What problem does this solve or what need does it fill?

Our requirements are too strict. This stops us from using types which are Send but not Sync in components, such as Cell<T>.

What solution would you like?

Remove the bound of Sync on component, and audit all the methods of World to ensure that any which go from &World->&T require T: Sync (primarily the SystemParam impl for &'_ T).

Note that the impl for &'_ mut T would not require Sync, since we guarantee that this reference is only on one thread (since the resultant Mut<T> can be used to get an &mut T directly).

What alternative(s) have you considered?

Keep things as they are - we haven't had a need for this yet, and it seems unlikely that we ever will.

However, philosophically there's no reason to have these bounds, and it strictly increases our usefulness.

Additional context

https://doc.rust-lang.org/nomicon/send-and-sync.html

https://doc.rust-lang.org/std/marker/trait.Sync.html