Merge pull request #910 from artemcm/EmitSupportedFeaturesLib

Query compiler supported features from `libSwiftScan`, when one is present

Author: artemcm

Sources/SwiftDriver/Jobs/EmitSupportedFeaturesJob.swift

@@ -59,18 +59,18 @@ extension Driver {
                                                fileSystem: FileSystem,
                                                executor: DriverExecutor, env: [String: String])
   throws -> Set<String> {
-    // TODO: Once we are sure libSwiftScan is deployed across supported platforms and architectures
-    // we should deploy it here.
-//    let swiftScanLibPath = try Self.getScanLibPath(of: toolchain,
-//                                                   hostTriple: hostTriple,
-//                                                   env: env)
-//
-//    if fileSystem.exists(swiftScanLibPath) {
-//      let libSwiftScanInstance = try SwiftScan(dylib: swiftScanLibPath)
-//      if libSwiftScanInstance.canQuerySupportedArguments() {
-//        return try libSwiftScanInstance.querySupportedArguments()
-//      }
-//    }
+    // if libSwiftScan is present in the toolchain, query the compiler supported features
+    // using the SwiftScan interface, instead of calling out to swift-frontend.
+    let swiftScanLibPath = try Self.getScanLibPath(of: toolchain,
+                                                   hostTriple: hostTriple,
+                                                   env: env)
+
+    if fileSystem.exists(swiftScanLibPath) {
+      let libSwiftScanInstance = try SwiftScan(dylib: swiftScanLibPath)
+      if libSwiftScanInstance.canQuerySupportedArguments() {
+        return try libSwiftScanInstance.querySupportedArguments()
+      }
+    }
 
     // Invoke `swift-frontend -emit-supported-features`
     let frontendOverride = try FrontendOverride(&parsedOptions, diagnosticsEngine)