I'm trying to implement a trait for all types that are Sync in a generic manner. The same trait would also be implemented for some specific non-Sync types.
The compiler is however fighting me on this, claiming that the non-Sync implementation is conflicting with the first one. See for instance: Rust Playground
Thanks!
Xavier
There is nothing preventing the standard library from adding unsafe impl<T> Sync for Rc<T> in the future (other than that being unsound, but the compiler doesn’t know this), if it did then there would be multiple applicable implementations of A for B.
Sure, but that isn't the case today, so I'm not sure why the compiler effectively pretend that B is Sync. My reasoning on this is that B is not Sync, and therefore the trait A shouldn't be implemented for it, due to the generic implementation requiring a Sync type. Thus, the specialized implementation of A for B shouldn't be conflicting with anything.
Yes, I could do that, but the point here is to have a "default" implementation of a trait for all Sync types, and I would then have a specific implementation of that trait for types that aren't Sync.
The compiler has to treat these potential future conflicts as errors. Otherwise, the standard library could never add a new impl to an existing type, because any new impl could break existing crates.
When specialization is stabilized, you'll be able to write both of these impls without an error.
Right! sorry now I get your question.