Let's write a macro in Rust
48 comments
·October 10, 2025TheSoftwareGuy
For those interested, this blog post also has a part 2 and 3:
gnatolf
Which confusingly are linked to on the 'prev' link at the bottom of each part...
the__alchemist
I like using or exposing macros when appropriate. They can simplify messy syntax, or prevent repetitions not allowed by the compiler directly. I can't get the syntax for writing them to stick. So, I am happy with letting LLMs handle this.
Sometimes the macros are for large chunks of code that uses a different type in a key place, or sometimes they're for cleaning up verbose function calls that will be made multiple times, so the important parts (e.g. the params) are obvious, instead of being mixed with boilerplate. In general, they're nice for papering over boilerplate. In particular, there's a pattern in embedded that involves a long expression with `try_into().unwrap()` for parsing bytes that I have a macro for. And another for managing RefCell<Mutex<Option>>> locks.
I prompt the LLM with the working, but repetitive or boilerplate-laden code, and it macros it: "Please write a macro that stops the repetition in this code block: ```rust ```. Here's a example of calling it: `do_thing!(a, b, c)`
Works every time!
afavour
> I can't get the syntax for writing them to stick
Oh man, same. I wrote a macro crate then went back to it months later and couldn't make heads nor tails of anything I wrote. Something about the syntax just doesn't stick in my brain.
iambacktomyword
True, macros in Rust can feel heavy, but they also enable some powerful abstractions you just can’t get otherwise. I guess the “don’t write macros” advice is more about avoiding overuse than rejecting them completely.
kachapopopow
The only macro I ever use is json! or for applying 10 derives to more than 1 struct
stephenlf
vec!? println!?
kachapopopow
those are language macros iirc (hardcoded in the language) hence I consider those the same as when, fn etc.
adregan
One neat thing about the referenced chapter on macros from Practical Common Lisp is that it's chapters 7 and 8 of a 30+ chapter book. You're only learning about variables in the chapter before and then macros. I like to think about that when conversations about macros always devolve into talking about how you should never use macros.
Now apologies for the aside—and I know I'm likely wading into a very expansive topic and reducing it down to something simple—but why in Rust, if the types are known:
struct Song {
title: String,
artist: String,
rating: i64,
}
Song::new("Hate Me".to_string(), "Blue October".to_string(), 9)
LtdJorge
Because to_string allocates. And if a function requires a String (owned), it cannot accept a str reference (borrowed), it would defeat the purpose of the strong type system. String is moved, while str is passed by reference.
There is the exception of Deref. If the function requires type A, and you pass it type B, which Derefs into type A, the compiler will Deref it for you. But that is zero cost and panic free, whereas allocating (and copying) an owned type from a reference isn't. In Rust you have to be explicit.
Anyway, using String in function signatures is most often not the best choice. If you will internally be required to use a String, it's better to ask for a type "impl Into<String>", you'd call into() inside your function. And in the most common case, where you require a &str or can convert to your type from it, the best choice is an "impl AsRef<str>" and you can call as_ref() on it to get a str reference, which you can wrap in a Box, Rc, Arc, String, or your custom type, or pass it directly to other functions. All of those, Box<str>, Rc<str>, etc implement both traits.
Using impl Trait, you avoid having to declare generic parameters.
steveklabnik
They’re different kinds of strings, the String string means heap allocation, and Rust never allocates in the language, so the compiler automatically invoking allocation routines for you wouldn’t be good, in Rust’s view.
tialaramex
Those "things that look at a lot like strings already" are literals and thus constants, with the type `&'static str` which is how Rust spells an immutable reference to a string slice which lives forever. So, Rust is promising that the string slices "Hate Me" and "Blue October" exist, and in practice probably if you look inside the resulting executable it says "Hate MeBlue October" or similar.
On the other hand the String type is a growable array of bytes used to assemble strings, it's an owning type, so Song will own those two Strings, they can't go away or be altered via some other force. Owning a string would allow you to add to it, change it, or discard it entirely, you obviously can't do that to the text baked into the executable, so if you want to own a String it will have to be allocated at runtime.
You can also write "Some Words".to_owned() for example to the same effect.
estebank
To expand on the sibling comments, this is why Rust doesn't do implicit type conversions that could allocate:
https://groups.google.com/a/chromium.org/g/chromium-dev/c/EU...
rootnod3
To be fair, as much as Rust macros are nice, it is a losing battle to go up against the Lisp macro system. Lisp might have its flaws, but macros in Lisp are second to none.
When to use them is a whole different story. But examples of macros I like are `when` and `unless`. Yes, simple, but they show a nice example of their power.
For more complicated once, love it or hate it, but the `loop` macro is probably THE prime example of a powerful macro.
