The new validator doesn't work with rootDir tsconfig property

Link to the code that reproduces this issue

https://github.com/ecl1ps/next-validator-rootdir/commit/3490369c43c6d8915cb8279ec9b9cc0a86d2cb5e

To Reproduce

1. pnpm install
2. pnpm build

Current vs. Expected behavior

The new validator breaks the build when you use rootDir and your sources are not at the top level of the repo.

 > next build

   ▲ Next.js 15.5.1-canary.2

   Linting and checking validity of types  ...Failed to compile.

Type error: File 'C:/data/repos/next-validator-rootdir/.next/types/validator.ts' is not under 'rootDir' 'C:/data/repos/next-validator-rootdir/src'. 'rootDir' is expected to contain all source files.
  The file is in the program because:
    Root file specified for compilation

Next.js build worker exited with code: 1 and signal: null

It is expected to work with the rootDir.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 32401
  Available CPU cores: 20
Binaries:
  Node: 24.6.0
  npm: 11.5.1
  Yarn: N/A
  pnpm: 10.0.0
Relevant Packages:
  next: 15.5.1-canary.2 // Latest available version is detected (15.5.1-canary.2).
  eslint-config-next: N/A
  react: 19.1.1
  react-dom: 19.1.1
  typescript: 5.9.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Output

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

BTW, why don't you export types like ApiRouteConfig, PagesPageConfig, and AppPageConfig (which you AFAIK define only inside the new validator). We could use them to type our pages and API routes and get the validation even earlier, in the IDE. This was a breaking change for us as well, because the new validator is stricter, and specific types were not defined by Next. :(