Using Rust at a startup: A cautionary tale

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Rust has a huge learning curve.

I’ve worked in dozens of languages in my career, and with few exceptions most modern, procedural languages (C++, Go, Python, Java, etc.) all very similar in terms of their basic concepts. Each language has its differences but usually it’s a matter of learning a few key patterns that differ across languages and then one can be productive pretty quickly. With Rust, though, one needs to learn entirely new ideas — things like lifetimes, ownership, and the borrow checker. These are not familiar concepts to most people working in other common languages, and there is a pretty steep learning curve, even for experienced programmers.

There are other ways to fix the problems that Rust is trying to solve.

As mentioned above, the service we were building was a fairly straightforward CRUD app. The expected load on this service was going to be on the order no more than a few queries per second, max, through the lifetime of this particular system. The service was a frontend to a fairly elaborate data-processing pipeline that could take many hours to run, so the service itself was not expected to be a performance bottleneck. There was no particular concern that a conventional language like Python would have any trouble delivering good performance. There were no special safety or concurrency needs beyond what any web-facing service needs to deal with. The only reason we were using Rust was because the original authors of the system were Rust experts, not because it was an especially good fit for building this kind of service.

You will have a hard time hiring Rust developers.

We hired a ton of people during my time at this company, but only about two or three of the 60+ people that joined the engineering team had previous experience with Rust. This was not for want of trying to find Rust devs — they just aren’t out there. (By the same token we were hesitant to hire people who only wanted to code in Rust, since I think that’s a bad expectation to set in a startup setting where language and other technology choices need to be made in an agile way.) This paucity of Rust dev talent will change over time, as Rust becomes more mainstream, but building around Rust on the assumption you’ll be able to hire people who already know it seems risky.

Libraries and documentation are immature.

This is a problem that (I hope!) will be fixed over time, but compared to, say, Go, Rust’s library and documentation ecosystem are incredibly immature. Now, Go had the benefit that it was developed and supported by an entire dedicated team at Google before it was released to the world, so docs and the libraries were fairly polished. Rust, by comparison, has long felt like a work in progress. The docs for a lot of popular libraries are pretty sparse, and one often needs to read the source code of a given library to understand how to use it. This is bad.

Rust makes roughing out new features very hard.

I don’t know about anyone else, but at least for me, when I’m building a new feature I usually don’t have all the data types, APIs, and other fine details worked out up front. I’m often just farting out code trying to get some basic idea working and checking whether my assumptions about how things should work are more-or-less correct. Doing this in, say, Python is extremely easy, because you can play fast and loose with things like typing and not worry if certain code paths are broken while you rough out your idea. You can go back later and make it all tidy and fix all the type errors and write all the tests.

What is Rust good at?

There are definitely things I like about Rust, and features from Rust that I’d love to have in other languages. The match syntax is great. The Option, Result, and Error traits are really powerful, and the ? operator is an elegant way of handling errors. Many of these ideas have counterparts in other languages, but Rust’s approach with them is particularly elegant.



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Matt Welsh

Matt Welsh

CEO and Co-Founder Ex-OctoML SVP, Ex-Google engineering director, Ex-Apple. Systems hacker and drinker of beer.