diff --git a/Sources/SKCore/BuildSystem.swift b/Sources/SKCore/BuildSystem.swift
index 05434701..0e693c5c 100644
--- a/Sources/SKCore/BuildSystem.swift
+++ b/Sources/SKCore/BuildSystem.swift
@@ -11,35 +11,26 @@
 //===----------------------------------------------------------------------===//
 
 import LanguageServerProtocol
+import Basic
 
-/// Provider of build settings.
-public protocol BuildSettingsProvider {
+/// Provider of FileBuildSettings and other build-related information.
+///
+/// The primary role of the build system is to answer queries for FileBuildSettings and (TODO) to
+/// notify clients when they change. The BuildSystem is also the source of related informatino,
+/// such as where the index datastore is located.
+///
+/// For example, a SwiftPMWorkspace provides compiler arguments for the files contained in a
+/// SwiftPM package root directory.
+public protocol BuildSystem {
+
+  /// The path to the raw index store data, if any.
+  var indexStorePath: AbsolutePath? { get }
+
+  /// The path to put the index database, if any.
+  var indexDatabasePath: AbsolutePath? { get }
 
   /// Returns the settings for the given url and language mode, if known.
-  func settings(for: URL, language: Language) -> FileBuildSettings?
+  func settings(for: URL, _ language: Language) -> FileBuildSettings?
 
   // TODO: notifications when settings change.
 }
-
-/// Build settings for a single file.
-public struct FileBuildSettings {
-
-  /// The identifier of the toolchain that is preferred for compiling this file, if any.
-  public var preferredToolchain: String? = nil
-
-  /// The compiler arguments to use for this file.
-  public var compilerArguments: [String]
-
-  /// The working directory to resolve any relative paths in `compilerArguments`.
-  public var workingDirectory: String? = nil
-
-  public init(
-    preferredToolchain: String? = nil,
-    compilerArguments: [String],
-    workingDirectory: String? = nil
-  ) {
-    self.preferredToolchain = preferredToolchain
-    self.compilerArguments = compilerArguments
-    self.workingDirectory = workingDirectory
-  }
-}
diff --git a/Sources/SKCore/BuildSettingsProviderList.swift b/Sources/SKCore/BuildSystemList.swift
similarity index 52%
rename from Sources/SKCore/BuildSettingsProviderList.swift
rename to Sources/SKCore/BuildSystemList.swift
index 6503d9f6..6c8c9ec0 100644
--- a/Sources/SKCore/BuildSettingsProviderList.swift
+++ b/Sources/SKCore/BuildSystemList.swift
@@ -10,21 +10,29 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Basic
 import LanguageServerProtocol
 
-/// Provides build settings from a list of providers in priority order.
-public final class BuildSettingsProviderList: BuildSettingsProvider {
+/// Provides build settings from a list of build systems in priority order.
+public final class BuildSystemList {
 
-  /// The build settings providers to try (in order).
-  public var providers: [BuildSettingsProvider] = [
-    FallbackBuildSettingsProvider()
+  /// The build systems to try (in order).
+  public var providers: [BuildSystem] = [
+    FallbackBuildSystem()
   ]
 
   public init() {}
+}
 
-  public func settings(for url: URL, language: Language) -> FileBuildSettings? {
+extension BuildSystemList: BuildSystem {
+
+  public var indexStorePath: AbsolutePath? { return providers.first?.indexStorePath }
+
+  public var indexDatabasePath: AbsolutePath? { return providers.first?.indexDatabasePath }
+
+  public func settings(for url: URL, _ language: Language) -> FileBuildSettings? {
     for provider in providers {
-      if let settings = provider.settings(for: url, language: language) {
+      if let settings = provider.settings(for: url, language) {
         return settings
       }
     }
diff --git a/Sources/SKCore/CompilationDatabaseBuildSystem.swift b/Sources/SKCore/CompilationDatabaseBuildSystem.swift
index 4960bcb7..03073788 100644
--- a/Sources/SKCore/CompilationDatabaseBuildSystem.swift
+++ b/Sources/SKCore/CompilationDatabaseBuildSystem.swift
@@ -18,7 +18,7 @@ import LanguageServerProtocol
 ///
 /// Provides build settings from a `CompilationDatabase` found by searching a project. For now, only
 /// one compilation database, located at the project root.
-public final class CompilationDatabaseBuildSystem: BuildSettingsProvider {
+public final class CompilationDatabaseBuildSystem {
 
