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Comment by steveklabnik | Hacker News Reader

TL;DR: don't underestimate the power of technical debt.

Longer answer: The Rust compiler was written in Rust before the Rust language was even what it was today. As one particular example, the AST used to be garbage collected! And when you have a codebase that's 320,000 lines long, you don't update the entire codebase to the latest idiom, because if you did, you'd never actually ship the language.

Basically no other Rust program will ever have to contend with this kind of situation. As such, while I can sympathize with this line of argument, I don't think it's representative.

EDIT: I previously had 750,000 but it's actually 323,326 LOC as of right now. Whoops!

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Legacy code and inexperience with new technologies are not problems that are unique to Rust and the Rust compiler. And presumably Rust will continue to evolve, will it not? If my team starts a large, multi-year project using Rust 1.1 today, does Rust offer anything that will help us avoid the sorts of crashes that the Rust compiler suffers from?

Your language changing literally every other day is a problem unique to a pre-1.0 language. You're right that technical debt in general is not.

> And presumably Rust will continue to evolve, will it not?

Right, but it will evolve in a backwards-compatible way, rather than in a "the fundamental semantics of the language have changed Yet Again" kind of way.

Just to expand on what Steve said.

An evolving language is one thing. Pre-1.0 was more like many stages of violent metamorphasis. Some changes to the language required changing almost every line of code in the tree. Normal technical debt is when you find an old approach doesn't work any more and you need to fix it. Much of rustc's technical debt is from things like features disappearing from underneath it, or things designed before the features that you would use now even existed.

Looking at rustc as a regular large Rust project is unfair. A new project can at least assume that the features you are using today will exist tomorrow and design with that in mind. Your designs may become obsolete due to new features, but you aren't going to have to pick between a hacky workaround and a redesign/rewrite of 1000's of lines of code.

Sure, Rust will continue to evolve. But many changes that happened in the past were on the scale of "Java 10 decides to get rid of classes".

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