Describe the bug

When using build mode with following tsdown config to build .mts files, rolldown-plugin-dts errors with a RollupError: Unable to build .d.ts file for /path/to/index.mts error.

import { defineConfig } from 'tsdown'

export default defineConfig({
  entry: 'src/index.mts',
  dts: { build: true }
});

🧐 Likely Cause

Under build mode, the plugin only looks for .d.ts files whereas tsc is emitting .d.mts files for .mts source files.

⛰️ Context

Why am I running the build mode? → seemingly it is the only way to have rolldown-plugin-dts emit .tsbuildinfo files. Only incremental: true wasn't effective for this purpose.

Why do I need .tsbuildinfo files? → I want to mimic tsc behavior with tsdown so that project references properly work.

Reproduction

https://github.com/uhyo/tsdown-dmts-repro

System Info

System:
    OS: Linux 6.6 Ubuntu 22.04.1 LTS 22.04.1 LTS (Jammy Jellyfish)
    CPU: (16) x64 AMD Ryzen 7 PRO 6850U with Radeon Graphics
    Memory: 11.43 GB / 15.01 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
  Binaries:
    Node: 22.20.0 - /home/uhyo/.volta/tools/image/node/22.20.0/bin/node
    Yarn: 4.1.1 - /home/uhyo/.volta/tools/image/yarn/4.1.1/bin/yarn
    npm: 11.4.2 - /home/uhyo/.volta/tools/image/npm/11.4.2/bin/npm
    pnpm: 10.25.0 - /home/uhyo/.volta/bin/pnpm
    Deno: 2.2.1 - /home/uhyo/.deno/bin/deno

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A, please open a GitHub Discussion instead.
• The provided reproduction is a minimal reproducible of the bug.

Contributions

• I am willing to submit a PR to fix this issue
• I am willing to submit a PR with failing tests (actually just go ahead and do it, thanks!)