal enum PluginMessage {
-  static var PROTOCOL_VERSION_NUMBER: Int { 3 }  // Renamed 'customAttributeSyntax' to 'attributeSyntax'.
+  static var PROTOCOL_VERSION_NUMBER: Int { 4 }  // Added 'loadPluginLibrary'.
 
   struct PluginCapability: Codable {
     var protocolVersion: Int
-  }
 
-  static var capability: PluginCapability {
-    PluginCapability(protocolVersion: PluginMessage.PROTOCOL_VERSION_NUMBER)
+    /// Optional features this plugin provides.
+    ///  * 'load-plugin-library': 'loadPluginLibrary' message is implemented.
+    var features: [String]?
   }
 
   struct MacroReference: Codable {