   /// The compilation database.
   var compdb: CompilationDatabase? = nil
@@ -31,8 +31,15 @@ public final class CompilationDatabaseBuildSystem: BuildSettingsProvider {
       self.compdb = tryLoadCompilationDatabase(directory: path, fileSystem)
     }
   }
+}
 
-  public func settings(for url: URL, language: Language) -> FileBuildSettings? {
+extension CompilationDatabaseBuildSystem: BuildSystem {
+
+  // FIXME: derive from the compiler arguments.
+  public var indexStorePath: AbsolutePath? { return nil }
+  public var indexDatabasePath: AbsolutePath? { return nil }
+
+  public func settings(for url: URL, _ language: Language) -> FileBuildSettings? {
     guard let db = database(for: url),
           let cmd = db[url].first else { return nil }
     return FileBuildSettings(
diff --git a/Sources/SKCore/ExternalWorkspace.swift b/Sources/SKCore/ExternalWorkspace.swift
deleted file mode 100644
index 86561b03..00000000
--- a/Sources/SKCore/ExternalWorkspace.swift
+++ /dev/null
@@ -1,29 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See https://swift.org/LICENSE.txt for license information
-// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-import Basic
-
-/// Connection to an external workspace/project, providing access to settings, etc.
-///
-/// For example, a swiftpm package loaded from disk can provide command-line arguments for the files
-/// contained in its package root directory.
-public protocol ExternalWorkspace {
-
-  /// The build system, providing access to compiler arguments.
-  var buildSystem: BuildSettingsProvider { get }
-
-  /// The path to the raw index store data, if any.
-  var indexStorePath: AbsolutePath? { get }
-
-  /// The path to put the index database, if any.
-  var indexDatabasePath: AbsolutePath? { get }
-}
diff --git a/Sources/SKCore/FallbackBuildSettingsProvider.swift b/Sources/SKCore/FallbackBuildSystem.swift
similarity index 61%
rename from Sources/SKCore/FallbackBuildSettingsProvider.swift
rename to Sources/SKCore/FallbackBuildSystem.swift
index 412afde4..1e838e8e 100644
--- a/Sources/SKCore/FallbackBuildSettingsProvider.swift
+++ b/Sources/SKCore/FallbackBuildSystem.swift
@@ -14,34 +14,41 @@ import LanguageServerProtocol
 import Basic
 import enum Utility.Platform
 
-/// A simple build settings provider suitable as a fallback when accurate settings are unknown.
-public final class FallbackBuildSettingsProvider: BuildSettingsProvider {
+/// A simple BuildSystem suitable as a fallback when accurate settings are unknown.
+public final class FallbackBuildSystem: BuildSystem {
 
+  /// The path to the SDK.
   lazy var sdkpath: AbsolutePath? = {
-    if case .darwin? = Platform.currentPlatform {
-      if let str = try? Process.checkNonZeroExit(args: "/usr/bin/xcrun", "--show-sdk-path", "--sdk", "macosx"), let path = try? AbsolutePath(validating: str.spm_chomp()) {
-        return path
-      }
+    if case .darwin? = Platform.currentPlatform,
+       let str = try? Process.checkNonZeroExit(
+         args: "/usr/bin/xcrun", "--show-sdk-path", "--sdk", "macosx"),
+       let path = try? AbsolutePath(validating: str.spm_chomp())
+    {
+      return path
     }
     return nil
   }()
 
