Updating `syn` to v3 in `zerocopy-derive`

[AudaciousAxiom]

Following the recent release of syn v3, would it be possible to update it in zerocopy-derive (and prettyplease to v0.3), with the aim of reducing dependency duplication in dependents? (I would PR it but I don't want to sign the CLA.)

[xtqqczze]

We’re currently pinned to syn v2.0.56 because of our MSRV. syn v3 raises the MSRV to Rust 1.71, while zerocopy still supports Rust 1.56, so we can’t upgrade yet.

[xtqqczze]

We can do this in zerocopy v0.9.

[AudaciousAxiom]

We can do this in zerocopy v0.9.

Bumping syn to v3 (and prettyplease) will indeed raise the MSRV of zerocopy-derive, and of zerocopy with the derive Cargo feature, but isn't this considered acceptable and not a breaking change by the MSRV policy? My reading of the second paragraph is that this is exactly what it is intended to allow for (and the zerocopy-derive package doesn't have a rust-version key). Or is it because zerocopy-derive is also used as a dev-dependency of zerocopy and because rust-version applies to all Cargo targets (including tests)?

[joshlf]

We can do this in zerocopy v0.9.

Bumping syn to v3 (and prettyplease) will indeed raise the MSRV of zerocopy-derive, and of zerocopy with the derive Cargo feature, but isn't this considered acceptable and not a breaking change by the MSRV policy? My reading of the second paragraph is that this is exactly what it is intended to allow for (and the zerocopy-derive package doesn't have a rust-version key). Or is it because zerocopy-derive is also used as a dev-dependency of zerocopy and because rust-version applies to all Cargo targets (including tests)?

You're right that this would be consistent with our MSRV policy. The only subtlety is that, currently, our syn dep (2.0.46) is low enough that its MSRV is Rust 1.56, and so users can opt to pin to the lowest-supported syn version and continue to receive zerocopy updates. If we were to upgrade to syn 3, such users would no longer be able to upgrade zerocopy. That's technically consistent with our MSRV policy, but it would still be painful for some users, so it's worth some consideration.

[xtqqczze]

I've opened #3498 for the missing rust-version metadata in the zerocopy-derive crate.

[joshlf]

I've opened #3500, which does the same as #3498 but also documents a new MSRV policy: That we will always support some set of package resolutions which satisfy the current 1.56 MSRV. Note that despite this policy, the plan is still to have a different, more relaxed MSRV policy when we release zerocopy 0.9.

@jswrenn How do you feel about this proposed policy? Merging #3500 will have the effect of closing us off from upgrading our syn dep to 3.0, which will affect our ability to support unsafe fields.