Allow loading the SourceKit plugin from the toolchain during testing

Author: ahoppen

Documentation/Environment Variables.md

@@ -18,4 +18,4 @@ The following environment variables can be used to control some behavior in Sour
 - `SOURCEKIT_LSP_KEEP_TEST_SCRATCH_DIR`: Does not delete the temporary files created during test execution. Allows inspection of the test projects after the test finishes.
 - `SOURCEKIT_LSP_TEST_MODULE_CACHE`: Specifies where tests should store their shared module cache. Defaults to writing the module cache to a temporary directory. Intended so that CI systems can clean the module cache directory after running.
 - `SOURCEKIT_LSP_TEST_TIMEOUT`: Override the timeout duration for tests, in seconds.
-- `SOURCEKIT_LSP_TEST_PLUGIN_PATHS`: Load the SourceKit plugins from this path instead of relative to the package's build folder.
+- `SOURCEKIT_LSP_TEST_PLUGIN_PATHS`: Load the SourceKit plugins from this path instead of relative to the package's build folder. If set to `RELATIVE_TO_SOURCEKITD`, the SourceKit plugin is found from the toolchain, relative to sourcekitd.

Sources/SKTestSupport/PluginPaths.swift

@@ -109,7 +109,7 @@ private func pluginPaths(relativeTo base: URL) -> PluginPaths? {
 
 /// Returns the paths from which the SourceKit plugins should be loaded or throws an error if the plugins cannot be
 /// found.
-package var sourceKitPluginPaths: PluginPaths {
+package var sourceKitPluginPaths: PluginPaths? {
   get throws {
     struct PluginLoadingError: Error, CustomStringConvertible {
       let searchBase: URL
@@ -126,12 +126,16 @@ package var sourceKitPluginPaths: PluginPaths {
       }
     }
 
-    let base =
-      if let pluginPaths = ProcessInfo.processInfo.environment["SOURCEKIT_LSP_TEST_PLUGIN_PATHS"] {
-        URL(fileURLWithPath: pluginPaths)
+    var base: URL
+    if let pluginPaths = ProcessInfo.processInfo.environment["SOURCEKIT_LSP_TEST_PLUGIN_PATHS"] {
+      if pluginPaths == "RELATIVE_TO_SOURCEKITD" {
+        return nil
       } else {
-        xctestBundle
+        base = URL(fileURLWithPath: pluginPaths)
       }
+    } else {
+      base = xctestBundle
+    }
     var searchPath = base
     while searchPath.pathComponents.count > 1 {
       if let paths = pluginPaths(relativeTo: searchPath) {

Sources/SKTestSupport/TestSourceKitLSPClient.swift

@@ -39,8 +39,8 @@ extension SourceKitLSPOptions {
   ) -> SourceKitLSPOptions {
     return SourceKitLSPOptions(
       sourcekitd: SourceKitDOptions(
-        clientPlugin: try! sourceKitPluginPaths.clientPlugin.filePath,
-        servicePlugin: try! sourceKitPluginPaths.servicePlugin.filePath
+        clientPlugin: try! sourceKitPluginPaths?.clientPlugin.filePath,
+        servicePlugin: try! sourceKitPluginPaths?.servicePlugin.filePath
       ),
       backgroundIndexing: backgroundIndexing,
       experimentalFeatures: experimentalFeatures,