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.

Inizia esaminando il comportamento richiesto di inlay-hint e i quattro esempi di varianza nell'issue. Determina come devono essere visualizzate le annotazioni mancanti per i parametri di tipo, inclusi i casi in, out e in out. Il lavoro è completato quando gli hint di varianza vengono visualizzati per i tipi applicabili senza modificare il JavaScript emesso o il comportamento esistente di TypeScript/JavaScript.