CI job lowest versions is broken

[konstin]

The CI job lowest versions is broken on dev, it's now failing when it didn't use to: https://github.com/pubgrub-rs/pubgrub/actions/runs/16957511945/job/48412686398

[mpizenberg]

Do you have an idea of what could be wrong? Seems like a serde round trip test is failing. BTW, it feels a little weird to be testing old version, but still need nightly to run it? Is nightly needed for the cargo test command also, or just for the cargo update?

[konstin]

I haven't yet gotten around to investigating the problem. For the Rust nightly, it's cause the Cargo feature that let's us use minimal versions is on unstable (https://doc.rust-lang.org/cargo/reference/unstable.html?highlight=minimal#minimal-versions), so we need a nightly cargo to do the lowest version resolution.

[nataziel]

I'm 99% sure this a bug in nightly. I used cargo bisect-rustc after running cargo +nightly update -Zminimal-versions and it pointed to this commit rust-lang/rust@425a9c0

In the short term you could change the CI script to run the tests against stable rust after using nightly to downgrade the lockfile:

cargo +nightly upgrade -Zminimal-versions

cargo +nightly build --all-features --workspace

cargo test --all-features --workspace

It seems that running the build with nightly doesn't seem to cause the issue, only building/running the tests.

edit: further investigation found that even without downgrading the lockfile the error occurs when using nightly

[nataziel]

rust-lang/rust#145873

[nataziel]

Turns out this is caused by a hack used by ron being broken by an internal compiler change in nightly: ron-rs/ron#579

That issue is now patched awaiting release.

For pubgrub I think the fix is still to run the tests on stable after using nightly to downgrade to minimal versions, but if the internal compiler change is released to stable rust you will need to specify a lower bound on ron with the fix