-  public func settings(for url: URL, language: Language) -> FileBuildSettings? {
+  public var indexStorePath: AbsolutePath? { return nil }
+
+  public var indexDatabasePath: AbsolutePath? { return nil }
+
+  public func settings(for url: URL, _ language: Language) -> FileBuildSettings? {
     guard let path = try? AbsolutePath(validating: url.path) else {
       return nil
     }
 
     switch language {
     case .swift:
-      return settingsSwift(path: path)
+      return settingsSwift(path)
     case .c, .cpp, .objective_c, .objective_cpp:
-      return settingsClang(path: path, language: language)
+      return settingsClang(path, language)
     default:
       return nil
     }
   }
 
-  func settingsSwift(path: AbsolutePath) -> FileBuildSettings {
+  func settingsSwift(_ path: AbsolutePath) -> FileBuildSettings {
     var args: [String] = []
     if let sdkpath = sdkpath {
       args += [
@@ -53,7 +60,7 @@ public final class FallbackBuildSettingsProvider: BuildSettingsProvider {
     return FileBuildSettings(preferredToolchain: nil, compilerArguments: args)
   }
 
-  func settingsClang(path: AbsolutePath, language: Language) -> FileBuildSettings {
+  func settingsClang(_ path: AbsolutePath, _ language: Language) -> FileBuildSettings {
     var args: [String] = []
     if let sdkpath = sdkpath {
       args += [
diff --git a/Sources/SKCore/FileBuildSettings.swift b/Sources/SKCore/FileBuildSettings.swift
new file mode 100644
index 00000000..b04482e7
--- /dev/null
+++ b/Sources/SKCore/FileBuildSettings.swift
@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// Build settings for a single file.
+///
+/// Encapsulates all the settings needed to compile a single file, including the compiler arguments,
+/// working directory, and preferred toolchain if any. FileBuildSettings are typically the result
+/// of a BuildSystem query.
+public struct FileBuildSettings {
+
+  /// The identifier of the toolchain that is preferred for compiling this file, if any.
+  public var preferredToolchain: String? = nil
+
+  /// The compiler arguments to use for this file.
+  public var compilerArguments: [String]
+
+  /// The working directory to resolve any relative paths in `compilerArguments`.
+  public var workingDirectory: String? = nil
+
+  public init(
+    preferredToolchain: String? = nil,
+    compilerArguments: [String],
+    workingDirectory: String? = nil)
+  {
+    self.preferredToolchain = preferredToolchain
+    self.compilerArguments = compilerArguments
+    self.workingDirectory = workingDirectory
+  }
+}
diff --git a/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift b/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
index 48d8ccfa..1a27502c 100644
--- a/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
+++ b/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
@@ -40,7 +40,7 @@ public final class SwiftPMWorkspace {
   var fileToTarget: [AbsolutePath: TargetBuildDescription] = [:]
   var sourceDirToTarget: [AbsolutePath: TargetBuildDescription] = [:]
 
-  /// Creates a `BuildSettingsProvider` using the Swift Package Manager, if this workspace is part of a package.
+  /// Creates a `BuildSystem` using the Swift Package Manager, if this workspace is part of a package.
   ///
   /// - returns: nil if `workspacePath` is not part of a package or there is an error.
   public convenience init?(url: LanguageServerProtocol.URL, toolchainRegistry: ToolchainRegistry) {
@@ -52,7 +52,7 @@ public final class SwiftPMWorkspace {
     }
   }
 
-  /// Creates a `BuildSettingsProvider` using the Swift Package Manager, if this workspace is part of a package.
+  /// Creates a `BuildSystem` using the Swift Package Manager, if this workspace is part of a package.
   ///
   /// - returns: nil if `workspacePath` is not part of a package.
   /// - throws: If there is an error loading the package.
@@ -163,9 +163,7 @@ public final class SwiftPMWorkspace {
   }
 }
 
-extension SwiftPMWorkspace: ExternalWorkspace, BuildSettingsProvider {
-
-  public var buildSystem: BuildSettingsProvider { return self }
+extension SwiftPMWorkspace: BuildSystem {
 
   public var buildPath: AbsolutePath {
     return buildParameters.buildPath
@@ -179,13 +177,16 @@ extension SwiftPMWorkspace: ExternalWorkspace, BuildSettingsProvider {
     return buildPath.appending(components: "index", "db")
   }
 
