Update serial_test_derive syn dependency to syn 2

[jtroo]

Syn 2 was released recently and some crates, e.g. thiserror, are already migrated. To reduce clean compile times, should upgrade dependency to syn 2.

[palfrey]

This won't help until tokio-macros upgrades. See https://github.com/tokio-rs/tokio/blob/master/tokio-macros/Cargo.toml#L27

[jtroo]

This is now unblocked as tokio-macros has upgraded 🙂. No pressure to rush on this, just a friendly unblocking notification.

[jtroo]

One thing I noticed in other repos is that syn 2's MSRV is 1.56, so according to the README,

We require at least Rust 1.51. Upgrades to this will require at a major version bump.

Updating to syn 2 will require a major version bump.

[CosmicHorrorDev]

I'm up for giving the update a go if ya want the help @palfrey

[palfrey]

I'm up for giving the update a go if ya want the help @palfrey

No, that's fine. The upgrade here is pretty simple, but I want to write some specific docs about the MSRV bump as I'm planning on bumping it all the way to 1.68.2. This is more than compilation requires right now, but mostly for a future-proofing perspective i.e. reducing the odds of having to release a 3.x when something else bumps.

[palfrey]

@CosmicHorrorDev If you felt like helping this path though, actix-macros needs that work as well but that's a much larger piece. Technically we rely on that similarly to tokio, but given they're both just test dependencies I'm going to do the upgrade here now for the core and fix actix when that's available without needing to do a new release.

[palfrey]

#96 fixes this

[palfrey]

Released as 2.0.0