Define __SOLID_SERVER_COMPONENTS__ at build time so libraries can drop server-component-only client code #396

Problem

@solidjs/router ships a client fallback for server-component forms. These forms arrive as URLs in the HTML with no client reference. When no action() handler is installed, the router intercepts POSTs under actionBase and lazily imports its action layer.

Only server-component apps need this, but every router app pays for it. In a minimal router app with no server functions (Vite 8, gzip):

• about 0.85 KB eager (up to 1.4 KB when it's the app's only dynamic import);
• a 16.8 KB lazy chain: the serverForms chunk (8.6 KB) plus the decode chunk (7.0 KB) and a server chunk (1.2 KB) behind it.

The router's flat build inlines all of it eagerly (about 8 KB).

Proposal

The plugin defines a global constant __SOLID_SERVER_COMPONENTS__. Core owns and documents the name, with the meaning "server components are enabled for this build".

• Value: true when serverFunctions.components is truthy (including 'external'), otherwise false. It must always be defined: if it's left undefined, nothing can be removed.
• Set it in both top-level define (build, and dev source via /@vite/env) and optimizeDeps.rolldownOptions.transform.define (dev pre-bundled dependencies; the optimizer ignores top-level define).
• Respect a user-provided define[__SOLID_SERVER_COMPONENTS__].
• Applying it to all environments is fine; it's harmless on the server.

Why a define

Whether a check folds away depends on the form:

Cost

Zero for apps. One config entry in the plugin.

Router follow-up

declare const __SOLID_SERVER_COMPONENTS__: boolean | undefined;
if (typeof __SOLID_SERVER_COMPONENTS__ !== "undefined" && __SOLID_SERVER_COMPONENTS__) {
  /* intercept */ import("./serverForms.js")...
}

• With true, output matches today and serverForms stays lazy, so server-component apps without action() still don't pay for it eagerly.
• In the flat build, false saves about 7–8 KB eager.
• The router's own build (tsc + Rollup) keeps the guard; no config change is needed.
• Without the plugin, the fallback is off at runtime and the bytes are the same as today. Server-component setups without the plugin define the constant themselves, as documented in core.

Alternatives considered

• configureClient setup modules: too much wiring through start mode.
• A runtime flag in server-functions/frames: doesn't fold under Rolldown or esbuild (measured above).
• The router installing the fallback from serverRouteComponent/action: leaves a coverage gap, because forms can arrive with neither present.
• Asking Rolldown upstream to fold imported constants: out of our control.

Implementation sketch

This goes in the main plugin's config() hook, next to the existing optimizeDeps.rolldownOptions.transform.jsx, using the serverFunctions.components value resolved in solidPlugin().

const SC_KEY = "__SOLID_SERVER_COMPONENTS__";
// inside config():
const scDefine = { [SC_KEY]: userConfig.define?.[SC_KEY] ?? JSON.stringify(serverComponents) };
return {
  define: scDefine,
  optimizeDeps: {
    // ...existing
    rolldownOptions: { transform: { jsx: { runtime: "classic" }, define: scDefine }, plugins: [/* existing */] }
  }
};

Checklist

• Plugin: define plus optimizeDeps.rolldownOptions.transform.define; respect the user's value.
• Core docs: RFC 11 integration surface and the server-components skill (cross-link).
• Router: wrapped guard; vitest define true plus an off-path test; changeset.