feat: support server-only and client-only modules (#2162)

.changeset/support-server-client-only.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/start": minor
+---
+
+Add support for `server-only` and `client-only` modules

packages/start/env.d.ts

@@ -7,3 +7,15 @@ declare namespace App {
     [key: string | symbol]: any;
   }
 }
+
+/**
+ * Import `server-only` to ensure this module is never bundled for the client.
+ * Importing it in a client module will throw a build error.
+ */
+declare module "server-only" {}
+
+/**
+ * Import `client-only` to ensure this module is never bundled for the server.
+ * Importing it in a server module will throw a build error.
+ */
+declare module "client-only" {}

packages/start/src/config/index.ts

@@ -209,6 +209,29 @@ export function solidStart(options?: SolidStartOptions): Array<PluginOption> {
     }),
     lazy(),
     envPlugin(options?.env),
+    {
+      name: "solid-start:boundary-modules",
+      enforce: "pre",
+      resolveId(id, importer, { ssr }) {
+        if (id === "server-only") {
+          if (!ssr)
+            this.error(
+              `Attempt to import 'server-only' in a client module: ${importer}`,
+            );
+        } else if (id === "client-only") {
+          if (ssr)
+            this.error(
+              `Attempt to import 'client-only' in a server module: ${importer}`,
+            );
+        } else {
+          return null;
+        }
+        return "\0solid-start:boundary-modules:id";
+      },
+      load(id) {
+        if (id === "\0solid-start:boundary-modules:id") return "export {}";
+      },
+    },
     {
       name: "solid-start:virtual-modules",
       async resolveId(id) {