Inlay variance hints

[bgenia]

🔍 Search Terms

inlay hints, variance annotations

✅ Viability Checklist

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

⭐ Suggestion

Display missing variance annotations for type parameters as inlay hints.

Examples:

type Ex1<T> = {}

type Ex2</* out */ T> = () => T

type Ex3</* in */ T> = (in: T) => void

type Ex4</* in out */ T> = (in: T) => T

📃 Motivating Example

It is convenient to know variance of specific type parameters when you're trying to understand how a type works/should be used.

💻 Use Cases

Examining complex 3rd party types.