refactor(config): flatten nested shared config layers

packages/rstack/src/configLayers.ts

@@ -6,7 +6,11 @@ import type { ConfigParams as LibConfigParams, RslibConfig } from '@rslib/core';
 import type { RslintConfig } from '@rslint/core';
 import type { UserConfig as RspressConfig } from '@rspress/core';
 import type { RstestConfig } from '@rstest/core';
-import type { Configs } from './config.ts';
+import {
+  type Configs,
+  type RstackConfig,
+  normalizeRstackConfig,
+} from './config.ts';
 import type { FmtConfig } from './fmt/types.ts';
 import type { StagedConfig } from './staged.ts';
 
@@ -26,6 +30,35 @@ type ConfigArgs<K extends keyof Configs> = K extends 'app'
     ? [params: LibConfigParams]
     : [];
 
+/** Expand inherited configs before their children, preserving every occurrence. */
+export const flattenConfigLayers = (
+  configs: readonly RstackConfig[],
+): Configs[] => {
+  const layers: Configs[] = [];
+  const ancestors = new Map<RstackConfig, string>();
+
+  const visit = (config: RstackConfig, path: string): void => {
+    if (config.extends?.length) {
+      const ancestorPath = ancestors.get(config);
+      if (ancestorPath !== undefined) {
+        throw new Error(
+          `Circular config inheritance at ${path}: references ${ancestorPath}.`,
+        );
+      }
+
+      ancestors.set(config, path);
+      config.extends.forEach((inherited, index) => {
+        visit(inherited, `${path}.extends[${index}]`);
+      });
+      ancestors.delete(config);
+    }
+    layers.push(normalizeRstackConfig(config));
+  };
+
+  configs.forEach((config, index) => visit(config, `extends[${index}]`));
+  return layers;
+};
+
 /**
  * Resolve one tool from ordered, normalized config layers. Lint factories are
  * already wrapped by define.lint. This function does not merge the results.

packages/rstack/tests/config/flatten.test.ts

@@ -0,0 +1,49 @@
+import { expect, rs, test } from 'rstack/test';
+import type { RstackConfig } from '../../src/config.ts';
+import { flattenConfigLayers } from '../../src/configLayers.ts';
+
+test('flattens nested configs in order and preserves repeated references without resolving factories', () => {
+  const app = rs.fn(() => ({}));
+  const base = { app, fmt: { printWidth: 80 } };
+  const left = { extends: [base], fmt: { singleQuote: true } };
+  const nested = { extends: [left], fmt: { semi: false } };
+  const right = { extends: [base], fmt: { tabWidth: 4 } };
+  const configs: readonly RstackConfig[] = [nested, right, base];
+
+  expect(flattenConfigLayers(configs)).toEqual([
+    base,
+    { fmt: { singleQuote: true } },
+    { fmt: { semi: false } },
+    base,
+    { fmt: { tabWidth: 4 } },
+    base,
+  ]);
+  expect(app).not.toHaveBeenCalled();
+  expect(configs).toEqual([
+    {
+      extends: [{ extends: [base], fmt: { singleQuote: true } }],
+      fmt: { semi: false },
+    },
+    { extends: [base], fmt: { tabWidth: 4 } },
+    base,
+  ]);
+});
+
+test('reports the inheritance positions of a direct cycle', () => {
+  const config: RstackConfig = {};
+  config.extends = [config];
+
+  expect(() => flattenConfigLayers([config])).toThrow(
+    'Circular config inheritance at extends[0].extends[0]: references extends[0].',
+  );
+});
+
+test('reports the inheritance positions of an indirect cycle', () => {
+  const first: RstackConfig = {};
+  const second: RstackConfig = { extends: [first] };
+  first.extends = [second];
+
+  expect(() => flattenConfigLayers([first])).toThrow(
+    'Circular config inheritance at extends[0].extends[0].extends[0]: references extends[0].',
+  );
+});