Rust compiler support works differently

Posted Dec 16, 2025 14:29 UTC (Tue) by laarmen (subscriber, #63948)
In reply to: Rust compiler support works differently by taladar
Parent article: The state of the kernel Rust experiment

The whole "newer rustc is compatible with older code" isn't actually true. In practice, what holds is that at the time of its release, a new version of rustc will be able to compile the latest version of every crate on crates.io, as demonstrated by their crater runs. That's an impressive feat, to be sure. And from the standard dev perspective, both notions are fairly identical, as keeping your dependencies up to date is a common practice, and you don't have that many of them anyway.

Your LTS distro packages a *lot* of crates, mostly outdated, and you can't update them easily. In that setting, bumping the version of the default Rust compiler suddenly becomes much riskier.

to post comments

Rust compiler support works differently

Posted Dec 16, 2025 14:34 UTC (Tue) by farnz (subscriber, #17727) [Link] (1 responses)

I've dug out some rather old code (written for 1.24, based on the rust-toolchain file), and it still builds quite happily with current nightly.

Do you have examples of crates that were written for older versions of rustc, didn't use unstable features, but that don't work with current rustc? I'm curious to see what's actually broken - I know about the changes to std::env::set_var, for example, and I'm wondering what else has broken over the last decade or so.

Rust compiler support works differently

Posted Dec 16, 2025 14:48 UTC (Tue) by laarmen (subscriber, #63948) [Link]

Yes, most old code will compile without issues. AFAICT, the Rust compatibility story is one of the best out there. The only specific example that comes to mind right now is Rust 1.80 vs `time`, but that one was special in that it was actually detected and disregarded. However, I do remember reading several times in Rust changelogs things like "We detected XX crates that failed to compile with this change and worked with their authors to get a new, fixed version out".

Rust compiler support works differently

Posted Dec 16, 2025 15:03 UTC (Tue) by pizza (subscriber, #46) [Link] (2 responses)

> And from the standard dev perspective, both notions are fairly identical, as keeping your dependencies up to date is a common practice, and you don't have that many of them anyway.

...."hundreds" isn't "that many" ?

(this anectdata is brought to you by every Rust-based thing I have deployed right now)

Rust compiler support works differently

Posted Dec 16, 2025 15:23 UTC (Tue) by laarmen (subscriber, #63948) [Link]

Right, my bad, I should have qualified: not that many *compared to the entirety of crates.io*, meaning the odds of one of them being both out of date and incompatible are fairly low.

Don't take this as an endorsement of the sprawling dep trees of some Rust projects :-)

To give some hard numbers, Rust 1.85 in Ubuntu vendors 672 crates. Since that directory contains dependencies for the entire toolchain, including cargo and rustdoc, I think they're on the conservative side of the ecosystem.

Rust compiler support works differently

Posted Dec 17, 2025 9:02 UTC (Wed) by taladar (subscriber, #68407) [Link]

Hundreds of crates is probably still significantly less code than e.g. one Qt.