feat(config): support experimental.useExperimentalReact (#2109)

[Divkix]

Summary

Implements experimental.useExperimentalReact from Next.js (vercel/next.js#94861, closes #2109).

Next.js added experimental.useExperimentalReact as a direct opt-in to React's experimental release channel, replacing the need to enable an unrelated feature (taint/transitionIndicator/gestureTransition) as a side effect. The experimental React build emits <link rel="expect"> to hold first paint until the streamed shell is coherent.

vinext does not ship React's experimental build (and no existing flag like taint drives a React-channel switch), so this change matches the precedent set by appShells/cachedNavigations: accept the flag for config compatibility and replicate the assignDefaults contradiction warning, rather than wiring up a channel switch.

Changes

• config/next-config.ts
  • Add the typed experimental.useExperimentalReact?: boolean option with a doc-comment porting Next.js's description and documenting vinext's support level.
  • Replicate Next.js's assignDefaults warning: when useExperimentalReact is explicitly false but taint / transitionIndicator / gestureTransition still require the experimental channel, warn (an explicit false cannot disable the channel those features require).
• tests/next-config.test.ts — add coverage for accepting the flag (true/false without throwing), the contradiction warning (single + multiple dependents), and the no-warn cases. Ported from Next.js' rsc-basic-use-experimental-react.test.ts.

Support level

useExperimentalReact is accepted so user configs don't error, but it has no functional effect today because vinext does not ship React's experimental build. This is documented in the type doc-comment and surfaced in the warning text.

Verification

• vp test run tests/next-config.test.ts — 208 passed
• vp check — format, lint, and type checks clean

Upstream

• PR: Add experimental.useExperimentalReact to opt into React's experimental channel vercel/next.js#94861
• Commit: vercel/next.js@71b70e0

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@2167

npm i https://pkg.pr.new/vinext@2167

commit: 08713ed

[github-actions]

Performance benchmarks

Compared 08713ed against base 42f8700.

0 improved · 4 regressed · 2 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	Next.js	185.6 KB	185.6 KB	⚫ 0.0%
Client bundle size (gzip)	vinext	129.9 KB	129.9 KB	⚫ -0.0%
Dev server cold start	Next.js	2.26 s	2.45 s	🔴 +8.4%
Dev server cold start	vinext	2.44 s	2.67 s	🔴 +9.5%
Production build time	Next.js	3.98 s	4.37 s	🔴 +9.8%
Production build time	vinext	2.66 s	2.89 s	🔴 +8.7%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5%}