Fixes a bug where the errors variable could be mutated on multiple threads by switching to Structured Concurrency from GCD, allowing the compiler to prevent future bugs of this nature from being written

Author: benpious

CodeGeneration/Sources/generate-swift-syntax/GenerateSwiftSyntax.swift

@@ -153,11 +153,9 @@ struct GenerateSwiftSyntax: AsyncParsableCommand {
       }
     )
 
-    let specs = fileSpecs
-    await withTaskGroup(of: (URL, Error?).self) { group in
-      for index in 0..<fileSpecs.count {
+    await withTaskGroup(of: (url: URL, error: Error?).self) { group in
+      for fileSpec in fileSpecs {
         group.addTask {
-          let fileSpec = specs[index]
           do {
             var destination = destination
             for component in fileSpec.pathComponents {
@@ -181,9 +179,9 @@ struct GenerateSwiftSyntax: AsyncParsableCommand {
         }
       }
       for await result in group {
-        _ = previouslyGeneratedFiles.remove(result.0)
-        if let error = result.1 {
-          print(error)
+        _ = previouslyGeneratedFiles.remove(result.url)
+        if let error = result.error {
+          print("Error when generating file at \(result.url):", error)
         }
       }
     }
@@ -197,7 +195,7 @@ struct GenerateSwiftSyntax: AsyncParsableCommand {
 
 }
 
-func generateFile(
+fileprivate func generateFile(
   contents: @autoclosure () -> String,
   destination: URL,
   verbose: Bool