[verdagon]

I can imagine a Rust-like language where we have move-constructors (in TFA), and every generated Future subtype is opaque and also heap allocated for us.

I think the need for Pin could then disappear, because the user would have no way to destroy it, since it's opaque and elsewhere on the heap, and therefore no way to move it (because having move-constructors implies that moving is conceptually destroying then recreating things).

[pornel]

Pin is a state rather than a property of the data itself.

This has a very nice effect of allowing merging and inlining of Futures before they're executed.

It's similar to how Rust does immutability — there's no immutable memory, only immutable references.

[withoutboats3]

You don't need move constructors if every future is heap allocated. But then every call to an async function is a separate allocation, which is terrible for memory locality. Some kind of virtual stack would be much better than that (but then you need garbage collection if you want the stacks to be memory-optimized to be small by default).

[anonymoushn]

You could use an actual stack. As I understand it this was not done for questionable reasons relating to borrows of thread-locals. You could also allocate a top-level async function and all of its transitive async callees all at once, if you force the user to put all this information in 1 translation unit. Or you could use a bump allocator specifically for futures used in a certain part of the program, if you're willing to give up using a global allocator for everything. So it seems like there are a lot of options.

[josephg]

Yeah. I can guess how disruptive it would be, but I really wish rust bit the bullet and added a Move trait to std, baked into the language at a similar level as Copy. Move defines a function which moves a value from one address in memory to another. Structs without impl Move cannot be moved.

Almost all types would #[derive(Move)], which implements a trivial move function that copies the bytes. But this opens the door to self-referential types, futures, and lots of other things that need more complex move behaviour. (Actually, it might make more sense to define two traits, mirroring the difference between Copy and Clone. One is a marker trait which tells the compiler that the bytes can just be moved. The other allows custom "move constructor" implementation.)

I want move because pin is so hard to understand. Its a complex idea wrapped in double- or sometimes triple negatives. fn<X: !Unpin>(...). Wat? I drop off at unsafe pin-projecting. When is that safe? When is it not? Blah I'm out.

Moving from rust-without-move to rust-with-move would be inconvenient, because basically every struct anyone has written to date with rust needs #[derive(Move)] to be added. Including in std. And all types in existing editions that aren't pinned would need the compiler to infer a Move trait implementation. This should be possible to do mechanically. It would just be a lot of work.

Async rust is horrible. Especially compared to futures / promises in almost any other language. At some point, someone will make a new rust-like systems language which has an improved version of rust's memory safety model, a Move trait, and better futures. I'd personally also love comptime instead of rust's macro system.

I love rust. I love all the work the team has put into it over the years. But the language I’m really looking forward to is the language that comes after rust. Same idea, but something that has learned from rust’s mistakes. And it’s increasingly becoming clear what that better rust-like language might potentially look like. I can't wait.

[conradludgate]

I very much disagree that Pin is as hard as everyone makes it out to be. Using the pin! macros, the pin-project crate, and enough as_mut() to get it to compile and it's not hard at all to get a future impl working. It would be good to get this native (which is what boats wants) so it's easier to discover but it's not at all hard by any means

I think a lot of people think pin is confusing but don't actually try to learn it. When I've sat with people and helped them they understand pretty quickly what pin solves and how it works.

I very strongly think move constructors would be even more complex than pin.

[sophacles]

> Async rust is horrible. Especially compared to futures / promises in almost any other language.

Having written a bunch of async code over 20 years (first exposure was twisted in the early 2000s and all sorts of stuff since - including manual function/stack wrangling on my own reactor), async in rust is like many other things in rust: It forces you to deal with the problems up-front rather than 6-12 months later once something is in prod. It helps to stop and understand why the compiler is yelling at you - for me anyway once I do grok that I'm glad I didn't have a repeat of $BUG_THAT_TOOK_DOWN_PROD_FOR_A_WEEK - that was a bad week.