`LocalShared` similar to `futures_util`'s `Shared`

[andrewbaxter]

JsValue isn't Send/Sync since #955 . This prevents the use of OnceCell-like things and futures_util's Shared. I may hack up my own in the meantime, but would it make sense to have a similar LocalShared that can't be sent across threads?

I was trying to figure out the best place to ask about this:

• wasm-bindgen has good reasons for making JsValue non sync/send, so probably not there
• I was going to post in futures_util, but
• it felt like LocalShared would parallel spawn_local which is here (edit: wasm-bindgen-futures) and not in tokio/futures

Edit: Oops, I thought wasm_bindgen_futures was a different repo and didn't notice.

[guybedford]

#4770 adds back Send and Sync support for JsValue with that configuration. Would that resolve your use case here?

[andrewbaxter]

Oh yeah!

I'm working in the browser, where IIUC there may be multi-threaded environments at some point, so I think this may be riskier than having local-wrappers? But it'd work for me...