refactor: check output_dir is a subdirectory of the source dir

Author: vaind

generator/lib/src/code_builder.dart

@@ -112,9 +112,16 @@ class CodeBuilder extends Builder {
     // need to prefix imports with as many '../' to be relative from root.
     final rootPath = _dir(buildStep);
     final outPath = _outDir(buildStep);
-    final rootDir = Directory(rootPath);
-    var outDir = Directory(outPath);
+    final rootDir = Directory(rootPath).absolute;
+    var outDir = Directory(outPath).absolute;
     var prefix = '';
+
+    if (!outDir.path.startsWith(rootDir.path)) {
+      throw InvalidGenerationSourceError(
+          'configured output_dir ${outDir.path} is not a '
+          'subdirectory of the source directory ${rootDir.path}');
+    }
+
     while (outDir.path != rootDir.path) {
       final parent = outDir.parent;
       if (parent.path == outDir.path) {