feat: log warning if svelte field causes difference in resolve

.changeset/cold-horses-begin.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': patch
+---
+
+log deprecation warnings for packages that use the "svelte" field to resolve svelte files differently than standard vite resolve

docs/config.md

@@ -410,3 +410,10 @@ export default {
 - **Default:** `false`
 
   disable svelte compile statistics.
+
+### disableSvelteResolveWarnings
+
+- **Type** `boolean`
+- **Default:** `false`
+
+  disable svelte resolve warnings

docs/faq.md

@@ -208,3 +208,33 @@ module.exports = {
 +    }
 }
 ```
+
+<!-- the following header generates an anchor that is used in logging, do not modify!-->
+
+### deprecated "svelte" field
+
+In the past, Svelte recommended using the custom "svelte" field in package.json to allow libraries to point at .svelte source files.
+This field requires a custom implementation to resolve, so you have to use a bundler plugin and this plugin needs to implement resolving.
+Since then, node has added support for [conditional exports](https://nodejs.org/api/packages.html#conditional-exports), which have more generic support in bundlers and node itself. So to increase the compatibility with the wider ecosystem and reduce the implementation needs for current and future bundler plugins, it is recommended that packages use the "svelte" exports condition.
+
+Example:
+
+```diff
+// package.json
+  "svelte": "src/index.js",
++ "exports": {
++   "./*": {
++     "svelte": "./src/lib/components/*",
++   },
++   ".": {
++     "svelte": "./index.js"
++   }
+  }
+```
+
+> **Support for the svelte field is going to be deprecated and later removed in a future versions of vite-plugin-svelte.**
+>
+> Library authors are highly encouraged to update their packages to add the new exports condition as outlined above. Check out
+> [svelte-package](https://kit.svelte.dev/docs/packaging) which already supports it.
+>
+> For backwards compatibility, you can keep the svelte field in addition to the exports condition. But make sure that both always resolve to the same files.

packages/e2e-tests/_test_dependencies/svelte-exports-simple/index.svelte.js

@@ -0,0 +1 @@
+export { default as Dependency } from './src/components/Dependency.svelte';

packages/e2e-tests/_test_dependencies/svelte-exports-simple/package.json

@@ -6,6 +6,8 @@
     "e2e-test-dep-cjs-only": "file:../cjs-only"
   },
   "type": "module",
+	"files": ["src","index.svelte.js","index.js"],
+	"svelte": "./index.svelte.js",
 	"exports": {
 		"./*": {
 			"svelte": "./src/components/*"

packages/e2e-tests/resolve-exports-svelte/vite.config.js

@@ -3,5 +3,10 @@ import { svelte } from '@sveltejs/vite-plugin-svelte';
 
 // https://vitejs.dev/config/
 export default defineConfig({
-	plugins: [svelte({ compilerOptions: { css: 'none' } })]
+	plugins: [
+		svelte({
+			compilerOptions: { css: 'none' },
+			experimental: { disableSvelteResolveWarnings: false }
+		})
+	]
 });

packages/vite-plugin-svelte/src/index.ts

@@ -24,6 +24,7 @@ import { saveSvelteMetadata } from './utils/optimizer';
 import { svelteInspector } from './ui/inspector/plugin';
 import { VitePluginSvelteCache } from './utils/vite-plugin-svelte-cache';
 import { loadRaw } from './utils/load-raw';
+import { FAQ_LINK_DEPRECATED_SVELTE_FIELD } from './utils/constants';
 
 interface PluginAPI {
 	/**
@@ -50,6 +51,7 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 	/* eslint-enable no-unused-vars */
 
 	let resolvedSvelteSSR: Promise<PartialResolvedId | null>;
+	let packagesWithResolveWarnings: Set<string>;
 	const api: PluginAPI = {};
 	const plugins: Plugin[] = [
 		{
@@ -73,7 +75,7 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 			},
 
 			async configResolved(config) {
-				options = resolveOptions(options, config);
+				options = resolveOptions(options, config, cache);
 				patchResolvedViteConfig(config, options);
 				requestParser = buildIdParser(options);
 				compileSvelte = createCompileSvelte(options);
@@ -84,6 +86,7 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 			},
 
 			async buildStart() {
+				packagesWithResolveWarnings = new Set<string>();
 				if (!options.prebundleSvelteLibraries) return;
 				const isSvelteMetadataChanged = await saveSvelteMetadata(viteConfig.cacheDir, options);
 				if (isSvelteMetadataChanged) {
@@ -164,13 +167,36 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 				// for ssr, during scanning and non-prebundled, we do it
 				if (ssr || scan || !isPrebundled) {
 					try {
