What problem does this feature solve?

Improved compatibility with rollup

What does the proposed API look like?

Just a minor difference I've noticed, rolldown writes out the literal [format] to filenames if it is used in an output pattern. With rollup it is replaced with the format, like "es" or cjs.

It affects:

• output.entryFileNames - https://rollupjs.org/configuration-options/#output-entryfilenames
• (this does not seem to exist in rolldown though, so can be ignored) output.sourcemapFileNames - https://rollupjs.org/configuration-options/#output-sourcemapfilenames
• output.chunkFileNames - https://rollupjs.org/configuration-options/#output-chunkfilenames

It's easily worked around, but I thought I report it anyway.