Newest `proc_macro2` version does not work with kani-0.49.0

[tautschnig]

See https://github.com/model-checking/kani/actions/runs/8683193890/job/23808816781:

1. It seems that, on this occasion, a manual PR with cargo updates will be required.
2. We need to fix the CI job to make sure that compilation failures do not trigger an early abort, but instead automatically cut an issue.

[tautschnig]

Actually it seems that, even without any cargo update, suddenly various build actions fail: https://github.com/model-checking/kani/actions/runs/8692459550. No idea what changed here.

[joshlf]

We ran into this too: https://github.com/google/zerocopy/actions/runs/8695299265/job/23846059719?pr=1071

[celinval]

See this thread for the details: https://users.rust-lang.org/t/cannot-compile-simple-tokio-example/109956/3

But basically our nightly version matches the latest released Rust compiler, which has now stabilized proc_macro_byte_character feature. The proc_macro build script checks the Rust compiler version to see whether it needs to enable this feature.

I believe there are three different fixes we can apply:

1. Update our toolchain version, which will hopefully have that property stabilized.
2. Change Kani to enable the feature by default for now.
3. Downgrade the proc_macro version.

[tautschnig]

But I'm still with https://users.rust-lang.org/t/cannot-compile-simple-tokio-example/109956/6: what has changed? Kani neither changed its toolchain version nor the cargo dependencies. So what proc_macro version would be downgrading to when we had something that worked until a couple of hours ago?!

[celinval]

The proc_macro crate has a new minor version which includes this change. When building a project with cargo without a Cargo.lock file, cargo will download the latest minor version that matches your dependencies specification.

To go around this issue, an user can either ensure their build are locked via Cargo.lock or specify an exact version in their Cargo.toml. Any version previous to the change I just pasted should do the trick.

[celinval]

Note that the toolchain upgrade will fix this issue as expected.

[celinval]

@joshlf is this something that is blocking you?