[Bug]: `output.iife: false` option may result in duplicate inline data.

[micooz]

System Info

System:
OS: macOS 26.1
CPU: (12) arm64 Apple M4 Pro
Memory: 264.17 MB / 24.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 22.18.0 - /Users/micooz/.local/state/fnm_multishells/67176_1763266678252/bin/node
Yarn: 1.22.22 - /opt/homebrew/bin/yarn
npm: 10.9.3 - /Users/micooz/.local/state/fnm_multishells/67176_1763266678252/bin/npm
pnpm: 10.20.0 - /opt/homebrew/bin/pnpm
Browsers:
Chrome: 142.0.7444.162
Safari: 26.1
npmPackages:
@rspack/cli: ^1.6.3 => 1.6.3

Details

output.iife: false 选项可能导致内联重复数据

Reproduce link

https://github.com/micooz/rspack-repro-12212

Reproduce Steps

package.json

{
  "dependencies": {
    "@rspack/cli": "^1.6.3"
  }
}

rspack.config.ts

import { defineConfig } from '@rspack/cli';

export default defineConfig({
  output: { iife: false },
});

源文件

// src/index.js
const SOME_BASE64_DATA = 'aiVBORw0KGgoA...';

atob(SOME_BASE64_DATA);
// atob(SOME_BASE64_DATA);

构建产物

var __webpack_modules__ = {},
  __webpack_module_cache__ = {};
function __webpack_require__(_) {
  var e = __webpack_module_cache__[_];
  if (void 0 !== e) return e.exports;
  var r = (__webpack_module_cache__[_] = { exports: {} });
  return __webpack_modules__[_](r, r.exports, __webpack_require__), r.exports;
}
(__webpack_require__.rv = () => '1.6.3'),
  (__webpack_require__.ruid = 'bundler=rspack@1.6.3');
const SOME_BASE64_DATA = 'aiVBORw0KGgoA...';
atob('aiVBORw0KGgoA...');
//# sourceMappingURL=main.js.map

可以看到 SOME_BASE64_DATA 的值被内联到 atob 函数中，并且 SOME_BASE64_DATA 被保留且未被引用，出现了两份相同的数据。SOME_BASE64_DATA 的数据量可能很大，会对产物体积造成一定影响。

然而，如果设置 iife: true，或者再引用一次 SOME_BASE64_DATA，此问题会消失：

源文件

// src/index.js
const SOME_BASE64_DATA = 'aiVBORw0KGgoA...';

atob(SOME_BASE64_DATA);
atob(SOME_BASE64_DATA); // 再引用一次

构建产物

var __webpack_modules__ = {},
  __webpack_module_cache__ = {};
function __webpack_require__(_) {
  var e = __webpack_module_cache__[_];
  if (void 0 !== e) return e.exports;
  var r = (__webpack_module_cache__[_] = { exports: {} });
  return __webpack_modules__[_](r, r.exports, __webpack_require__), r.exports;
}
(__webpack_require__.rv = () => '1.6.3'),
  (__webpack_require__.ruid = 'bundler=rspack@1.6.3');
const SOME_BASE64_DATA = 'aiVBORw0KGgoA...';
atob(SOME_BASE64_DATA), atob(SOME_BASE64_DATA);
//# sourceMappingURL=main.js.map

这个行为非常令人困惑，是 by design 还是 bug 呢，希望得到解答。

[github-actions]

Hello @micooz, sorry we can't investigate the problem further without reproduction demo, please provide a repro demo by forking rspack-repro, or provide a minimal GitHub repository by yourself. Issues labeled by need reproduction will be closed if no activities in 14 days.

[micooz]

Hello @micooz, sorry we can't investigate the problem further without reproduction demo, please provide a repro demo by forking rspack-repro, or provide a minimal GitHub repository by yourself. Issues labeled by need reproduction will be closed if no activities in 14 days.

https://github.com/micooz/rspack-repro-12212

see dist/main.js.

[SyMind]

@CPunisher After taking a look, it's most likely a swc minify bug.

[CPunisher]

Sadly, it's not simple to decide whether the variable should be inlined here. Consider another case:

const LONG = "loooooooooooooooong"
if (!LONG) {
    veryLoooooooooooooooooooooongCode();
    // Many other codes
}

In this case, inlining LONG will make DCE possible and eliminate all codes in the if body.

const LONG_1 = "loooooooooooooooong"

Introducing an complicated inline cost model will help, but it's currently not possible for javascript minifiers.

Given that the long string literals are uncommon cases, manually marking it with NO_INLINE could help:

const SOME_BASE64_DATA = () => 'aiVBORw0KGgoA...';
atob(/*#__NOINLINE__*/ SOME_BASE64_DATA());

[micooz]

This works:

/*#__NOINLINE__*/
atob(SOME_BASE64_DATA());

But atob(/*#__NOINLINE__*/ SOME_BASE64_DATA()); not.

It seems there's no better solution in Rspack at the moment.

[CPunisher]

Let me provide another information you may be confused about:

并且 SOME_BASE64_DATA 被保留且未被引用

By default, Rspack provide non-esm artifacts that run in <script> html tag rather than <script type="module"> tag. This means all top-level declarations and symbols should be preserved:

<html>
  <script>const a = 1</script>
  <script>console.log(a)</script>
</html>

[CPunisher]

Probably, making /*#__NOINLINE__*/ available for variable inlining is another good way that should be implemented by minifier.