syn is a parser for Rust source code, most frequently used in procedural macros.
It got a v2 release some time ago and more and more other crates are switching over to that.
UniFFI, our Rust bindings generator, also has plans to upgrade.

Right now we only have syn v1 in-tree.
As we currently disallow duplicate crates it would be rejected unless we explicitly allow it (by modifying TOLERATED_DUPES).

Are we okay with pulling in syn v2?
(Actually pulling it in will happen later, only once UniFFI actually merges said PR, does a release and we upgrade a-s and Glean to that)

This sounds like a glandium question...

Let me give you a definite answer to this next week. I'm evaluating the situation.

Ok. Let's do this, but please update the askama dependency for it to pick syn2 too. I'll start by adding syn2 in tree alongside an upgrade of serde after the merge. I have local patches that get me to this point:

$ grep -c "syn 1" Cargo.lock 
15
$ grep -c "syn 2" Cargo.lock 
30

(and that's without uniffi/askama, which would make it 11/34)

Let's track progress with this bug, actually.

It's the last crate with a remaining dependency on syn v1. The
transition is over.

https://hg.mozilla.org/mozilla-central/rev/20af9af06587