-  public func settings(for url: LanguageServerProtocol.URL, language: Language) -> FileBuildSettings? {
+  public func settings(
+    for url: LanguageServerProtocol.URL,
+    _ language: Language) -> FileBuildSettings?
+  {
     guard let path = try? AbsolutePath(validating: url.path) else {
       return nil
     }
 
     if let td = self.fileToTarget[path] {
-      return settings(for: path, language: language, targetDescription: td)
+      return settings(for: path, language, td)
     }
 
     if path.basename == "Package.swift" {
@@ -193,7 +194,7 @@ extension SwiftPMWorkspace: ExternalWorkspace, BuildSettingsProvider {
     }
 
     if path.extension == "h" {
-      return settings(forHeader: path, language: language)
+      return settings(forHeader: path, language)
     }
 
     return nil
@@ -206,8 +207,8 @@ extension SwiftPMWorkspace {
 
   public func settings(
     for path: AbsolutePath,
-    language: Language,
-    targetDescription td: TargetBuildDescription
+    _ language: Language,
+    _ td: TargetBuildDescription
   ) -> FileBuildSettings? {
 
     let buildPath = self.buildPath
@@ -315,11 +316,11 @@ extension SwiftPMWorkspace {
     return nil
   }
 
-  public func settings(forHeader path: AbsolutePath, language: Language) -> FileBuildSettings? {
+  public func settings(forHeader path: AbsolutePath, _ language: Language) -> FileBuildSettings? {
     var dir = path.parentDirectory
     while !dir.isRoot {
       if let td = sourceDirToTarget[dir] {
-        return settings(for: path, language: language, targetDescription: td)
+        return settings(for: path, language, td)
       }
       dir = dir.parentDirectory
     }
diff --git a/Sources/SourceKit/SourceKitServer.swift b/Sources/SourceKit/SourceKitServer.swift
index af8edea7..31773e49 100644
--- a/Sources/SourceKit/SourceKitServer.swift
+++ b/Sources/SourceKit/SourceKitServer.swift
@@ -118,8 +118,8 @@ public final class SourceKitServer: LanguageServer {
     client.send(note.params)
   }
 
-  func toolchain(for url: URL, language: Language) -> Toolchain? {
-    if let id = workspace?.buildSettings.settings(for: url, language: language)?.preferredToolchain,
+  func toolchain(for url: URL, _ language: Language) -> Toolchain? {
+    if let id = workspace?.buildSettings.settings(for: url, language)?.preferredToolchain,
        let toolchain = toolchainRegistry.toolchain(identifier:id)
     {
       return toolchain
@@ -150,7 +150,7 @@ public final class SourceKitServer: LanguageServer {
     return nil
   }
 
-  func languageService(for toolchain: Toolchain, language: Language) -> Connection? {
+  func languageService(for toolchain: Toolchain, _ language: Language) -> Connection? {
     let key = LanguageServiceKey(toolchain: toolchain.identifier, language: language)
     if let service = languageService[key] {
       return service
@@ -158,7 +158,7 @@ public final class SourceKitServer: LanguageServer {
 
     // Start a new service.
     return orLog("failed to start language service", level: .error) {
-      guard let service = try SourceKit.languageService(for: toolchain, language: language, client: self) else {
+      guard let service = try SourceKit.languageService(for: toolchain, language, client: self) else {
         return nil
       }
 
@@ -183,12 +183,14 @@ public final class SourceKitServer: LanguageServer {
     }
   }
 
-  func languageService(for url: URL, language: Language, workspace: Workspace) -> Connection? {
+  func languageService(for url: URL, _ language: Language, in workspace: Workspace) -> Connection? {
     if let service = workspace.documentService[url] {
       return service
     }
 
-    guard let toolchain = toolchain(for: url, language: language), let service = languageService(for: toolchain, language: language) else {
+    guard let toolchain = toolchain(for: url, language),
+          let service = languageService(for: toolchain, language)
+    else {
       return nil
     }
 
