Continuing my series on ergonomic ref-counting, I want to explore another idea, one that I’m calling “just call clone (or alias)”. This proposal specializes the clone and alias methods so that, in a new edition, the compiler will (1) remove redundant or unnecessary calls (with a lint); and (2) automatically capture clones or aliases in move closures where needed.

The goal of this proposal is to simplify the user’s mental model: whenever you see an error like “use of moved value”, the fix is always the same: just call clone (or alias, if applicable). This model is aiming for the balance of “low-level enough for a Kernel, usable enough for a GUI” that I described earlier. It’s also making a statement, which is that the key property we want to preserve is that you can always find where new aliases might be created – but that it’s ok if the fine-grained details around exactly when the alias is created is a bit subtle.

Hmm, as I re-read the post I literally just posted a few minutes ago, I got to thinking. Maybe the right name is indeed Alias, and not Share. The rationale is simple: alias can serve as both a noun and a verb. It hits that sweet spot of “common enough you know what it means, but weird enough that it can be Rust Jargon for something quite specific”. In the same way that we talk about “passing a clone of foo” we can talk about “passing an alias to foo” or an “alias of foo”. Food for thought! I’m going to try Alias on for size in future posts and see how it feels.

There have been two major sets of responses to my proposal for a Handle trait. The first is that the Handle trait seems useful but doesn’t over all the cases where one would like to be able to ergonomically clone things. The second is that the name doesn’t seem to fit with our Rust conventions for trait names, which emphasize short verbs over nouns. The TL;DR of my response is that (1) I agree, this is why I think we should work to make Clone ergonomic as well as Handle; and (2) I agree with that too, which is why I think we should find another name. At the moment I prefer Share, with Alias coming in second.

In my previous post about Ergonomic Ref Counting, I talked about how, whatever else we do, we need a way to have explicit handle creation that is ergonomic. The next few posts are going to explore a few options for how we might do that.

This post focuses on explicit capture clauses, which would permit closures to be annotated with an explicit set of captured places. My take is that explicit capture clauses are a no brainer, for reasons that I’ll cover below, and we should definitely do them; but they may not be enough to be considered ergonomic, so I’ll explore more proposals afterwards.

This post presents a proposal to extend Rust to support a number of different kinds of destructors. This means we could async drop, but also prevent “forgetting” (leaking) values, enabling async scoped tasks that run in parallel à la rayon/libstd. We’d also be able to have types whose “destructors” require arguments. This proposal – an evolution of “must move” that I’ll call “controlled destruction” – is, I think, needed for Rust to live up to its goal of giving safe versions of critical patterns in systems programming. As such, it is needed to complete the “async dream”, in which async Rust and sync Rust work roughly the same.

Continuing my discussion on Ergonomic RC, I want to focus on the core question: should users have to explicitly invoke handle/clone, or not? This whole “Ergonomic RC” work was originally proposed by Dioxus and their answer is simple: definitely not. For the kind of high-level GUI applications they are building, having to call cx.handle() to clone a ref-counted value is pure noise. For that matter, for a lot of Rust apps, even cloning a string or a vector is no big deal. On the other hand, for a lot of applications, the answer is definitely yes – knowing where handles are created can impact performance, memory usage, and even correctness (don’t worry, I’ll give examples later in the post). So how do we reconcile this?