Detect dependency loops in module migrator

lib/src/migrators/module.dart

@@ -200,6 +200,19 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
   /// The values of the --forward flag.
   final Set<ForwardType> forwards;
 
+  //  Maps direct and indirect dependencies to prevent any potential loops.
+  final Map<Uri, Uri> _dependencies = {};
+
+  void _addDependency(Uri source, Uri importedPath) {
+      if (_dependencies.containsKey(importedPath) && _dependencies[importedPath] == source) {
+        // Throw an error indicating a potential loop.
+        var sourceUrl = _absoluteUrlToDependency(source);
+        var importedPathUrl = _absoluteUrlToDependency(importedPath);
+       throw MigrationException('Dependency loop detected: ${sourceUrl.item1} -> ${importedPathUrl.item1}');
+      }
+      _dependencies[source] = importedPath;
+  }
+
   /// Constructs a new module migration visitor.
   ///
   /// [importCache] must be the same one used by [references].
@@ -1239,6 +1252,13 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
     var url = declaration.sourceUrl;
     if (url == currentUrl) return null;
 
+    // Trace dependencies for loop detection.
+    try {
+      _addDependency(currentUrl, url);
+    } on Exception catch (e) {
+      throw MigrationSourceSpanException(e.toString(), declaration.member.span);
+    }
+
     // If we can load [declaration] from a library entrypoint URL, do so. Choose
     // the shortest one if there are multiple options.
     var libraryUrls = references.libraries[declaration];

test/migrators/module/namespace_references/loop_error.hrx

@@ -0,0 +1,21 @@
+<==> arguments
+--migrate-deps
+
+<==> input/entrypoint.scss
+@import "ejemplo";
+$var: $value;
+
+<==> input/_ejemplo.scss
+$value: blue;
+a {
+  color: $var;
+}
+
+<==> error.txt
+Error: Error: Dependency loop detected: entrypoint -> ejemplo
+  ,
+1 | $value: blue;
+  | ^^^^^^^^^^^^
+  '
+  _ejemplo.scss 1:1  root stylesheet
+Migration failed!