@@ -226,8 +228,7 @@ extension SourceKitServer {
       self.workspace = Workspace(
         rootPath: nil,
         clientCapabilities: req.params.capabilities,
-        external: nil,
-        buildSettings: BuildSettingsProviderList(),
+        buildSettings: BuildSystemList(),
         index: nil
       )
     }
@@ -274,7 +275,7 @@ extension SourceKitServer {
   func openDocument(_ note: Notification<DidOpenTextDocument>, workspace: Workspace) {
     workspace.documentManager.open(note)
 
-    if let service = languageService(for: note.params.textDocument.url, language: note.params.textDocument.language, workspace: workspace) {
+    if let service = languageService(for: note.params.textDocument.url, note.params.textDocument.language, in: workspace) {
       service.send(note.params)
     }
   }
@@ -448,11 +449,16 @@ extension SourceKitServer {
   }
 }
 
-/// Creates a new connection from `client` to a service for `language` if available, and launches the service. Does *not* send the initialization request.
+/// Creates a new connection from `client` to a service for `language` if available, and launches
+/// the service. Does *not* send the initialization request.
 ///
 /// - returns: The connection, if a suitable language service is available; otherwise nil.
 /// - throws: If there is a suitable service but it fails to launch, throws an error.
-public func languageService(for toolchain: Toolchain, language: Language, client: MessageHandler) throws -> Connection? {
+public func languageService(
+  for toolchain: Toolchain,
+  _ language: Language,
+  client: MessageHandler) throws -> Connection?
+{
   switch language {
 
   case .c, .cpp, .objective_c, .objective_cpp:
diff --git a/Sources/SourceKit/Workspace.swift b/Sources/SourceKit/Workspace.swift
index c0409c16..237c245f 100644
--- a/Sources/SourceKit/Workspace.swift
+++ b/Sources/SourceKit/Workspace.swift
@@ -23,9 +23,7 @@ import SKSwiftPMWorkspace
 /// In LSP, this represents the per-workspace state that is typically only available after the
 /// "initialize" request has been made.
 ///
-/// Typically a workspace is contained in a root directory, and may be represented by an
-/// `ExternalWorkspace` if this workspace is part of a workspace in another tool such as a swiftpm
-/// package.
+/// Typically a workspace is contained in a root directory.
 public final class Workspace {
 
   /// The root directory of the workspace.
@@ -33,14 +31,8 @@ public final class Workspace {
 
   public let clientCapabilities: ClientCapabilities
 
-  /// The external workspace connection, if any.
-  public let external: ExternalWorkspace?
-
   /// The build settings provider to use for documents in this workspace.
-  ///
-  /// If `external` is not `nil`, this will typically include `external.buildSystem`. It may also
-  /// provide settings for files outside the workspace using additional providers.
-  public let buildSettings: BuildSettingsProvider
+  public let buildSettings: BuildSystem
 
   /// The source code index, if available.
   public var index: IndexStoreDB? = nil
@@ -54,13 +46,11 @@ public final class Workspace {
   public init(
     rootPath: AbsolutePath?,
     clientCapabilities: ClientCapabilities,
-    external: ExternalWorkspace?,
-    buildSettings: BuildSettingsProvider,
+    buildSettings: BuildSystem,
     index: IndexStoreDB?)
   {
     self.rootPath = rootPath
     self.clientCapabilities = clientCapabilities
-    self.external = external
     self.buildSettings = buildSettings
     self.index = index
   }
@@ -79,21 +69,17 @@ public final class Workspace {
 
     self.rootPath = try AbsolutePath(validating: url.path)
     self.clientCapabilities = clientCapabilities
-    self.external = SwiftPMWorkspace(url: url, toolchainRegistry: toolchainRegistry)
-
-    let settings = BuildSettingsProviderList()
+    let settings = BuildSystemList()
     self.buildSettings = settings
 
     settings.providers.insert(CompilationDatabaseBuildSystem(projectRoot: rootPath), at: 0)
 