QuantumNomad_
When you have a &str (like "Blue October") and pass it to something that wants a String, you can do .into() instead of .to_string()
It’s shorter to write and takes up a little less space on screen to. I almost always use .into() when like in your example I initialize String field members of a struct in Rust.
marcosdumay
That's the price you pay for a low level language where things like memory usage are visible for you to optimize.
If you don't see the gain, maybe Rust is not the right language for your use-case.
throwawaymaths
the reasons to not use macros have to do with hygeinicity? -- macros can do things like introduce code that is hard to understand, perhaps a dynamically named function, or a difficult to chase dependency or import... moreover these can make grepping harder.
BetaDeltaAlpha
There are so many typos in here it makes it difficult to focus on the content. Is this a deliberate signal that the article was human written?
I learned a lot regardless and I'm adding this to my blogroll.
127
Having done some Rust macros, the entire thing is such a huge hidden wart. Even very simple meta-programming in Rust need to be in their separate crate and has to use very complex syntax and invocation.
Also the whole "don't write macros" is such a hilarious statement given that entire Rust ecosystem is built on them.
airstrike
I agree Rust macros are cursed in many different ways but
> given that entire Rust ecosystem is built on them.
I don't think the people saying "don't write macros" are the same people building an entire ecosystem on top of them
The iced crate has virtually zero macros by design.
ameliaquining
Macros by example, which is what this post is about, don't have to be in a separate crate.
pjmlp
It is its own can of worms, and maybe it is due to me using the language on and off since 1992, but I find what is possible in C++23, much easier for metaprogramming as the few times I had to understand trying to implement Rust macros.
C++26 reflection will make this much easier, without having to depend in stuff like the syn crate.
I wonder if going with two macro systems, each with its own syntax and approach, was such a good idea.
spoiler
Eh. I agree there's complexity in them, and that a lot of the ecosystem indirectly use them because of derive macros, but I wouldn't call it a wart, and the "syntax" for writing proc macros isn't that bad if you use the syn/quote crates. I agree that decl macros usually make me sigh and open the docs though
nicoburns
> I agree that decl macros usually make me sigh and open the docs though
Declarative macros are very like regex's: both in that at first they seem incredibly dense and arcane but once you work out how to read them they're actually very simple, and in that the syntax is literally similar in that it's a list of tokens that must match sequentially.
pjmlp
If regexp was simple there would not exist universities dedicating a semester to its theory and various implementation algorithms, alongside a famous quote. :)
IshKebab
Maybe "don't write your own macros". I guess it's like C++ template meta programming. If you find yourself using that a lot, it's probably a bad sign. But also Eigen is pretty great.
TinkersW
I dunno, the C++ template implementation of this macro would be far easier to read as it is just normal C++.
IshKebab
C++ templates are totally different feature. You couldn't do this with C++ templates.
It was just an example of something that gets very hairy for library implementers but is still quite nice for users.
Communitivity
Bjarne Stroustrup said "C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do it blows your whole leg off". The same is true of Rust macros. When you need them they're awesome, but you should almost never need them - add a new macro as a very last resort.
galangalalgol
I agree with this, mostly because all the macros I would use are mostly written already. If const generic expressions arrive, a lot of those could go away too. In languages like lisp or zig where metaprogramming is a central feature, what do they do differently to make it better? Would those things have worked for rust?
jacquesm
That's a hilarious quote. Thank you.
tcfhgj
> The same is true of Rust macros.
can't follow
stared
`macro_rules!` - I always read “rules” as a verb.
darkest_ruby
Omg, was it not a verb all this time??!
iambacktomyword
Yeah, Rust’s macro system feels like a double-edged sword — amazing power, but with a big learning curve. The tooling and syntax definitely make simple meta-programming harder than it should be.
tracker1
I liken it to a bit less extreme a version of the "don't write your own cryptography." Macros are definitely useful, and they are pretty widely used and adopted for several things. That said, actually creating your own tends to take a slightly different mindset than general application development.
iambacktomyword
Interesting points — I always thought macros were core to Rust’s design philosophy. Do you think procedural macros will ever get simpler, or is the complexity kind of intentional for safety?
null
So, this article is writing a "by example" macro, and my impression is that many comments here are thinking about Rust's procedural or "proc" macros and most particularly its entirely general procedural macros which are indeed arbitrary code executing in the compiler. That's not what the article is about at all.
"by example" macros are actually just a fancier version of templating systems I'd expect most of you have used. Their biggest fancy feature is probably repetition, it's easy for Rust's macro to say if we've got a list of N things, we're going to emit this same code N times, but with each thing in turn filled out.
The syntax doesn't look that much like the rest of Rust but that's mostly an ergonomic consideration, so you can distinguish between your macro and the code your macro is spitting out. In fact "at least the syntax is the same" is one of the few ways the procedural macros are simpler, since they're literally some Rust run by your compiler.