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Analysis of various tricky Rust code
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Case studies

This repository showcases some examples of tricky Rust code that I have encountered during my years working with a variety of advanced macro libraries in Rust (my own and others').

📯 The point

This project is dedicated to the one profound insight about Rust macro development: the difference between someone who is competent with macros vs an expert at macros mostly has nothing to do with how good they are "at macros".

90% of what enables people to push the limits of possibility in pursuit of a powerful and user-friendly macro library API is in their mastery of everything else about Rust outside of macros, and their creativity to put together ordinary language features in interesting ways that may not occur in handwritten code.

You may occasionally come across Rust macros that you feel are really advanced or magical. If you ever feel this way, I encourage you to take a closer look and you'll discover that as far as the macro implementation itself is concerned, none of those libraries are doing anything remotely interesting. If it is a procedural macro, they always just parse some input in a boring way, crawl some syntax trees in a boring way to find out about the input, and paste together some output code in a boring way exactly like what you would learn in a few hours by working through any part of my procedural macro workshop. If it is a macro_rules macro, everything is conceptually just as boring but when stretched to its limits it becomes a write-only syntax that poses a challenge for even the author to follow and understand later, let alone someone else not already fluent in the basics of macro_rules.

To the extent that there are any tricks to macro development, all of them revolve around what code the macros emit, not how the macros emit the code. This realization can be surprising to people who entered into macro development with a vague notion of procedural macros as a "compiler plugin" which they imagine must imply all sorts of complicated APIs for how to integrate with the rest of the compiler. That's not how it works. The only thing macros do is emit code that could have been written by hand. If you couldn't have come up with some piece of tricky code from one of those magical macros, learning more "about macros" won't change that; but learning more about every other part of Rust will. Inversely, once you come up with what code you want to generate, writing the macro to generate it is generally the easy part.

👢 Focus

Yes, these case studies are drawn from use cases that arise from work on macros, but the macros are never the interesting part. The ingenuity and sophistication always lie in what Rust code ultimately gets emitted by the macro, and I think you will find that those are fully possible to appreciate even if you know nothing about macros.

To that end, I make an effort to minimize the role of macros in these case studies. For each one I give only enough context about the relevant macro to explain a set of constraints that the generated code will need to comply with. The focus is on the generated code, which somehow solves the constraints using a clever combination of Rust language features unrelated to macros. Lastly and least importantly, I tie it back to the macro to point out that making a macro produce the generated code we came up with would be the easy part.

Read and enjoy; I hope you find these an enlightening window into this corner of Rust that has so far not been put into words.

🎃 Case studies

Function epilogue
Topics: borrow checker, no_std, closures, lifetime elision
Multiple of 8 const assertion
Topics: diagnostics, name resolution, const evaluation, traits
Unit struct with type parameters
Topics: namespaces, glob imports, layout optimization, autotraits, documentation
Read-only fields of mutable struct
Topics: deref coercion, borrow checker, repr, unsafe code, documentation
Consecutive integer match patterns
Topics: macro_rules, const
User-defined callable types
Topics: deref coercion, closures, trait objects, repr, unsafe code


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