-    guard let external = self.external else {
-      return
+    if let swiftpm = SwiftPMWorkspace(url: url, toolchainRegistry: toolchainRegistry) {
+      settings.providers.insert(swiftpm, at: 0)
     }
 
-    settings.providers.insert(external.buildSystem, at: 0)
-
-    if let storePath = external.indexStorePath,
-       let dbPath = external.indexDatabasePath,
+    if let storePath = buildSettings.indexStorePath,
+       let dbPath = buildSettings.indexDatabasePath,
        let libPath = toolchainRegistry.default?.libIndexStore
     {
       do {
diff --git a/Sources/SourceKit/clangd/ClangLanguageServer.swift b/Sources/SourceKit/clangd/ClangLanguageServer.swift
index b16f05de..5efae901 100644
--- a/Sources/SourceKit/clangd/ClangLanguageServer.swift
+++ b/Sources/SourceKit/clangd/ClangLanguageServer.swift
@@ -22,13 +22,13 @@ final class ClangLanguageServerShim: LanguageServer {
 
   let clangd: Connection
 
-  let buildSettingsProvider: BuildSettingsProvider
+  let buildSystem: BuildSystem
 
   /// Creates a language server for the given client using the sourcekitd dylib at the specified path.
-  public init(client: Connection, clangd: Connection, buildSettingsProvider: BuildSettingsProvider) throws {
+  public init(client: Connection, clangd: Connection, buildSystem: BuildSystem) throws {
 
     self.clangd = clangd
-    self.buildSettingsProvider = buildSettingsProvider
+    self.buildSystem = buildSystem
     super.init(client: client)
   }
 
@@ -65,7 +65,7 @@ final class ClangLanguageServerShim: LanguageServer {
 
   func openDocument(_ note: Notification<DidOpenTextDocument>) {
     let url = note.params.textDocument.url
-    let settings = buildSettingsProvider.settings(for: url, language: note.params.textDocument.language)
+    let settings = buildSystem.settings(for: url, note.params.textDocument.language)
 
     logAsync(level: settings == nil ? .warning : .debug) { _ in
       let settingsStr = settings == nil ? "nil" : settings!.compilerArguments.description
@@ -82,7 +82,7 @@ final class ClangLanguageServerShim: LanguageServer {
   }
 }
 
-func makeJSONRPCClangServer(client: MessageHandler, clangd: AbsolutePath, buildSettings: BuildSettingsProvider?) throws -> Connection {
+func makeJSONRPCClangServer(client: MessageHandler, clangd: AbsolutePath, buildSettings: BuildSystem?) throws -> Connection {
 
   let clientToServer: Pipe = Pipe()
   let serverToClient: Pipe = Pipe()
@@ -98,7 +98,7 @@ func makeJSONRPCClangServer(client: MessageHandler, clangd: AbsolutePath, buildS
   let shim = try ClangLanguageServerShim(
     client: connectionToClient,
     clangd: connection,
-    buildSettingsProvider: buildSettings ?? BuildSettingsProviderList()
+    buildSystem: buildSettings ?? BuildSystemList()
   )
 
   connectionToShim.start(handler: shim)
diff --git a/Sources/SourceKit/sourcekitd/SwiftLanguageServer.swift b/Sources/SourceKit/sourcekitd/SwiftLanguageServer.swift
index 24355dfe..c568f01e 100644
--- a/Sources/SourceKit/sourcekitd/SwiftLanguageServer.swift
+++ b/Sources/SourceKit/sourcekitd/SwiftLanguageServer.swift
@@ -21,7 +21,7 @@ public final class SwiftLanguageServer: LanguageServer {
 
   let sourcekitd: SwiftSourceKitFramework
 
-  let buildSettingsProvider: BuildSettingsProvider
+  let buildSystem: BuildSystem
 
   // FIXME: ideally we wouldn't need separate management from a parent server in the same process.
   var documentManager: DocumentManager
@@ -34,10 +34,10 @@ public final class SwiftLanguageServer: LanguageServer {
   var values: sourcekitd_values { return sourcekitd.values }
 
