Make FallbackBuildSystem not conform to BuildSystem and remove no-op functions from it

Sources/SKCore/BuildSystemManager.swift

@@ -120,7 +120,6 @@ public actor BuildSystemManager {
 
   public func filesDidChange(_ events: [FileEvent]) async {
     await self.buildSystem?.filesDidChange(events)
-    self.fallbackBuildSystem?.filesDidChange(events)
   }
 }
 
@@ -206,7 +205,6 @@ extension BuildSystemManager {
     }
 
     await buildSystem?.registerForChangeNotifications(for: mainFile, language: language)
-    fallbackBuildSystem?.registerForChangeNotifications(for: mainFile, language: language)
   }
 
   /// Return settings for `file` based on  the `change` settings for `mainFile`.
@@ -248,7 +246,7 @@ extension BuildSystemManager {
   public func fileHandlingCapability(for uri: DocumentURI) async -> FileHandlingCapability {
     return max(
       await buildSystem?.fileHandlingCapability(for: uri) ?? .unhandled,
-      fallbackBuildSystem?.fileHandlingCapability(for: uri) ?? .unhandled
+      fallbackBuildSystem != nil ? .fallback : .unhandled
     )
   }
 }

Sources/SKCore/FallbackBuildSystem.swift

@@ -21,7 +21,7 @@ import class TSCBasic.Process
 import struct TSCBasic.AbsolutePath
 
 /// A simple BuildSystem suitable as a fallback when accurate settings are unknown.
-public final class FallbackBuildSystem: BuildSystem {
+public final class FallbackBuildSystem {
 
   let buildSetup: BuildSetup
 
@@ -59,13 +59,6 @@ public final class FallbackBuildSystem: BuildSystem {
     }
   }
 
-  public func registerForChangeNotifications(for uri: DocumentURI, language: Language) {
-    // Fallback build systems never change.
-  }
-
-  /// We don't support change watching.
-  public func unregisterForChangeNotifications(for: DocumentURI) {}
-
   func settingsSwift(_ file: String) -> FileBuildSettings {
     var args: [String] = []
     args.append(contentsOf: self.buildSetup.flags.swiftCompilerFlags)
@@ -98,10 +91,4 @@ public final class FallbackBuildSystem: BuildSystem {
     args.append(file)
     return FileBuildSettings(compilerArguments: args)
   }
-
-  public func filesDidChange(_ events: [FileEvent]) {}
-
-  public func fileHandlingCapability(for uri: DocumentURI) -> FileHandlingCapability {
-    return .fallback
-  }
 }

Tests/SKCoreTests/BuildSystemManagerTests.swift

@@ -34,8 +34,8 @@ final class BuildSystemManagerTests: XCTestCase {
     ]
 
     let bsm = await BuildSystemManager(
-      buildSystem: FallbackBuildSystem(buildSetup: .default),
-      fallbackBuildSystem: nil,
+      buildSystem: nil,
+      fallbackBuildSystem: FallbackBuildSystem(buildSetup: .default),
       mainFilesProvider: mainFiles)
     defer { withExtendedLifetime(bsm) {} } // Keep BSM alive for callbacks.