fix(pages-router): keep 'use server' files outside app/ as normal modules (#1476)

[james-elicx]

Summary

• Server Actions are an App Router-only feature. When a Pages Router page imports a file containing "use server", @vitejs/plugin-rsc's rsc:use-server transform rewrites the exports into server-reference proxies. Calling such a function from getServerSideProps then throws "Server Functions cannot be called during initial render."
• Add vinext:strip-non-app-use-server (an enforce: "pre" transform) that strips the "use server" directive prologue from any file outside the app/ directory before plugin-rsc sees it. Files inside app/ are unaffected, so existing server-action behaviour is preserved.
• This mirrors Next.js's SWC pipeline, where the server-actions transform only runs for the App Router layer (packages/next/src/build/swc/options.ts isAppRouterPagesLayer).

Fixes #1476.

Test plan

• New regression test in tests/app-router.test.ts reproduces the failure on main and passes with the fix:
  • pnpm test tests/app-router.test.ts -t "use server"
• Full tests/app-router.test.ts suite still green (333 tests, including the existing server-action coverage).
• Full tests/pages-router.test.ts suite still green (265 tests).
• tests/form.test.ts, tests/app-rsc-handler.test.ts, tests/app-server-action-execution.test.ts still green.
• pnpm run check (format, lint, types) green.
• CI runs the full Vitest + Playwright matrices.

Ported from Next.js: test/e2e/app-dir/action-in-pages-router/action-in-pages-router.test.ts.

[james-elicx]

/bigbonk review

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vinext@1679

commit: 9cfed98

[github-actions]

Example	Preview	Production	Original
app-router-cloudflare	preview	production
pages-router-cloudflare	preview	production
app-router-playground	preview	production	original
realworld-api-rest	preview	production
nextra-docs-template	preview	production
benchmarks	preview	production
hackernews	preview	production	original
web	preview	production

[ask-bonk]

Clean fix — the approach is correct, the implementation is careful, and the test coverage is sufficient for the stated problem.

What works well:

• Correct scoping. Stripping the file-level directive prologue (not inline "use server" inside function bodies) is exactly right — inline directives inside function bodies within pages code are already harmless because they don't trigger @vitejs/plugin-rsc's module-level transform gate.

• Proper plugin ordering. enforce: "pre" ensures this runs before the RSC plugin's rsc:use-server transform. The cheap code.includes("use server") pre-check avoids cost on the vast majority of files.

• Robust directive parsing. The stripUseServerDirective function correctly handles BOM, hashbang, comments, whitespace, and other directives preceding "use server" (e.g. "use strict"). Follows ECMA-262 Directive Prologue grammar faithfully.

• Timing safety. appDir is assigned in config() → the strip plugin reads it in configResolved() → Vite guarantees all config() hooks complete before any configResolved() fires. The closure-based getAppDir/hasAppDir callbacks are the same pattern used by createMiddlewareServerOnlyPlugin.

• Good test. The fixture mirrors the Next.js test (test/e2e/app-dir/action-in-pages-router/) well, and the test assertion checks both the presence of the test element and the real return value ("action:foo"), which would fail if the export were still a server-reference proxy.

One thing to be aware of (not blocking):

There's an inherent trade-off in the path-based scoping: if a file with "use server" lives in a shared directory (e.g. lib/) and is imported by both a pages route and an app route, the directive gets stripped for both importers since Vite's module graph is shared. The app route consumer silently loses the server action. This matches Next.js behavior (SWC's isAppRouterPagesLayer gate is per-compilation-layer, and Next.js has separate bundler pipelines for pages vs app), but is worth noting for anyone who later tries to share "use server" files across routers. Not actionable now — just documenting the design constraint.

LGTM.

[ask-bonk]

Review posted and approved on PR #1679.

Summary: This is a clean, well-scoped fix. The plugin strips file-level "use server" directives from modules outside the app/ directory so that @vitejs/plugin-rsc doesn't rewrite them into server-reference proxies when they're imported by Pages Router code. The implementation follows existing plugin patterns, has correct timing guarantees, robust directive parsing, and a good regression test ported from the Next.js test suite. Approved with a non-blocking note about the inherent design constraint when sharing "use server" files across both routers (which matches Next.js behavior).

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