This type doesn’t have any methods accessing the inner value with &self, so by SyncWrapper logic it can be unconditionally Sync — however currently, it never implements the trait.

Options to fix this include:

• Add an extra dependency on sync_wrapper;
• Add unsafe code to tower;
• Wait for SyncWrapper to be added to std.

Opinions on which would be (un)desirable?

Edit: Realized that this doesn’t apply to BoxCloneService, since that type needs Clone and so obviously must be !Sync.