feat(next): make defineConfig generic

.changeset/forty-trains-notice.md

@@ -0,0 +1,5 @@
+---
+'astro': minor
+---
+
+Improves `defineConfig` type safety. TypeScript will now error if a group of related options do not have coherent types. For example, it will now error if `i18n.defaultLocale` is not one of the locales specified in `i18n.locales`

packages/astro/package.json

@@ -46,10 +46,7 @@
     },
     "./compiler-runtime": "./dist/runtime/compiler/index.js",
     "./runtime/*": "./dist/runtime/*",
-    "./config": {
-      "types": "./config.d.ts",
-      "default": "./config.mjs"
-    },
+    "./config": "./dist/config/entrypoint.js",
     "./container": {
       "types": "./dist/container/index.d.ts",
       "default": "./dist/container/index.js"
@@ -93,8 +90,6 @@
     "types",
     "astro.js",
     "index.d.ts",
-    "config.d.ts",
-    "config.mjs",
     "zod.d.ts",
     "zod.mjs",
     "env.d.ts",

packages/astro/src/config/entrypoint.ts

@@ -0,0 +1,30 @@
+// IMPORTANT: this file is the entrypoint for "astro/config". Keep it as light as possible!
+
+import type { SharpImageServiceConfig } from '../assets/services/sharp.js';
+import type { ImageServiceConfig } from '../types/public/index.js';
+
+export { defineConfig, getViteConfig } from './index.js';
+export { envField } from '../env/config.js';
+
+/**
+ * Return the configuration needed to use the Sharp-based image service
+ */
+export function sharpImageService(config: SharpImageServiceConfig = {}): ImageServiceConfig {
+	return {
+		entrypoint: 'astro/assets/services/sharp',
+		config,
+	};
+}
+
+/**
+ * Return the configuration needed to use the passthrough image service. This image services does not perform
+ * any image transformations, and is mainly useful when your platform does not support other image services, or you are
+ * not using Astro's built-in image processing.
+ * See: https://docs.astro.build/en/guides/images/#configure-no-op-passthrough-service
+ */
+export function passthroughImageService(): ImageServiceConfig {
+	return {
+		entrypoint: 'astro/assets/services/noop',
+		config: {},
+	};
+}

packages/astro/src/config/index.ts

@@ -1,13 +1,22 @@
 import type { UserConfig as ViteUserConfig, UserConfigFn as ViteUserConfigFn } from 'vite';
 import { Logger } from '../core/logger/core.js';
 import { createRouteManifest } from '../core/routing/index.js';
-import type { AstroInlineConfig, AstroUserConfig } from '../types/public/config.js';
+import type { AstroInlineConfig, AstroUserDefineConfig, Locales } from '../types/public/config.js';
 import { createDevelopmentManifest } from '../vite-plugin-astro-server/plugin.js';
 
-export function defineConfig(config: AstroUserConfig) {
+/**
+ * See the full Astro Configuration API Documentation
+ * https://astro.build/config
+ */
+export function defineConfig<const TLocales extends Locales = never>(
+	config: AstroUserDefineConfig<TLocales>,
+) {
 	return config;
 }
 
+/**
+ * Use Astro to generate a fully resolved Vite config
+ */
 export function getViteConfig(
 	userViteConfig: ViteUserConfig,
 	inlineAstroConfig: AstroInlineConfig = {},

packages/astro/src/env/config.ts

@@ -10,7 +10,7 @@ import type {
 } from './schema.js';
 
 /**
- * Return a valid env field to use in this Astro config for `experimental.env.schema`.
+ * Return a valid env field to use in this Astro config for `env.schema`.
  */
 export const envField = {
 	string: (options: StringFieldInput): StringField => ({

packages/astro/src/types/public/config.ts

@@ -16,6 +16,14 @@ import type { AstroIntegration } from './integrations.js';
 
 export type Locales = (string | { codes: string[]; path: string })[];
 
