Rust Quiz asks what this program prints, then explains why

Every page is one question and one output

The whole format fits on a screen: a heading asking what the output of a Rust program is, a short listing, and multiple choice answers. The published site lives at dtolnay.github.io/rust-quiz, and the README's entire pitch is that single question.

The contributing guide prefers questions that compile and are well-defined, rather than questions about compilation or undefined behaviour; it frames a fun question as choosing between obvious outputs A and B. That constraint comes straight from the contributing guide, which lists what is not fun: asking whether a program compiles, or whether behaviour is undefined. Fun, in this project's terms, is whether the program prints obvious possibility A or obvious possibility B.

The result is a quiz you can answer without touching a compiler, which is what makes it usable on a phone or during a screening call, and also what makes it different from a test suite. Nothing here verifies your answer against a real compiler, so a question written years ago can drift from current behaviour.

The house style is narrow on purpose

The contributing guide is the most detailed document in the repository, and it reads as a style guide rather than a legal document.

Questions should be no more than 25 lines including blank lines, with shorter preferred. Up to 35 lines may be accepted only when there is no way to frame the same idea more concisely. Width is capped at 40 columns, with the only exception being boilerplate that is not necessary to read in order to solve the question.

The stated source of good questions is personal experience, writing or reading Rust code and being bewildered by its behaviour. That origin story matters for judging the catalogue: these are the questions that surprised one experienced Rust programmer, which biases the set toward the parts of the language where the rules are subtle rather than the parts everyone learns in the first week.

The README also points at cppquiz.org as the inspiration for the project, for readers who know C++.

Each question is a .rs file plus a .md file

Adding a question means adding exactly two files under the questions/ directory, a Rust file and a Markdown file. Naming uses a brief two to four word description, and the number in the filename must be 000 to avoid collisions between concurrent pull requests; the maintainer assigns the real number when merging.

The Markdown file has a required structure, and the guide says to copy an existing one. The first line is the correct answer, the second line is a difficulty rating of 1, 2 or 3, and there must be a Hint section and an Explanation section. The rating is meant to reflect how obscure the required knowledge is rather than how hard the code looks.

Hints have their own limit: a maximum of three lines, with two sentences considered ideal. Explanations have the opposite instruction, be as thorough as possible, but without dwelling on concepts irrelevant to the crux. That asymmetry is deliberate and produces the site's best reading experience, since the hint has to be short enough not to give the answer away and the explanation has to answer the follow-up question properly.

Previewing locally packages the questions into one file

To see a question rendered before opening a pull request, you run the site locally from the repository root:

cargo run -- serve

That packages all the questions into a single JavaScript file and serves the site over HTTP on port 8000. A question you numbered 000 is then reachable at http://localhost:8000/rust-quiz/0.

The build is a real Rust binary rather than a static site generator script. Cargo.toml names the package rust-quiz at version 0.0.6, edition 2021, with publish = false and an exclude entry for the rust-quiz binary, so it is a tool for the site rather than a library anyone depends on.

The dependency list explains the architecture: hyper and hyper-util with http1, http2 and server features, hyper-staticfile for serving assets, pulldown-cmark for Markdown rendering, tokio with the full feature set, rayon for parallel work, oqueue as a queue, clap for the CLI, serde and serde_json for the packaged question data, and thiserror plus remain for error handling. num_cpus is there for sizing the parallelism.

A JavaScript linter in a Rust repository

The repository root has a package.json, and it is not part of the site build:

{
  "devDependencies": {
    "@eslint/js": "^9.19.0",
    "eslint": "^9.19.0",
    "globals": "^15.14.0"
  }
}

Other root entries include.gitlab-ci.yml alongside.github/, so there is CI configuration for two forges, plus a.cargo/ directory, docs/, src/ and the rust-quiz binary.

Licence, cadence and the limits of a quiz

Everything in the repository, questions, explanations, website and other intellectual property, is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License. That is unusual for a code project, though Cargo.toml also sets the same identifier as the package license.

The practical consequence for reuse is that you may copy and adapt the questions with attribution provided, and that adaptations must be shared under the same licence. Fine for study material, worth knowing before you build an internal quiz from it.

Cadence has slowed. Releases are named after questions rather than versions, and the three most recent are question35 and question36 both dated 2022-09-26, then question37 on 2025-01-25. The last push was on 2026-07-30, so the repository is not abandoned, but new questions are rare.

Two limits follow from all this. A question is written against the Rust of its moment, so behaviour around newer language features may not be what the answer says. And a quiz verifies reasoning, not code: nothing here will tell you whether your compiler agrees.

Editorial conclusion

Read Rust Quiz if you want a working test of Rust semantics rather than a reference manual, and submit a question if you have one that puzzled you in real code. The site is useful for interview preparation and for catching habits carried over from C++. Verify first that the question you are looking at matches your compiler version, since the repository has not added a question since 2025-01-25.

Frequently asked questions

Official sources

1. dtolnay/rust-quiz on GitHub
2. License: CC-BY-SA-4.0
3. Project website
4. README
5. Releases