Wednesday, April 12, 2023

Improving build times for derive macros by 3x or more

I was dissatisfied with argument parsing crates. There didn't seem to be one that checked all the boxes I cared about:

  • Simple. Don't overcomplicate it! All I want to do is provide a rudimentary human interface to my app. If I wanted interface complexity, I'd put a whole TUI library in it.
  • Don't make me repeat myself, and don't make me write unnecessary boilerplate. The only thing a parser needs to do is compare given args to a known list and populate some state based on it. It shouldn't require adding a bunch of attributes to define how that state is populated, and it shouldn't require writing the application "help text" that describes the arguments separately from struct declaration.
  • Correctness. If I give my app invalid UTF-8 and it panics or returns an error inappropriately, I'm going to be furious.
  • Small! I once used clap as a young Rustacean and a third of my binary size was for parsing arguments! The app was fairly sophisticated, doing things like parsing ELF files. A whole third of the size of the binary was only used in the first few milliseconds of the app's entire runtime. (I may be exaggerating a bit, and I can't find any evidence to back the claim. The very first commit in the project in question doesn't contain any argument parsing library. No doubt due to the experience described.)
  • Fast! Build times are terrible for all arg parsing #[derive] macros that I've seen. Even on my relatively modern/high-end desktop (with a Ryzen 9 5900X running Windows 11 and Ubuntu 22.04 in WSL2), the fastest derive crate (argh) takes 2.15 seconds for the initial build. For comparison, the fastest all-around crate is pico-args at 370 ms.

There's a list of popular argument parsing crates along with various benchmarks at These results sparked my curiosity. If we eliminate all crates that don't support derive and invalid UTF-8 from this list, we're left with just two options: bpaf_derive and clap_derive. Both of these fail the "small" and "fast" requirements, though bpaf_derive is the better of the two.