+type NormalizeLocales<T extends Locales> = {
+	[K in keyof T]: T[K] extends string
+		? T[K]
+		: T[K] extends { codes: Array<string> }
+			? T[K]['codes'][number]
+			: never;
+}[number];
+
 export interface ImageServiceConfig<T extends Record<string, any> = Record<string, any>> {
 	entrypoint: 'astro/assets/services/sharp' | (string & {});
 	config?: T;
@@ -102,7 +110,10 @@ export interface ViteUserConfig extends OriginalViteUserConfig {
  * Astro User Config
  * Docs: https://docs.astro.build/reference/configuration-reference/
  */
-export interface AstroUserConfig {
+export type AstroUserConfig = AstroUserDefineConfig<never>;
+
+/** @internal */
+export interface AstroUserDefineConfig<TLocales extends Locales = never> {
 	/**
 	 * @docs
 	 * @kind heading
@@ -1205,30 +1216,31 @@ export interface AstroUserConfig {
 	i18n?: {
 		/**
 		 * @docs
-		 * @name i18n.defaultLocale
-		 * @type {string}
+		 * @name i18n.locales
+		 * @type {Locales}
 		 * @version 3.5.0
 		 * @description
 		 *
-		 * The default locale of your website/application. This is a required field.
+		 * A list of all locales supported by the website. This is a required field.
 		 *
-		 * No particular language format or syntax is enforced, but we suggest using lower-case and hyphens as needed (e.g. "es", "pt-br") for greatest compatibility.
+		 * Languages can be listed either as individual codes (e.g. `['en', 'es', 'pt-br']`) or mapped to a shared `path` of codes (e.g.  `{ path: "english", codes: ["en", "en-US"]}`). These codes will be used to determine the URL structure of your deployed site.
+		 *
+		 * No particular language code format or syntax is enforced, but your project folders containing your content files must match exactly the `locales` items in the list. In the case of multiple `codes` pointing to a custom URL path prefix, store your content files in a folder with the same name as the `path` configured.
 		 */
-		defaultLocale: string;
+		locales: [TLocales] extends [never] ? Locales : TLocales;
+
 		/**
 		 * @docs
-		 * @name i18n.locales
-		 * @type {Locales}
+		 * @name i18n.defaultLocale
+		 * @type {string}
 		 * @version 3.5.0
 		 * @description
 		 *
-		 * A list of all locales supported by the website, including the `defaultLocale`. This is a required field.
+		 * The default locale of your website/application, that is one of the specified `locales`. This is a required field.
 		 *
-		 * Languages can be listed either as individual codes (e.g. `['en', 'es', 'pt-br']`) or mapped to a shared `path` of codes (e.g.  `{ path: "english", codes: ["en", "en-US"]}`). These codes will be used to determine the URL structure of your deployed site.
-		 *
-		 * No particular language code format or syntax is enforced, but your project folders containing your content files must match exactly the `locales` items in the list. In the case of multiple `codes` pointing to a custom URL path prefix, store your content files in a folder with the same name as the `path` configured.
+		 * No particular language format or syntax is enforced, but we suggest using lower-case and hyphens as needed (e.g. "es", "pt-br") for greatest compatibility.
 		 */
-		locales: Locales;
+		defaultLocale: [TLocales] extends [never] ? string : NormalizeLocales<NoInfer<TLocales>>;
 
 		/**
 		 * @docs
@@ -1258,7 +1270,14 @@ export interface AstroUserConfig {
 		 * })
 		 * ```
 		 */
-		fallback?: Record<string, string>;
+		fallback?: [TLocales] extends [never]
+			? Record<string, string>
+			: {
+					[Locale in NormalizeLocales<NoInfer<TLocales>>]?: Exclude<
+						NormalizeLocales<NoInfer<TLocales>>,
+						Locale
+					>;
+				};
 
 		/**
 		 * @docs
@@ -1444,7 +1463,9 @@ export interface AstroUserConfig {
 		 *
 		 * See the [Internationalization Guide](https://docs.astro.build/en/guides/internationalization/#domains) for more details, including the limitations of this feature.
 		 */
-		domains?: Record<string, string>;
+		domains?: [TLocales] extends [never]
+			? Record<string, string>
+			: Partial<Record<NormalizeLocales<NoInfer<TLocales>>, string>>;
 	};
 
 	/** ! WARNING: SUBJECT TO CHANGE */

packages/astro/test/types/define-config.ts

@@ -0,0 +1,41 @@
+import { describe, it } from 'node:test';
+import { defineConfig } from '../../dist/config/index.js';
+import type { AstroUserDefineConfig } from '../../dist/types/public/config.js';
+import { expectTypeOf } from 'expect-type';
+
+describe('defineConfig()', () => {
+	it('Infers generics correctly', () => {
+		const config_0 = defineConfig({});
+		expectTypeOf(config_0).toEqualTypeOf<AstroUserDefineConfig<never>>();
+		expectTypeOf(config_0.i18n!.defaultLocale).toEqualTypeOf<string>();
+
+		const config_1 = defineConfig({
+			i18n: {
+				locales: ['en'],
+				defaultLocale: 'en',
+			},
+		});
+		expectTypeOf(config_1).toEqualTypeOf<AstroUserDefineConfig<['en']>>();
+		expectTypeOf(config_1.i18n!.defaultLocale).toEqualTypeOf<'en'>();
+
+		const config_2 = defineConfig({
+			i18n: {
+				locales: ['en', 'fr'],
+				defaultLocale: 'fr',
+			},
+		});
+		expectTypeOf(config_2).toEqualTypeOf<AstroUserDefineConfig<['en', 'fr']>>();
+		expectTypeOf(config_2.i18n!.defaultLocale).toEqualTypeOf<'en' | 'fr'>();
+
+		const config_3 = defineConfig({
+			i18n: {
+				locales: ['en', { path: 'french', codes: ['fr', 'fr-FR'] }],
+				defaultLocale: 'en',
+			},
+		});
+		expectTypeOf(config_3).toEqualTypeOf<
+			AstroUserDefineConfig<['en', { readonly path: 'french'; readonly codes: ['fr', 'fr-FR'] }]>
+		>();
+		expectTypeOf(config_3.i18n!.defaultLocale).toEqualTypeOf<'en' | 'fr' | 'fr-FR'>();
+	});
+});