## Summary

Implements the feature requested in #7583 (comment)

This PR adds a warning when both the source code and `postBanner` contain shebang lines.

## Changes

- [x] Add `DuplicateShebang` event kind to `EventKind` enum
- [x] Create new `duplicate_shebang.rs` event module
- [x] Update `mod.rs` to export the new event
- [x] Update `constructors.rs` to add constructor for DuplicateShebang
- [x] Modify `post_banner_footer.rs` to detect duplicate shebangs and emit warning
- [x] Add test case for duplicate shebang scenario
- [x] Run tests to verify the implementation - ✅ All tests pass
- [x] Run code review and address feedback
- [x] Address PR review feedback
- [x] Run format and generator
- [x] Fix lint and update CLI snapshot
- [x] Update JS snapshots
- [x] Fix test to run sequentially

## Behavior

When both source code and `postBanner` contain shebang:
- ✅ Original shebang preserved as first line
- ✅ postBanner placed after shebang (per PR #7583)
- ✅ Warning emitted: `[DUPLICATE_SHEBANG] Warning: Both the code and postBanner contain shebang in "main.js". This will cause a syntax error.`

## Example

**Input file:**
```javascript
#!/usr/bin/env node
console.log('hello');
```

**postBanner:**
```javascript
#!/usr/bin/env bun
/* Version 1.0.0 */
```

**Output:**
```javascript
#!/usr/bin/env node
#!/usr/bin/env bun
/* Version 1.0.0 */
console.log('hello');
```

**Warning:**
```
[DUPLICATE_SHEBANG] Warning: Both the code and postBanner contain shebang in "main.js". This will cause a syntax error.
```

<!-- START COPILOT CODING AGENT TIPS -->
---

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## [1.0.0-rc.2] - 2026-01-28

⚡ Lazy Barrel Optimization

- Skips compilation of unused re-export modules in side-effect-free barrel modules
- Particularly beneficial for projects importing from large component libraries

For large component libraries like Ant Design, when you import just one component:

```js
import { Button } from 'antd';
Button;
```

| Metric                    | Without lazy barrel | With lazy barrel |
| -------------------- | --------------------- | ---------------- |
| Modules compiled | 2986                       | 250              |
| Build time               | ~210ms                  | ~50ms            |

By enabling lazy barrel, Rolldown reduces the number of compiled modules by 90% and speeds up the build by 2-4x. 

Enable it in your config:

```js
export default {
  experimental: {
    lazyBarrel: true,
  },
};
```

For more details, see the https://rolldown.rs/in-depth/lazy-barrel-optimization

### 💥 BREAKING CHANGES

- expose `\0rolldown/runtime` in transform hook (#8068) by @hyf0
- rename `rolldown:runtime` to `\0rolldown/runtime.js` (#8067) by @hyf0

### 🚀 Features

- remove inlined constants in smart mode (#8085) by @sapphi-red
- allow more options for `this.emitFile` with `type: 'prebuilt-chunk'` (#8062) by @sapphi-red
- warn when both code and postBanner contain shebang (#8039) by @Copilot

### 🐛 Bug Fixes

- update the links to Rolldown docs in the error messages (#8103) by @sapphi-red
- handle tsconfig.json load errors (#8105) by @sapphi-red
- include inlined constants in namespace object (#8099) by @sapphi-red
- vite test ci (#8084) by @IWANABETHATGUY
- renamer: nested binding shadowing external module namespace in UMD/IIFE formats (#8083) by @Dunqing
- deduplicate ESM chunk imports by canonical symbol (#8059) by @IWANABETHATGUY
- refine side-effect detection for BigInt and RegExp (#8060) by @IWANABETHATGUY
- rust: use string literal span for `new URL` error diagnostic (#8043) by @valadaptive
- rust: use ModuleType::Asset for `new URL` imports (#8035) by @valadaptive
- CJS-ESM interop - property assignment on CJS module exports (#8006) by @IWANABETHATGUY
- eliminate the facade chunk if the dynamic entry module has been merged into common chunk (#8046) by @IWANABETHATGUY
- Inlining dynamic imports broken with multiple entry points (#8037) by @IWANABETHATGUY
- devtools: revert `Chunk#id` to `Chunk#chunk_id` (#8040) by @hyf0
- invert `__exportAll` parameter logic to reduce default output size (#8036) by @Copilot
- `</script` tag search should be case insensitive (#8033) by @IWANABETHATGUY
- use directory name as-is for the variable name even if the name contained `.` (#8029) by @Copilot
- dev/lazy: remove unnecessary rewrite from top level `this` to `undefined` (#8020) by @hyf0
- dev/lazy: should keep lazy entries imports for patch file (#8019) by @hyf0
- `output.generatedCode.preset: 'es2015'` was not set by default (#8026) by @sapphi-red
- node: align option validator to types (#8023) by @sapphi-red
- node: allow `output.strictExecutionOrder` by the option validator (#8022) by @sapphi-red
- types: return `this` from on / off methods of `RolldownWatcher` (#8015) by @sapphi-red

