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[Flight] Add bundler-less version of RSC using plain ESM (#26889)
This isn't really meant to be actually used, there are many issues with this approach, but it shows the capabilities as a proof-of-concept. It's a new reference implementation package `react-server-dom-esm` as well as a fixture in `fixtures/flight-esm` (fork of `fixtures/flight`). This works pretty much the same as pieces we already have in the Webpack implementation but instead of loading modules using Webpack on the client it uses native browser ESM. To really show it off, I don't use any JSX in the fixture and so it also doesn't use Babel or any compilation of the files. This works because we don't actually bundle the server in the reference implementation in the first place. We instead use [Node.js Loaders](https://nodejs.org/api/esm.html#loaders) to intercept files that contain `"use client"` and `"use server"` and replace them. There's a simple check for those exact bytes, and no parsing, so this is very fast. Since the client isn't actually bundled, there's no module map needed. We can just send the file path to the file we want to load in the RSC payload for client references. Since the existing reference implementation for Node.js already used ESM to load modules on the server, that all works the same, including Server Actions. No bundling. There is one case that isn't implemented here. Importing a `"use server"` file from a Client Component. We don't have that implemented in the Webpack reference implementation neither - only in Next.js atm. In Webpack it would be implemented as a Webpack loader. There are a few ways this can be implemented without a bundler: - We can intercept the request from the browser importing this file in the HTTP server, and do a quick scan for `"use server"` in the file and replace it just like we do with loaders in Node.js. This is effectively how Vite works and likely how anyone using this technique would have to support JSX anyway. - We can use native browser "loaders" once that's eventually available in the same way as in Node.js. - We can generate import maps for each file and replace it with a pointer to a placeholder file. This requires scanning these ahead of time which defeats the purposes. Another case that's not implemented is the inline `"use server"` closure in a Server Component. That would require the existing loader to be a bit smarter but would still only "compile" files that contains those bytes in the fast path check. This would also happen in the loader that already exists so wouldn't do anything substantially different than what we currently have here. DiffTrain build for commit f181ba8.
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