Almost five years ago, Saoirse “boats” wrote “Notes on a smaller Rust”, and a year after that, revisited the idea.
The basic idea is a language that is highly inspired by Rust but doesn’t have the strict constraint of being a “systems” language in the vein of C and C++; in particular, it can have a nontrivial (or “thick”) runtime and doesn’t need to limit itself to “zero-cost” abstractions.
What languages are being designed that fit this description? I’ve seen a few scripting languages written in Rust on GitHub, but none of them have been very active. I also recently learned about Hylo, which does have some ideas that I think are promising, but it seems too syntactically alien to really be a “smaller Rust.”
Edit to add: I think Graydon Hoare’s post about language design choices he would have preferred for Rust also sheds some light on the kind of things a hypothetical “Rust-like but not Rust” language could do differently: https://graydon2.dreamwidth.org/307291.html
Go is a “small” language in the sense that it has an exceptionally small number of concepts (i.e. features and syntactic constructs); someone else in this thread made a comment to the effect that it takes almost no time to learn because there’s nothing to learn if you’ve already learned a different language. This is of course an exaggeration, but only slightly: Go was very intentionally and explicitly designed to be as simple as possible to learn and to use. As an example, it famously had no support for generics until almost 10 years after its 1.0 release. I think that when talking about the size of a language, some people do include the standard library while others don’t; Go has quite a large standard library, but you don’t actually have to learn the whole library or even be aware of what’s available in order to be productive.
I personally don’t think it makes sense to include the standard library in the “size” of a language for the purpose of this thread, or Boats’ original blog posts. The fundamental point is about the learning curve of the language and the amount of work it takes to produce working code, and a large standard library tends to be more convenient, not less. Common functionality that isn’t in Rust’s standard library tends to come from libraries that become almost ubiquitous, such as serde, regex, crossbeam, and itertools. From the user’s perspective, this isn’t much more or less complicated than having the same functionality available via the standard library. (Of course, a large standard library certainly makes languages more difficult to implement and/or specify; if I recall correctly, about half the size of the C++ standard is dedicated to the standard library.)
I don’t really know how to fairly compare the “size” of Rust and C++, partly because Rust is so much younger, and several C++ “features” overlap with each other or are attempts to replace others (e.g. brace-initialization as a replacement for parentheses). But I don’t think I’ve ever heard someone claim that C++ is “small” or “minimal” in this sense, so it’s in no way a good point of comparison for determining whether Rust is “small”.
Edit to add: for what it’s worth, if I weren’t quoting Boats’ blog post (which is sort of the “canonical” post on this concept), I probably would have opted for “simpler (to learn & use)” rather than “smaller.”