[ID-Prep] Preserve all triple slash directives in declarations files

[dragomirtitian]

The [ID-prep] set of issues aligns Declaration Emit with the forthcoming Isolated Declarations feature.

🔍 Search Terms

preserve triple slash directives

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Right now TypeScript will remove unused /// <reference types="..." /> directives.

📃 Motivating Example

// @declaration: true

// @filename: /a/node_modules/@types/node/index.d.ts
interface Error {
    stack2: string;
}

// @filename: /a/app.ts
/// <reference types="node"/>
// ^ is currently removed
function foo(): Error {
    return undefined;
}

Playground

💻 Use Cases

1. What do you want to use this for?

   Preserving all references would make it easier for other tools to create the same output as typescript for declaration files

2. What shortcomings exist with current approaches?

   Other tools don't have enough type information to determine if the reference is used or not if they only look at one file.

3. What workarounds are you using in the meantime?

   Currently other tools can't emit declaration files.