[fix] disable illegal import detection when running tests (#8365)

Fixes #8180

Author: Rich-Harris

.changeset/big-cobras-sip.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Disable illegal import detection when running unit tests

documentation/docs/30-advanced/50-server-only-modules.md

@@ -51,4 +51,6 @@ Cannot import $lib/server/secrets.js into public-facing code:
 
 Even though the public-facing code — `src/routes/+page.svelte` — only uses the `add` export and not the secret `atlantisCoordinates` export, the secret code could end up in JavaScript that the browser downloads, and so the import chain is considered unsafe.
 
-This feature also works with dynamic imports, even interpolated ones like ``await import(`./${foo}.js`)``, with one small caveat: during development, if there are two or more dynamic imports between the public-facing code and the server-only module, the illegal import will not be detected the first time the code is loaded.
+This feature also works with dynamic imports, even interpolated ones like ``await import(`./${foo}.js`)``, with one small caveat: during development, if there are two or more dynamic imports between the public-facing code and the server-only module, the illegal import will not be detected the first time the code is loaded.
+
+> Unit testing frameworks like Vitest do not distinguish between server-only and public-facing code. For this reason, illegal import detection is disabled when running tests, as determined by `process.env.TEST === 'true'`.

packages/kit/src/exports/vite/index.js

@@ -301,7 +301,7 @@ function kit({ svelte_config }) {
 		},
 
 		async load(id, options) {
-			if (options?.ssr === false) {
+			if (options?.ssr === false && process.env.TEST !== 'true') {
 				const normalized_cwd = vite.normalizePath(cwd);
 				const normalized_lib = vite.normalizePath(svelte_config.kit.files.lib);
 				if (