Emitting metadata early makes building/checking Rust up to twice as fast

There’s an interesting comment thread (including potential downsides) on this technique in the submission for “How to speed up the Rust compiler in September 2026” from a couple days ago: https://news.ycombinator.com/item?id=49923594

Huh, I kinda thought something like this was already being done, but perhaps it was a bit later stage. Neat!

A kind of related shower thought I had, could rustc not instantiate generic methods, but write out what it expects to find (std::Vec<String>::push), and a separate build process listens to these, but makes sure already built ones are not built multiple times from the whole build.

I don’t actually know how much of duplication there usually is, but I had gotten the impression that it would be part of the problem.

Of course I’m no compiler engineer, and I’m sure there are complications like per crate build profiles, etc.

Excited to see if there is a path to getting this in the mainline compiler

So this is like a cache? Reminded me of https://turborepo.dev for TS, is it a similar concept?

Woah nice thought! :) good idea

Those are big improvements!

As an aside, it's just silly to make the repository a bunch of checked-in patches. Git already does revision control. You don't need to do revision control in your revision control.

[flagged]

Seems like an easy win! I'm kind of surprised nobody did this already. I guess someone will need to reimplement this by hand given Rust's AI policy, but this is still great because it demonstrates that it's a really good idea.