+						const isFirstResolve = !cache.hasResolvedSvelteField(importee, importer);
 						const resolved = await resolveViaPackageJsonSvelte(importee, importer, cache);
-						if (resolved) {
-							log.debug(
-								`resolveId resolved ${resolved} via package.json svelte field of ${importee}`
+						if (isFirstResolve && resolved) {
+							const packageInfo = await cache.getPackageInfo(resolved);
+							const packageVersion = `${packageInfo.name}@${packageInfo.version}`;
+							log.debug.once(
+								`resolveId resolved ${importee} to ${resolved} via package.json svelte field of ${packageVersion}`
 							);
-							return resolved;
+
+							try {
+								const viteResolved = (
+									await this.resolve(importee, importer, { ...opts, skipSelf: true })
+								)?.id;
+								if (resolved !== viteResolved) {
+									packagesWithResolveWarnings.add(packageVersion);
+									log.debug.enabled &&
+										log.debug.once(
+											`resolve difference for ${packageVersion} ${importee} - svelte: "${resolved}", vite: "${viteResolved}"`
+										);
+								}
+							} catch (e) {
+								packagesWithResolveWarnings.add(packageVersion);
+								log.debug.enabled &&
+									log.debug.once(
+										`resolve error for ${packageVersion} ${importee} - svelte: "${resolved}", vite: ERROR`,
+										e
+									);
+							}
 						}
+						return resolved;
 					} catch (e) {
 						log.debug.once(
 							`error trying to resolve ${importee} from ${importer} via package.json svelte field `,
@@ -224,6 +250,16 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 			},
 			async buildEnd() {
 				await options.stats?.finishAll();
+				if (
+					!options.experimental?.disableSvelteResolveWarnings &&
+					packagesWithResolveWarnings?.size > 0
+				) {
+					log.warn(
+						`WARNING: The following packages use a svelte resolve configuration in package.json that is going to cause problems in the future.\n\n${[
+							...packagesWithResolveWarnings
+						].join('\n')}\n\nPlease see ${FAQ_LINK_DEPRECATED_SVELTE_FIELD} for details.`
+					);
+				}
 			}
 		}
 	];

packages/vite-plugin-svelte/src/utils/constants.ts

@@ -20,3 +20,6 @@ export const SVELTE_HMR_IMPORTS = [
 ];
 
 export const SVELTE_EXPORT_CONDITIONS = ['svelte'];
+
+export const FAQ_LINK_DEPRECATED_SVELTE_FIELD =
+	'https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#deprecated-svelte-field';

packages/vite-plugin-svelte/src/utils/log.ts

@@ -73,7 +73,7 @@ function createLogger(level: string): LogFn {
 	const logFn: LogFn = _log.bind(null, logger) as LogFn;
 	const logged = new Set<String>();
 	const once = function (message: string, payload?: any) {
-		if (logged.has(message)) {
+		if (!logger.enabled || logged.has(message)) {
 			return;
 		}
 		logged.add(message);

packages/vite-plugin-svelte/src/utils/options.ts

@@ -34,6 +34,7 @@ import {
 
 import { isCommonDepWithoutSvelteField } from './dependencies';
 import { VitePluginSvelteStats } from './vite-plugin-svelte-stats';
+import { VitePluginSvelteCache } from './vite-plugin-svelte-cache';
 
 const allowedPluginOptions = new Set([
 	'include',
@@ -178,7 +179,8 @@ function mergeConfigs<T>(...configs: (Partial<T> | undefined)[]): T {
 // also validates the final config.
 export function resolveOptions(
 	preResolveOptions: PreResolvedOptions,
-	viteConfig: ResolvedConfig
+	viteConfig: ResolvedConfig,
+	cache: VitePluginSvelteCache
 ): ResolvedOptions {
 	const css = preResolveOptions.emitCss ? 'external' : 'injected';
 	const defaultOptions: Partial<Options> = {
@@ -211,7 +213,7 @@ export function resolveOptions(
 	const statsEnabled =
 		disableCompileStats !== true && disableCompileStats !== (merged.isBuild ? 'build' : 'dev');
 	if (statsEnabled && isLogLevelInfo) {
-		merged.stats = new VitePluginSvelteStats();
+		merged.stats = new VitePluginSvelteStats(cache);
 	}
 	return merged;
 }
@@ -732,6 +734,13 @@ export interface ExperimentalOptions {
 	 * @default false
 	 */
 	disableCompileStats?: 'dev' | 'build' | boolean;
+
+	/**
+	 * disable svelte field resolve warnings
+	 *
+	 * @default false
+	 */
+	disableSvelteResolveWarnings?: boolean;
 }
 
 export interface InspectorOptions {

packages/vite-plugin-svelte/src/utils/vite-plugin-svelte-cache.ts

@@ -1,5 +1,17 @@
 import { SvelteRequest } from './id';
 import { Code, CompileData } from './compile';
+import { readFileSync } from 'fs';
+import { dirname } from 'path';
+//eslint-disable-next-line node/no-missing-import
+import { findClosestPkgJsonPath } from 'vitefu';
