The Anatomy of a 500ns Parser: Porting libphonenumber to Rust

It started with a freelance project. I was writing a backend service in Rust and needed to validate international phone numbers. Like any Rust developer, I headed to crates.io and pulled the most popular library for the job. Then, I opened their GitHub issues. What I saw was a graveyard of long-standing, unhandled bugs. "National part of the number is truncated in some cases" (Open). "Numbers starting with the same sequence as country prefix are parsed incorrectly" (Open). "00-prefixed international numbers don't parse" (Open). Porting Google's massive C++ libphonenumber library is an incredibly complex task, and I deeply respect the authors of that crate for undertaking it. But I couldn't ship my client's project with those bugs. So I decided to do something slightly crazy: I was going to write a bug-to-bug compatible port of the C++ libphonenumber, built from the ground up for maximum performance. Here is the story of how making the library progressively "dumber" about regular expres