cargo-semver-checks v0.37 can now scan Cargo.toml files for breakage! In this post: a primer on Rust package features, and how innocuous-looking Cargo.toml changes can break your users.

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cargo-semver-checks v0.35 can determine whether Rust traits are "sealed", allowing it to catch many tricky new instances of SemVer breakage. Why is accurate sealed trait detection so important, and why is implementing it correctly so hard?

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In 2022, I gave a talk at a virtual conference with an unforgettable name: HYTRADBOI, which stands for "Have You Tried Rubbing a Database On It?" Its goal was to discuss unconventional uses of database-like technology, and featured many excellent talks.

My talk "How to Query (Almost) Everything" received copious praise. It describes the Trustfall query engine's architecture, and includes real-world examples of how my (now-former) employer relies on it to statically catch and prevent cross-domain bugs across a monorepo with hundreds of services and shared libraries. For example:

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Happy April 1st! This post is part of April Cools Club: an April 1st effort to publish genuine essays on unexpected topics. Please enjoy this true story, and rest assured that the tech content will be back soon! That's what my dad said when I asked what was wrong with our home internet connection. "… read more

Last month, I gave a talk titled "SemVer in Rust: Breakage, Tooling, and Edge Cases" at the FOSDEM 2024 conference. The talk is a practical look at what semantic versioning (SemVer) buys us, why SemVer goes wrong in practice, and how the cargo-semver-checks linter can help prevent the damage caused … read more

My last post covered the key cargo-semver-checks achievements from 2023. Here are the biggest challenges that lie ahead!

Many of the remaining challenges in cargo-semver-checks are obvious: we all want more lints, fewer false-positives, etc. etc. Let's set those aside.

Instead, let's talk about four non-obvious challenges we have yet to tackle:

• The Obvious in Retrospect
• The Blocked Upstream
• The Surprising Limitation
• The Existential Threat

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2023 was a big year for cargo-semver-checks! We saw ecosystem-wide adoption in projects of all shapes and sizes: the tokio and PyO3 ecosystems, company-backed OSS projects from companies like Amazon and Google, and even in cargo itself. Here's a look back at the highlights of 2023!

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cargo-semver-checks v0.25 squashes nearly all bugs related to doc(hidden) items — its most common source of false-positives. What does doc(hidden) mean in Rust, and why was handling it correctly so hard?

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This post is coauthored by Tomasz Nowak and Predrag Gruevski. It describes work the two of us did together with Bartosz Smolarczyk, Michał Staniewski, and Mieszko Grodzicki. Anecdotally, cargo-semver-checks is a helpful tool for preventing the semver violations that every so often cause ecosystem-wi… read more

A few days ago, I started polls on Mastodon and Twitter whether adding a new private type, or an import, can ever be a major breaking change. The consensus was that this should be impossible.

I agree with that. It should be impossible.

I've discovered a way to cause a previously-public type or function to disappear from a crate's public API by making innocuous-seeming changes like adding a private type or adding an import, etc. It is not a hypothetical problem, either — I've found at least one real-world Rust project that has been affected by it.

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For the longest time, I thought that "sealed trait" in Rust was a singular concept implementable in one specific way. To prevent downstream crates from implementing your traits, you make the traits sealed — done, end of story. I was wrong! It turns out there are multiple ways to seal traits, forming a pleasant spectrum of options:

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Happy April 1st! This post is part of April Cools Club: an effort to publish genuine posts on topics our usual audience would find unexpected. The tech content will be back soon! Over the many years I spent heavily involved in intern and full-time recruiting at $PREVIOUS_JOB, multiple people have co… read more

We've already explored some of the dark corners of Rust semantic versioning on this blog:

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This post describes work in progress: how cargo-semver-checks will benefit from the upcoming query optimization API in the Trustfall query engine. Read on to learn how a modern linter works under the hood, and how ideas from the world of databases can improve its performance.

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I recently embarked on a quest: revamp the cargo-semver-checks import-handling system so that moving and re-exporting an item stops being incorrectly flagged as a major breaking change. This is how crate authors can reorganize or rename items: just re-export the items in the original location under … read more

I've been saying for a while now that semantic versioning in Rust is tricky and full of unexpected edge cases. My last post mentioned that some Rust structs can be converted into enums without requiring a major version bump. It introduced a non-exhaustive struct called Chameleon that had no public f… read more

EDIT: The Rust API evolution RFC distinguishes between breaking changes and changes that require a new semver-major version (called major changes). All major changes are breaking, but not all breaking changes are major. Changing a struct to an enum is always breaking (as pointed out on r/rust) but i… read more

cargo-semver-checks ends 2022 with 40,000 downloads from crates.io, able to prevent 30 different kinds of semver issues, and having done so in real-world use cases. Inspired by Yoshua Wuyts' "Rust in 2023 (by Yosh)" post, here are my thoughts on cargo-semver-checks in 2022, and what I look forward t… read more

Undefined behavior (UB) is a tricky concept in programming languages and compilers. Over the many years I've been an industry mentor for MIT's 6.172 Performance Engineering course, An excellent class that I highly recommend. It's very thorough and hands-on, at the expense of also requiring a lot of… read more

I had a lot of fun spending nights-and-weekends time participating in the HYTRADBOI Jam, a global hack week aimed at building "exciting and weird" data-centric solutions to familiar problems. The name HYTRADBOI might sound familiar: the jam is associated with the same conference where I gave my "How… read more

The saying usually goes: "If at first you don't succeed, try, try again." But in the Safari web browser under the right conditions, trying again after succeeding once can get you in trouble. This is my recent debugging adventure. As requested on Twitter. While building Trustfall Playground, I noti… read more

I recently built cargo-semver-checks, a linter that ensures crates adhere to semantic versioning. This is why and how I built it.

Fearless development is a key theme throughout Rust. "If it compiles, it works", fearless concurrency, etc.

But there's one aspect of Rust (and nearly all other languages) that isn't entirely fearless yet: cargo update, upgrading the versions of the project's dependencies.

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A beginner-friendly introduction to compilers: follow along as we build a compiler from scratch, or fork the code on GitHub and add your own optimizations too! In this episode: value numbering helps track how values are used in the program. Last time on Compiler Adventures, we implemented constant p… read more

Happy April 1st! A group of folks and I decided to do something different this year: instead of publishing fake things, we’re publishing real posts on very different topics than our readers usually expect from our blogs. The tech content will be back soon! Check out the other April Cools posts here.… read more

A beginner-friendly introduction to compilers: follow along as we build a compiler from scratch, or fork the code on GitHub and add your own optimizations too! In this episode: propagating constant values to eliminate more instructions. Last time on Compiler Adventures, we wrote an optimization that… read more

A beginner-friendly introduction to compilers: follow along as we build a compiler from scratch, or fork the code on GitHub and add your own optimizations too! In this episode: eliminating no-op instructions. What part of computer science feels most like arcane magic? I'd say compilers. "The magical… read more

Distributed systems courses frequently introduce the Paxos and two-phase commit (2PC) protocols in quick succession. On one hand, this is a reasonable educational choice, as Paxos and 2PC are both examples of consensus algorithms used in the real world. On the other, it leads many students toward th… read more