Produce a warning when the compiler is too old

If VALIDATE_MODULE_DEPENDENCIES is enabled but the version of Clang
in the toolchain does not support the necessary header tracing, then
report a warning.

Author: bob-wilson

Sources/SWBCore/Dependencies.swift

@@ -80,10 +80,17 @@ public struct ModuleDependenciesContext: Sendable, SerializableCodable {
         self.init(validate: validate, moduleDependencies: settings.moduleDependencies, fixItContext: fixItContext)
     }
 
-    // For Clang imports, we do not get the import locations and do not know whether it was a public
-    // import (which depends on whether the import is in an installed header file).
-    public func makeDiagnostics(files: [Path]) -> [Diagnostic] {
+    /// For Clang imports, we do not get the import locations and do not know whether it was a public
+    /// import (which depends on whether the import is in an installed header file).
+    /// Nil `files` means the current toolchain doesn't have the features to trace imports. This is temporarily required to support running against older toolchains.
+    public func makeDiagnostics(files: [Path]?) -> [Diagnostic] {
         guard validate != .no else { return [] }
+        guard let files else {
+            return [Diagnostic(
+                behavior: .warning,
+                location: .unknown,
+                data: DiagnosticData("The current toolchain does not support \(BuiltinMacros.VALIDATE_MODULE_DEPENDENCIES.name)"))]
+        }
 
         // The following is a provisional/incomplete mechanism for resolving a module dependency from a file path.
         // For now, just grab the framework name and assume there is a module with the same name.

Sources/SWBTaskExecution/TaskActions/ClangCompileTaskAction.swift

@@ -322,14 +322,20 @@ public final class ClangCompileTaskAction: TaskAction, BuildValueValidatingTaskA
                 }
             }
 
-            if let moduleDependenciesContext, let traceFilePath, lastResult == .succeeded {
+            if let moduleDependenciesContext, lastResult == .succeeded {
                 // Verify the dependencies from the trace data.
-                let fs = executionDelegate.fs
-                let traceData = try JSONDecoder().decode(Array<TraceData>.self, from: Data(fs.read(traceFilePath)))
-
-                var allFiles = Set<Path>()
-                traceData.forEach { allFiles.formUnion(Set($0.includes)) }
-                let diagnostics = moduleDependenciesContext.makeDiagnostics(files: Array(allFiles))
+                let files: [Path]?
+                if let traceFilePath {
+                    let fs = executionDelegate.fs
+                    let traceData = try JSONDecoder().decode(Array<TraceData>.self, from: Data(fs.read(traceFilePath)))
+
+                    var allFiles = Set<Path>()
+                    traceData.forEach { allFiles.formUnion(Set($0.includes)) }
+                    files = Array(allFiles)
+                } else {
+                    files = nil
+                }
+                let diagnostics = moduleDependenciesContext.makeDiagnostics(files: files)
                 for diagnostic in diagnostics {
                     outputDelegate.emit(diagnostic)
                 }