feat(next): make defineConfig generic

.changeset/forty-trains-notice.md

@@ -0,0 +1,9 @@
+---
+'astro': minor
+---
+
+Makes `defineConfig` generic
+
+TypeScript will now provide feedback at the type level when using `defineConfig`. For example, it will make sure `i18n.locales` contains `i18n.defaultLocale`.
+
+This also makes the `AstroUserConfig` generic, but both changes are backward compatible.

packages/astro/config.d.ts

@@ -2,6 +2,7 @@
 type ViteUserConfig = import('vite').UserConfig;
 type ViteUserConfigFn = import('vite').UserConfigFn;
 type AstroUserConfig = import('./dist/types/public/config.js').AstroUserConfig;
+type Locales = import('./dist/types/public/config.js').Locales;
 type AstroInlineConfig = import('./dist/types/public/config.js').AstroInlineConfig;
 type ImageServiceConfig = import('./dist/types/public/config.js').ImageServiceConfig;
 type SharpImageServiceConfig = import('./dist/assets/services/sharp.js').SharpImageServiceConfig;
@@ -11,7 +12,13 @@ type EnvField = typeof import('./dist/env/config.js').envField;
  * See the full Astro Configuration API Documentation
  * https://astro.build/config
  */
-export function defineConfig(config: AstroUserConfig): AstroUserConfig;
+export function defineConfig<
+		TDefaultLocale extends string,
+		const TLocales extends [
+			TDefaultLocale | { codes: [TDefaultLocale, ...Array<string>]; path: string },
+			...Locales,
+		],
+	>(config: AstroUserConfig<TDefaultLocale, TLocales>): AstroUserConfig<TDefaultLocale, TLocales>;
 
 /**
  * Use Astro to generate a fully resolved Vite config

packages/astro/src/config/index.ts

@@ -1,10 +1,16 @@
 import type { UserConfig as ViteUserConfig } 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, AstroUserConfig, Locales } from '../types/public/config.js';
 import { createDevelopmentManifest } from '../vite-plugin-astro-server/plugin.js';
 
-export function defineConfig(config: AstroUserConfig) {
+export function defineConfig<
+	TDefaultLocale extends string,
+	const TLocales extends [
+		TDefaultLocale | { codes: [TDefaultLocale, ...Array<string>]; path: string },
+		...Locales,
+	],
+>(config: AstroUserConfig<TDefaultLocale, TLocales>) {
 	return config;
 }
 

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 +110,13 @@ export interface ViteUserConfig extends OriginalViteUserConfig {
  * Astro User Config
  * Docs: https://docs.astro.build/reference/configuration-reference/
  */
-export interface AstroUserConfig {
+export interface AstroUserConfig<
+	TDefaultLocale extends string = never,
+	TLocales extends [
+		TDefaultLocale | { codes: [TDefaultLocale, ...Array<string>]; path: string },
+		...Locales,
+	] = never,
+> {
 	/**
 	 * @docs
 	 * @kind heading
@@ -1214,7 +1228,7 @@ export interface AstroUserConfig {
 		 *
 		 * 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.
 		 */
-		defaultLocale: string;
+		defaultLocale: [TDefaultLocale] extends [never] ? string : TDefaultLocale;
 		/**
 		 * @docs
 		 * @name i18n.locales
@@ -1228,7 +1242,7 @@ export interface AstroUserConfig {
 		 *
 		 * 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.
 		 */
-		locales: Locales;
+		locales: [TLocales] extends [never] ? Locales : TLocales;
 
 		/**
 		 * @docs
@@ -1258,7 +1272,11 @@ export interface AstroUserConfig {
 		 * })
 		 * ```
 		 */
-		fallback?: Record<string, string>;
+		fallback?: [TLocales] extends [never]
+			? Record<string, string>
+			: {
+					[Locale in NormalizeLocales<TLocales>]?: Exclude<NormalizeLocales<TLocales>, Locale>;
+				};
 
 		/**
 		 * @docs
@@ -1444,7 +1462,9 @@ 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<TLocales>, string>>;
 	};
 
 	/** ! WARNING: SUBJECT TO CHANGE */