feat(preprocess): check preprocess dependencies for anomalies

.changeset/funny-waves-poke.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': minor
+---
+
+feat(preprocess): add warnings in case preprocess dependencies contain anomalies

packages/vite-plugin-svelte/src/utils/compile.js

@@ -4,7 +4,10 @@ import { createMakeHot } from 'svelte-hmr';
 import { safeBase64Hash } from './hash.js';
 import { log } from './log.js';
 
-import { createInjectScopeEverythingRulePreprocessorGroup } from './preprocess.js';
+import {
+	checkPreprocessDependencies,
+	createInjectScopeEverythingRulePreprocessorGroup
+} from './preprocess.js';
 import { mapToRelative } from './sourcemaps.js';
 import { enhanceCompileError } from './error.js';
 
@@ -22,7 +25,10 @@ export const _createCompileSvelte = (makeHot) => {
 	return async function compileSvelte(svelteRequest, code, options) {
 		const { filename, normalizedFilename, cssId, ssr, raw } = svelteRequest;
 		const { emitCss = true } = options;
+		/** @type {string[]} */
 		const dependencies = [];
+		/** @type {import('svelte/types/compiler/interfaces').Warning[]} */
+		const warnings = [];
 
 		if (options.stats) {
 			if (options.isBuild) {
@@ -87,7 +93,16 @@ export const _createCompileSvelte = (makeHot) => {
 				throw e;
 			}
 
-			if (preprocessed.dependencies) dependencies.push(...preprocessed.dependencies);
+			if (preprocessed.dependencies?.length) {
+				const checked = checkPreprocessDependencies(filename, preprocessed.dependencies);
+				if (checked.warnings.length) {
+					warnings.push(...checked.warnings);
+				}
+				if (checked.dependencies.length) {
+					dependencies.push(...checked.dependencies);
+				}
+			}
+
 			if (preprocessed.map) compileOptions.sourcemap = preprocessed.map;
 		}
 		if (typeof preprocessed?.map === 'object') {
@@ -134,6 +149,12 @@ export const _createCompileSvelte = (makeHot) => {
 		}
 		mapToRelative(compiled.js?.map, filename);
 		mapToRelative(compiled.css?.map, filename);
+		if (warnings.length) {
+			if (!compiled.warnings) {
+				compiled.warnings = [];
+			}
+			compiled.warnings.push(...warnings);
+		}
 		if (!raw) {
 			// wire css import and code for hmr
 			const hasCss = compiled.css?.code?.trim().length > 0;

packages/vite-plugin-svelte/src/utils/preprocess.js

@@ -1,6 +1,7 @@
 import MagicString from 'magic-string';
 import { log } from './log.js';
 import path from 'node:path';
+import { normalizePath } from 'vite';
 
 /**
  * this appends a *{} rule to component styles to force the svelte compiler to add style classes to all nodes
@@ -121,3 +122,52 @@ export function addExtraPreprocessors(options, config) {
 		}
 	}
 }
+
+/**
+ *
+ * @param filename {string}
+ * @param dependencies  {string[]}
+ * @returns {({dependencies: string[], warnings:import('svelte/types/compiler/interfaces').Warning[] })}
+ */
+export function checkPreprocessDependencies(filename, dependencies) {
+	/** @type {import('svelte/types/compiler/interfaces').Warning[]} */
+	const warnings = [];
+
+	// to find self, we have to compare normalized filenames, but must keep the original values in `dependencies`
+	// because otherwise file watching on windows doesn't work
+	// so we track idx and filter by that in the end
+	/** @type {number[]} */
+	const selfIdx = [];
+	const normalizedFullFilename = normalizePath(filename);
+	const normalizedDeps = dependencies.map(normalizePath);
+	for (let i = 0; i < normalizedDeps.length; i++) {
+		if (normalizedDeps[i] === normalizedFullFilename) {
+			selfIdx.push(i);
+		}
+	}
+	const hasSelfDependency = selfIdx.length > 0;
+	if (hasSelfDependency) {
+		warnings.push({
+			code: 'vite-plugin-svelte-preprocess-depends-on-self',
+			message:
+				'svelte.preprocess returned this file as a dependency of itself. This can be caused by invalid configuration or importing generated code that depends on .svelte files (eg. tailwind base css)',
+			filename
+		});
+	}
+
+	if (dependencies.length > 10) {
+		warnings.push({
+			code: 'vite-plugin-svelte-preprocess-many-dependencies',
+			message: `preprocess depends on more than 10 external files which can cause slow builds and poor DX, try to reduce them. Found: ${dependencies.join(
+				', '
+			)}`,
+			filename
+		});
+	}
+	return {
+		dependencies: hasSelfDependency
+			? dependencies.filter((_, i) => !selfIdx.includes(i)) // remove self dependency
+			: dependencies,
+		warnings
+	};
+}