Wire RSC loader for both SWC and Babel transpilers

The previous implementation only handled Array.isArray(rule.use) and
only looked for babel-loader. The tutorial uses swc as its transpiler
(shakapacker.yml: javascript_transpiler: swc), which makes Shakapacker
generate rule.use as a FUNCTION, not an array. The RSC loader was
therefore never attached to the transpilation rule — 'use client'
files were left untransformed in the RSC bundle, producing 134
webpack warnings (export 'useState' not found in 'react' etc.) and
setting up a runtime error when the RSC renderer would try to call
client components directly instead of emitting client references.

Follow the pattern from docs/oss/migrating/rsc-preparing-app.md:167
verbatim, which handles both forms and both loader names.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ihabadham

config/webpack/rscWebpackConfig.js

@@ -8,16 +8,29 @@ const configureRsc = () => {
   };
   rscConfig.entry = rscEntry;
 
-  // Runs before babel-loader (webpack loaders execute right-to-left) to
-  // detect 'use client' directives in raw source before transpilation.
+  // Runs before babel/swc (webpack loaders execute right-to-left) to detect
+  // 'use client' directives in raw source before transpilation. Shakapacker
+  // generates rule.use as an array for Babel and as a function for SWC, so
+  // handle both forms.
   const { rules } = rscConfig.module;
   rules.forEach((rule) => {
-    if (Array.isArray(rule.use)) {
-      const babelLoader = extractLoader(rule, 'babel-loader');
-      if (babelLoader) {
-        rule.use.push({
-          loader: 'react-on-rails-rsc/WebpackLoader',
-        });
+    if (typeof rule.use === 'function') {
+      const originalUse = rule.use;
+      rule.use = function rscLoaderWrapper(data) {
+        const result = originalUse.call(this, data);
+        const resultArray = Array.isArray(result) ? result : result ? [result] : [];
+        const resolvedRule = { use: resultArray };
+        const jsLoader =
+          extractLoader(resolvedRule, 'babel-loader') || extractLoader(resolvedRule, 'swc-loader');
+        if (jsLoader) {
+          return [...resultArray, { loader: 'react-on-rails-rsc/WebpackLoader' }];
+        }
+        return result;
+      };
+    } else if (Array.isArray(rule.use)) {
+      const jsLoader = extractLoader(rule, 'babel-loader') || extractLoader(rule, 'swc-loader');
+      if (jsLoader) {
+        rule.use = [...rule.use, { loader: 'react-on-rails-rsc/WebpackLoader' }];
       }
     }
   });