   /// Creates a language server for the given client using the sourcekitd dylib at the specified path.
-  public init(client: Connection, sourcekitd: AbsolutePath, buildSettingsProvider: BuildSettingsProvider, onExit: @escaping () -> () = {}) throws {
+  public init(client: Connection, sourcekitd: AbsolutePath, buildSystem: BuildSystem, onExit: @escaping () -> () = {}) throws {
 
     self.sourcekitd = try SwiftSourceKitFramework(dylib: sourcekitd)
-    self.buildSettingsProvider = buildSettingsProvider
+    self.buildSystem = buildSystem
     self.documentManager = DocumentManager()
     self.onExit = onExit
     super.init(client: client)
@@ -220,7 +220,7 @@ extension SwiftLanguageServer {
     req[keys.name] = note.params.textDocument.url.path
     req[keys.sourcetext] = snapshot.text
 
-    if let settings = buildSettingsProvider.settings(for: snapshot.document.url, language: snapshot.document.language) {
+    if let settings = buildSystem.settings(for: snapshot.document.url, snapshot.document.language) {
       req[keys.compilerargs] = settings.compilerArguments
     }
 
@@ -308,7 +308,7 @@ extension SwiftLanguageServer {
     skreq[keys.sourcefile] = snapshot.document.url.path
     skreq[keys.sourcetext] = snapshot.text
 
-    if let settings = buildSettingsProvider.settings(for: snapshot.document.url, language: snapshot.document.language) {
+    if let settings = buildSystem.settings(for: snapshot.document.url, snapshot.document.language) {
       skreq[keys.compilerargs] = settings.compilerArguments
     }
 
@@ -432,7 +432,7 @@ extension SwiftLanguageServer {
     skreq[keys.sourcefile] = snapshot.document.url.path
 
     // FIXME: should come from the internal document
-    if let settings = buildSettingsProvider.settings(for: snapshot.document.url, language: snapshot.document.language) {
+    if let settings = buildSystem.settings(for: snapshot.document.url, snapshot.document.language) {
       skreq[keys.compilerargs] = settings.compilerArguments
     }
 
@@ -510,7 +510,7 @@ extension SwiftLanguageServer {
     skreq[keys.sourcefile] = snapshot.document.url.path
 
     // FIXME: should come from the internal document
-    if let settings = buildSettingsProvider.settings(for: snapshot.document.url, language: snapshot.document.language) {
+    if let settings = buildSystem.settings(for: snapshot.document.url, snapshot.document.language) {
       skreq[keys.compilerargs] = settings.compilerArguments
     }
 
@@ -568,7 +568,7 @@ extension DocumentSnapshot {
   }
 }
 
-func makeLocalSwiftServer(client: MessageHandler, sourcekitd: AbsolutePath, buildSettings: BuildSettingsProvider?) throws -> Connection {
+func makeLocalSwiftServer(client: MessageHandler, sourcekitd: AbsolutePath, buildSettings: BuildSystem?) throws -> Connection {
 
   let connectionToSK = LocalConnection()
   let connectionToClient = LocalConnection()
@@ -576,7 +576,7 @@ func makeLocalSwiftServer(client: MessageHandler, sourcekitd: AbsolutePath, buil
   let server = try SwiftLanguageServer(
     client: connectionToClient,
     sourcekitd: sourcekitd,
-    buildSettingsProvider: buildSettings ?? BuildSettingsProviderList()
+    buildSystem: buildSettings ?? BuildSystemList()
   )
 
   connectionToSK.start(handler: server)
diff --git a/Tests/SKCoreTests/CompilationDatabaseTests.swift b/Tests/SKCoreTests/CompilationDatabaseTests.swift
index 44e015ec..5aafcac3 100644
--- a/Tests/SKCoreTests/CompilationDatabaseTests.swift
+++ b/Tests/SKCoreTests/CompilationDatabaseTests.swift
@@ -197,10 +197,10 @@ final class CompilationDatabaseTests: XCTestCase {
       ]
       """)
 
