Inlay variance hints

🔍 Search Terms

inlay hints, variance annotations

✅ 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

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.

Start by reviewing the requested inlay-hint behavior and the four variance examples in the issue. Determine how missing annotations should be displayed for type parameters, including the in, out, and in out cases. Done means variance hints appear for applicable types without changing emitted JavaScript or existing TypeScript/JavaScript behavior.