fix: insert conflict numbers before first extension in multi-extension filenames

[LeSingh1]

Closes #5822.

When rollup encounters a filename conflict it calls makeUnique, which
appended a counter just before the last file extension. For a
chunkFileNames pattern like chunk.d.ts that produced chunk.d2.ts
and chunk.d3.ts on subsequent conflicts instead of chunk2.d.ts and
chunk3.d.ts.

The problem is that path.extname only returns the last segment
(.ts), so the base after stripping becomes chunk.d and the number
lands in the wrong spot. TypeScript declaration files emitted by
rollup-plugin-dts are the most common case where this breaks things
because TypeScript's type resolver requires the .d.ts extension as a
whole.

The fix replaces the extname call with a small helper that scans for
the first dot in the filename (skipping a leading dot for hidden files
like .env.local) and treats everything from that position onwards as
the extension to preserve. Single-extension names are unaffected because
chunk.js has its first dot at the same position as its last dot.

A new chunking-form test verifies chunk.d.ts resolves conflicts as
chunk2.d.ts and chunk3.d.ts across all four output formats (es,
cjs, amd, system).

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[lukastaegert]

I like the idea, just some minor comments.

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Project	Deployment	Actions	Updated (UTC)
rollup	Ready	Preview, Comment	Jun 19, 2026 11:27am

[LeSingh1]

Thanks for the review. Applied both comments:

• Inlined the getFirstExtension logic directly into makeUnique and dropped the helper entirely. dotIndex is now computed inline with indexOf('.', 1), and base and extension are derived from it directly, so there's no longer a Math.max needed.
• Since the search always starts at index 1, the startsWith('.') conditional check was no longer needed and is gone.

Also fixed a prettier line-length violation in the test _config.js that was causing the Lint and Audit CI job to fail.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (2cd8194) to head (6fd0ec7).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6408   +/-   ##
=======================================
  Coverage   98.78%   98.78%           
=======================================
  Files         274      275    +1     
  Lines       10807    10815    +8     
  Branches     2882     2884    +2     
=======================================
+ Hits        10676    10684    +8     
  Misses         89       89           
  Partials       42       42           

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[github-actions]

This PR has been released as part of rollup@4.62.1. You can test it via npm install rollup.