Update syn to v2

[CosmicHorrorDev]

This updates syn to v2. Changes involved:

• Removing the Spanned requirement from get_metadata_inner() since it no longer seems needed and Spanned now requires ToTokens to be implemented on the type too
• Changing parse_terminated()'s arguments to take the delimiter instead of passing it with a turbo-fish
• Switching path to the method for Attributes
• Changing the structure of how literals are expressed for meta name-values
• Matching the old structure of how tokenstreams were represented for Attribute's
  • I tried to minimize the changes required to match the old parsing. A Meta::List(_) holds an internal tokenstream which could be used, but it's the tokenstream inside the group instead of the containing token-group itself. Instead I opted for taking the surrounding tokenstream and discarding the leading path to get at the group

[CosmicHorrorDev]

syn bumped its MSRV up to 1.56, so it's expected that the builds back on 1.32 would fail

[smoelius]

Can I please ask what the status of this PR is?

[Peternator7]

The PR is fine from an implementation standpoint. However, I try to maintain two years of backwards compatibility, and rust 1.56 is from 10/2021. Is there an urgency in merging this change?

[smoelius]

I wouldn't say it's urgent. It's more of a housekeeping matter. For one of my projects, this is the last dependency that still relies on syn 1.0.

But I can wait until October.

Thanks very much for your response.

[Peternator7]

Just release 0.25.0 which has this upgrade! Decided it's not that far off from October and it's impractical to maintain a project that's on an unsupported syn version.

[smoelius1]

Thank you!

[martin-g]

Thank you!