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Comment by insanitybit | Hacker News Reader

I think 90% of the reason why people have a hard time with GAT is because there aren't a ton of use cases. From what I can tell, GAT is quite simple - it is just like all other Rust generics except that now the generic can be in a new place.

Rust has made it years without needing this (granted, a few things have been less than ideal because of it) because, for the most part, people have not needed to reach for this type of abstraction. But it's really just "Generics in a new place".

Personally, I've accidentally written the GAT syntax years before I ever heard the term GAT because I thought a generic could go there.

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It's needed for a lot of features people desperately want. Async functions in traits, closures that return futures where you need to run the future in a loop, and array indexing that doesn't necessarily return a reference, are three examples of things that essentially need GATs in order to be implemented without allocating. You may not feel like you missed it but not having that leads to a ton of ugly workarounds in Rust code. Also, in a lot of cases where people would otherwise use GATs, people instead work around them by making traits generic and have super complicated towers of generic types, you've probably seen interfaces like this when using hyper for instance. GATs would simplify most of those interfaces.

The vast majority of users do not desperately want async functions in traits, they just use async_trait and go "it'll be cool when GATs are done". Besides, no one is going to use GAT for async traits, that will be done for them by the compiler.

Obviously there are use cases for GAT, what I'm saying is that they're going to be in very generic library code, which I don't think most users are writing.

For libraries like Hyper, which prioritize being extremely generic (it's split into a ton of reusable crates), which aims to serve as a very low level set of generic primitives for HTTP, yes it will come up.

Hyper is a great example of a use case that 99.99% of developers are not going to be able to relate to because they aren't building a set of completely generic http primitives. It's an important use case, it's just not relatable.

I know people who refuse to use async_trait for documentation reasons alone. The macro-based solutions aren't always pleasant to use and often come with a lot of weird tradeoffs. And yeah, I agree that most users don't need to write theme explicitly--I'm arguing that users are currently significantly impacted by the lack of them, even if they are never in a position where they'd consider writing them themselves, because it prevents library authors from writing precise APIs. For example, you might not be wanting to write GATs like the Hyper authors, but anyone using Hyper for anything nontrivial (and that's surprisingly common, IME) is certainly negatively impacted by the massive complexity caused by needing to work around their absence.

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