(not) Deprecating `--rootDirs`

[acutmore]

Acknowledgement

• [x] I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

@andrewbranch suggested that rootDirs be deprecated in 6.0 (https://github.com/microsoft/TypeScript/issues/54500#issuecomment-2450555269).

I started talking with @jakebailey about this and we decided that best action would be to discuss via an issue.

At Bloomberg, and I believe Google too. Heavy use of rootDirs is made for the exact reason the docs show as an example, copied here for reference:

 src
 └── views
     └── view1.ts (can import "./template1", "./view2`)
     └── view2.ts (can import "./template1", "./view1`)

 generated
 └── templates
         └── views
             └── template1.ts (can import "./view1", "./view2")

{
  "compilerOptions": {
    "rootDirs": ["src/views", "generated/templates/views"]
  }
}

https://github.com/microsoft/TypeScript-Website/blob/b0e9a5c007b3e953d409f989f6013078bdd363ed/packages/tsconfig-reference/copy/en/options/rootDirs.md?plain=1#L11-L29

The generated .d.ts files are in a separate 'mirror' directory for source control reasons. But we want them to 'appear' to TypeScript as if they are sibling files so they are picked up by the module resolution.