### 🚜 Refactor

- rolldown_plugin_vite_dynamic_import_vars: remove v1 implementation (#8096) by @shulaoda
- rolldown_plugin_vite_import_glob: remove v1 implementation (#8095) by @shulaoda
- lazy-barrel: restructure lazy barrel implementation (#8070) by @shulaoda
- remove `use_built_ins` and `use_spread` from internal JSX options (#8079) by @sapphi-red
- remove `experimental.transformHiresSourcemap` (#8055) by @Copilot
- rust: use `is_data_url` more consistently (#8042) by @valadaptive
- use `FxIndexMap` to store EntryPoint (#8032) by @IWANABETHATGUY
- node: add type checks that ensures validator schema is up to date with types (#8024) by @sapphi-red

### 📚 Documentation

- link to vite plugin registry (#8086) by @sapphi-red
- lazy-barrel: improve documentation and enable in sidebar (#8072) by @shulaoda
- add more examples and details (#8054) by @sapphi-red
- in-depth: add dead code elimination page (#8007) by @sapphi-red
- update status from beta to release candidate (#8012) by @shulaoda

### ⚡ Performance

- run inline-const pass for modules that are affected by inlining (#8064) by @sapphi-red

### 🧪 Testing

- lazy-barrel: use package.json sideEffects instead of plugin hook (#8077) by @shulaoda
- lazy-barrel: enable tests and add treeshake-behavior cases (#8071) by @shulaoda

### ⚙️ Miscellaneous Tasks

- deps: update crate-ci/typos action to v1.42.3 (#8087) by @renovate[bot]
- deps: update rollup submodule for tests to v4.56.0 (#8073) by @sapphi-red
- deps: update oxc to v0.111.0 (#8063) by @renovate[bot]
- deps: update dependency rolldown-plugin-dts to v0.21.6 (#8076) by @renovate[bot]
- deps: update test262 submodule for tests (#8074) by @sapphi-red
- deps: update crate-ci/typos action to v1.42.2 (#8069) by @renovate[bot]
- deps: update oxc apps (#8066) by @renovate[bot]
- remove `{@include ./foo.md}` from d.ts files (#8056) by @sapphi-red
- deps: update dependency oxlint-tsgolint to v0.11.2 (#8057) by @renovate[bot]
- deps: update github-actions (#8050) by @renovate[bot]
- deps: update npm packages (#8051) by @renovate[bot]
- deps: update rust crates (#8049) by @renovate[bot]
- debug: add IdxExt debug trait for human-readable index debugging (#8045) by @IWANABETHATGUY
- deps: update dependency rolldown-plugin-dts to v0.21.5 (#8034) by @renovate[bot]
- deps: update oxc resolver to v11.16.4 (#8031) by @renovate[bot]
- deps: update dependency rolldown-plugin-dts to v0.21.4 (#8030) by @renovate[bot]
- deps: update dependency rust to v1.93.0 (#8018) by @renovate[bot]
- archive 2025 beta changelog (#8014) by @shulaoda
- update release workflow version pattern from beta to rc (#8013) by @shulaoda

### ❤️ New Contributors

* @valadaptive made their first contribution in [#8043](#8043)

Co-authored-by: shulaoda <165626830+shulaoda@users.noreply.github.com>