only bounds are going to be the most impactful change to Rust that you’ve never heard of. They are currently being designed and developed by the Arm team (David Wood, Rémy Rakic, et al.) as part of the Sized Hierarchy and Scalable Vector Extension project goal. This post explores the feature and aims to answer a particular question about the design (the scope of bounds, I’ll explain). But before I dive in, I want to give a bit of context.

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.

Rust has lots of mechanisms that prevent you from doing something bad. But, right now, it has NO mechanisms that force you to do something good¹. I’ve been thinking lately about what it would mean to add “must move” types to the language. This is an idea that I’ve long resisted, because it represents a fundamental increase to complexity. But lately I’m seeing more and more problems that it would help to address, so I wanted to try and think what it might look like, so we can better decide if it’s a good idea.