Matthias Krüger 22e62a4fca
Rollup merge of #106944 - Nilstrieb:there-once-was-a-diagnostic, r=WaffleLapkin
Suggest using a lock for `*Cell: Sync` bounds

I mostly did this for `OnceCell<T>` at first because users will be confused to see that the `OnceCell<T>` in `std` isn't `Sync` but then extended it to `Cell<T>` and `RefCell<T>` as well.
2023-01-25 22:19:52 +01:00
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