-    let buildSystem: BuildSettingsProvider = CompilationDatabaseBuildSystem(
+    let buildSystem: BuildSystem = CompilationDatabaseBuildSystem(
       projectRoot: AbsolutePath("/a"), fileSystem: fs)
 
-    let settings = buildSystem.settings(for: URL(fileURLWithPath: "/a/a.swift"), language: .swift)
+    let settings = buildSystem.settings(for: URL(fileURLWithPath: "/a/a.swift"), .swift)
     XCTAssertNotNil(settings)
     XCTAssertEqual(settings?.workingDirectory, "/a")
     XCTAssertEqual(settings?.compilerArguments, ["-swift-version", "4", "/a/a.swift"])
diff --git a/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift b/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift
index 1e8691e8..a0e001a9 100644
--- a/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift
+++ b/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift
@@ -42,7 +42,7 @@ final class SwiftPMWorkspaceTests: XCTestCase {
 
     XCTAssertEqual(ws.buildPath, packageRoot.appending(components: ".build", "debug"))
     XCTAssertNotNil(ws.indexStorePath)
-    let arguments = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    let arguments = ws.settings(for: aswift.asURL, .swift)!.compilerArguments
 
     check(
       "-module-name", "lib", "-incremental", "-emit-dependencies",
@@ -87,10 +87,10 @@ final class SwiftPMWorkspaceTests: XCTestCase {
     let aswift = packageRoot.appending(components: "Sources", "lib", "a.swift")
     let bswift = packageRoot.appending(components: "Sources", "lib", "b.swift")
 
-    let argumentsA = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    let argumentsA = ws.settings(for: aswift.asURL, .swift)!.compilerArguments
     check(aswift.asString, arguments: argumentsA)
     check(bswift.asString, arguments: argumentsA)
-    let argumentsB = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    let argumentsB = ws.settings(for: aswift.asURL, .swift)!.compilerArguments
     check(aswift.asString, arguments: argumentsB)
     check(bswift.asString, arguments: argumentsB)
   }
@@ -124,12 +124,12 @@ final class SwiftPMWorkspaceTests: XCTestCase {
 
     let aswift = packageRoot.appending(components: "Sources", "libA", "a.swift")
     let bswift = packageRoot.appending(components: "Sources", "libB", "b.swift")
-    let arguments = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    let arguments = ws.settings(for: aswift.asURL, .swift)!.compilerArguments
     check(aswift.asString, arguments: arguments)
     checkNot(bswift.asString, arguments: arguments)
     check("-I", packageRoot.appending(components: "Sources", "libC", "include").asString, arguments: arguments)
 
-    let argumentsB = ws.settings(for: bswift.asURL, language: .swift)!.compilerArguments
+    let argumentsB = ws.settings(for: bswift.asURL, .swift)!.compilerArguments
     check(bswift.asString, arguments: argumentsB)
     checkNot(aswift.asString, arguments: argumentsB)
     checkNot("-I", packageRoot.appending(components: "Sources", "libC", "include").asString, arguments: argumentsB)
@@ -164,7 +164,7 @@ final class SwiftPMWorkspaceTests: XCTestCase {
 
     XCTAssertEqual(ws.buildPath, build)
     XCTAssertNotNil(ws.indexStorePath)
-    let arguments = ws.settings(for: acxx.asURL, language: .cpp)!.compilerArguments
+    let arguments = ws.settings(for: acxx.asURL, .cpp)!.compilerArguments
 
     check("-MD", "-MT", "dependencies",
       "-MF", build.appending(components: "lib.build", "a.cpp.d").asString,
@@ -210,7 +210,7 @@ final class SwiftPMWorkspaceTests: XCTestCase {
       fileSystem: fs)!
 
     let aswift = packageRoot.appending(components: "Sources", "lib", "a.swift")
-    let arguments = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    let arguments = ws.settings(for: aswift.asURL, .swift)!.compilerArguments
     check("-target", arguments: arguments) // Only one!
 #if os(macOS)
     check("-target", "x86_64-apple-macosx10.13", arguments: arguments)