+import { normalizePath } from 'vite';
+
+interface PackageInfo {
+	name: string;
+	version: string;
+	svelte?: string;
+	path: string;
+}
 
 export class VitePluginSvelteCache {
 	private _css = new Map<string, Code>();
@@ -8,6 +20,7 @@ export class VitePluginSvelteCache {
 	private _dependants = new Map<string, Set<string>>();
 	private _resolvedSvelteFields = new Map<string, string>();
 	private _errors = new Map<string, any>();
+	private _packageInfos: PackageInfo[] = [];
 
 	public update(compileData: CompileData) {
 		this._errors.delete(compileData.normalizedFilename);
@@ -110,6 +123,9 @@ export class VitePluginSvelteCache {
 		return this._resolvedSvelteFields.get(this._getResolvedSvelteFieldKey(name, importer));
 	}
 
+	public hasResolvedSvelteField(name: string, importer?: string) {
+		return this._resolvedSvelteFields.has(this._getResolvedSvelteFieldKey(name, importer));
+	}
 	public setResolvedSvelteField(
 		importee: string,
 		importer: string | undefined = undefined,
@@ -124,4 +140,43 @@ export class VitePluginSvelteCache {
 	private _getResolvedSvelteFieldKey(importee: string, importer?: string): string {
 		return importer ? `${importer} > ${importee}` : importee;
 	}
+
+	public async getPackageInfo(file: string): Promise<PackageInfo> {
+		let info = this._packageInfos.find((pi) => file.startsWith(pi.path));
+		if (!info) {
+			info = await findPackageInfo(file);
+			this._packageInfos.push(info);
+		}
+		return info;
+	}
+}
+
+/**
+ * utility to get some info from the package.json a file belongs to (closest one with a "name" set)
+ *
+ * @param {string} file to find info for
+ * @returns {PackageInfo}
+ */
+async function findPackageInfo(file: string): Promise<PackageInfo> {
+	const info: PackageInfo = {
+		name: '$unknown',
+		version: '0.0.0-unknown',
+		path: '$unknown'
+	};
+	let path = await findClosestPkgJsonPath(file, (pkgPath) => {
+		const pkg = JSON.parse(readFileSync(pkgPath, 'utf-8'));
+		if (pkg.name != null) {
+			info.name = pkg.name;
+			if (pkg.version != null) {
+				info.version = pkg.version;
+			}
+			info.svelte = pkg.svelte;
+			return true;
+		}
+		return false;
+	});
+	// return normalized path with appended '/' so .startsWith works for future file checks
+	path = normalizePath(dirname(path ?? file)) + '/';
+	info.path = path;
+	return info;
 }

packages/vite-plugin-svelte/src/utils/vite-plugin-svelte-stats.ts

@@ -1,10 +1,7 @@
 import { log } from './log';
-//eslint-disable-next-line node/no-missing-import
-import { findClosestPkgJsonPath } from 'vitefu';
-import { readFileSync } from 'fs';
-import { dirname } from 'path';
 import { performance } from 'perf_hooks';
 import { normalizePath } from 'vite';
+import { VitePluginSvelteCache } from './vite-plugin-svelte-cache';
 
 interface Stat {
 	file: string;
@@ -87,31 +84,13 @@ function formatPackageStats(pkgStats: PackageStats[]) {
 	return table;
 }
 
-/**
- * utility to get the package name a file belongs to
- *
- * @param {string} file to find package for
- * @returns {path:string,name:string} tuple of path,name where name is the parsed package name and path is the normalized path to it
- */
-async function getClosestNamedPackage(file: string): Promise<{ name: string; path: string }> {
-	let name = '$unknown';
-	let path = await findClosestPkgJsonPath(file, (pkgPath) => {
-		const pkg = JSON.parse(readFileSync(pkgPath, 'utf-8'));
-		if (pkg.name != null) {
-			name = pkg.name;
-			return true;
-		}
-		return false;
-	});
-	// return normalized path with appended '/' so .startsWith works for future file checks
-	path = normalizePath(dirname(path ?? file)) + '/';
-	return { name, path };
-}
-
 export class VitePluginSvelteStats {
 	// package directory -> package name
-	private _packages: { path: string; name: string }[] = [];
+	private _cache: VitePluginSvelteCache;
 	private _collections: StatCollection[] = [];
+	constructor(cache: VitePluginSvelteCache) {
+		this._cache = cache;
+	}
 	startCollection(name: string, opts?: Partial<CollectionOptions>) {
 		const options = {
 			...defaultCollectionOptions,
@@ -186,12 +165,7 @@ export class VitePluginSvelteStats {
 	private async _aggregateStatsResult(collection: StatCollection) {
 		const stats = collection.stats;
 		for (const stat of stats) {
-			let pkg = this._packages.find((p) => stat.file.startsWith(p.path));
-			if (!pkg) {
-				pkg = await getClosestNamedPackage(stat.file);
-				this._packages.push(pkg);
-			}
-			stat.pkg = pkg.name;
+			stat.pkg = (await this._cache.getPackageInfo(stat.file)).name;
 		}
 
 		// group stats