fix(rsc): keep CSS wrappers out of a trailing comment when a module has no final newline (fix #1495)

packages/plugin-rsc/src/transforms/wrap-export.test.ts

@@ -70,6 +70,26 @@ export class Cls {};
     `)
   })
 
+  test('module without a final newline', async () => {
+    // Published packages often end with a source map comment and no newline.
+    const input = `export const Arrow = () => {};\nexport function Fn() {};\n//# sourceMappingURL=index.js.map`
+    const output = await testTransform(input)
+    expect(output).toMatchInlineSnapshot(`
+      "let Arrow = () => {};
+      function Fn() {};
+      //# sourceMappingURL=index.js.map
+      Arrow = /* #__PURE__ */ $$wrap(Arrow, "<id>", "Arrow");
+      export { Arrow };
+      Fn = /* #__PURE__ */ $$wrap(Fn, "<id>", "Fn");
+      export { Fn };
+      "
+    `)
+    const lastLine = (output as string)
+      .split('\n')
+      .findLast((line) => line.includes('sourceMappingURL'))
+    expect(lastLine).toBe('//# sourceMappingURL=index.js.map')
+  })
+
   test('preserve reference', async () => {
     const input = `
 export let count = 0;

packages/plugin-rsc/src/transforms/wrap-export.ts

@@ -71,6 +71,10 @@ export function transformWrapExport(
   const exportNames: string[] = []
   const appendedCode: string[] = []
   const filter = options.filter ?? (() => true)
+  // Code moved to the end of a module that has no final newline would join its
+  // last line. If that line is a comment (often `//# sourceMappingURL=...`),
+  // the moved code becomes part of the comment and never runs.
+  let needsLeadingNewline = !input.endsWith('\n')
 
   /**
    * Strips a direct export declaration and emits assignments and exports at
@@ -113,8 +117,9 @@ export function transformWrapExport(
         .filter(Boolean)
         .join('') +
       `export { ${exports.map((e) => e.localName).join(', ')} };\n`
-    output.update(start, end, newCode)
+    output.update(start, end, (needsLeadingNewline ? '\n' : '') + newCode)
     output.move(start, end, input.length)
+    needsLeadingNewline = false
   }
 
   /**