Take run destinations into account for SwiftPM build targets

Documentation/Background Indexing.md

@@ -22,9 +22,10 @@ Next, point your editor to use the just-built copy of SourceKit-LSP and enable b
 "swift.sourcekit-lsp.serverArguments": [ "--experimental-feature", "background-indexing" ],
 ```
 
+Background indexing requires a Swift 6 toolchain. You can download Swift 6 nightly toolchains from https://www.swift.org/download/#swift-60-development.
+
 ## Known issues
 
-- The only supported toolchain for background indexing are currently [Swift 6.0 nightly toolchain snapshots](https://www.swift.org/download/#swift-60-development). Older toolchains are not supported and the nightly toolchains from `main` are having issues because building a target non-deterministically builds for tools or the destination [#1288](https://github.com/apple/sourcekit-lsp/pull/1288#issuecomment-2111400459) [rdar://128100158](rdar://128100158)
 - Not really a background indexing related issue but Swift nightly toolchain snapshots are crashing on macOS 14.4 and 14.5 (swift#73327)[https://github.com/apple/swift/issues/73327]
   - Workaround: Run the toolchains on an older version of macOS, if possible
 - Background Indexing is only supported for SwiftPM projects [#1269](https://github.com/apple/sourcekit-lsp/issues/1269), [#1271](https://github.com/apple/sourcekit-lsp/issues/1271)

Sources/SKCore/BuildServerBuildSystem.swift

@@ -278,6 +278,10 @@ extension BuildServerBuildSystem: BuildSystem {
     return nil
   }
 
+  public func toolchain(for uri: DocumentURI, _ language: Language) async -> SKCore.Toolchain? {
+    return nil
+  }
+
   public func configuredTargets(for document: DocumentURI) async -> [ConfiguredTarget] {
     return [ConfiguredTarget(targetID: "dummy", runDestinationID: "dummy")]
   }

Sources/SKCore/BuildSystem.swift

@@ -179,6 +179,11 @@ public protocol BuildSystem: AnyObject, Sendable {
   /// If `nil` is returned, the language based on the file's extension.
   func defaultLanguage(for document: DocumentURI) async -> Language?
 
+  /// The toolchain that should be used to open the given document.
+  ///
+  /// If `nil` is returned, then the default toolchain for the given language is used.
+  func toolchain(for uri: DocumentURI, _ language: Language) async -> Toolchain?
+
   /// Register the given file for build-system level change notifications, such
   /// as command line flag changes, dependency changes, etc.
   ///

Sources/SKCore/BuildSystemManager.swift

@@ -104,9 +104,18 @@ extension BuildSystemManager {
 
   /// Returns the toolchain that should be used to process the given document.
   public func toolchain(for uri: DocumentURI, _ language: Language) async -> Toolchain? {
-    // To support multiple toolchains within a single workspace, we need to ask the build system which toolchain to use
-    // for this document.
-    return await toolchainRegistry.defaultToolchain(for: language)
+    if let toolchain = await buildSystem?.toolchain(for: uri, language) {
+      return toolchain
+    }
+
+    switch language {
+    case .swift:
+      return await toolchainRegistry.preferredToolchain(containing: [\.sourcekitd, \.swift, \.swiftc])
+    case .c, .cpp, .objective_c, .objective_cpp:
+      return await toolchainRegistry.preferredToolchain(containing: [\.clang, \.clangd])
+    default:
+      return nil
+    }
   }
 
   /// - Note: Needed so we can set the delegate from a different isolation context.

Sources/SKCore/CompilationDatabaseBuildSystem.swift

@@ -119,6 +119,10 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
     return nil
   }
 
+  public func toolchain(for uri: DocumentURI, _ language: Language) async -> SKCore.Toolchain? {
+    return nil
+  }
+
   public func configuredTargets(for document: DocumentURI) async -> [ConfiguredTarget] {
     return [ConfiguredTarget(targetID: "dummy", runDestinationID: "dummy")]
   }

Sources/SKCore/ToolchainRegistry.swift

@@ -246,26 +246,14 @@ public final actor ToolchainRegistry {
     return darwinToolchainOverride ?? ToolchainRegistry.darwinDefaultToolchainIdentifier
   }
 
-  /// The toolchain to use for a document in the given language if the build system doesn't override it.
-  func defaultToolchain(for language: Language) -> Toolchain? {
-    let supportsLang = { (toolchain: Toolchain) -> Bool in
-      // FIXME: the fact that we're looking at clangd/sourcekitd instead of the compiler indicates this method needs a parameter stating what kind of tool we're looking for.
-      switch language {
-      case .swift:
-        return toolchain.sourcekitd != nil
-      case .c, .cpp, .objective_c, .objective_cpp:
-        return toolchain.clangd != nil
-      default:
-        return false
-      }
-    }
-
-    if let toolchain = self.default, supportsLang(toolchain) {
+  /// Returns the preferred toolchain that contains all the tools at the given key paths.
+  public func preferredToolchain(containing requiredTools: [KeyPath<Toolchain, AbsolutePath?>]) -> Toolchain? {
+    if let toolchain = self.default, requiredTools.allSatisfy({ toolchain[keyPath: $0] != nil }) {
       return toolchain
     }
 
     for toolchain in toolchains {
-      if supportsLang(toolchain) {
+      if requiredTools.allSatisfy({ toolchain[keyPath: $0] != nil }) {
         return toolchain
       }
     }