EmptyMutation breaks Sync trait

[j-maas]

Since Rocket requires the data it manages to be thread-safe, I encountered an issue where the Juniper Schema containing a thread-safe database pool would not be recognised as implementing Sync. I've initially opened an issue at the Rocket repo.

The gist is that I would get blocked by a compile error:

error[E0277]: `std::cell::Cell<i32>` cannot be shared between threads safely
  --> src/api.rs:27:1
   |
27 | / fn post_graphql_handler(
28 | |     context: Database,
29 | |     request: juniper_rocket::GraphQLRequest,
30 | |     schema: State<Schema>,
31 | | ) -> juniper_rocket::GraphQLResponse {
32 | |     request.execute(&schema, &context)
33 | | }
   | |_^ `std::cell::Cell<i32>` cannot be shared between threads safely
   |
   = help: within `juniper::schema::model::RootNode<'static, store::Query, juniper::types::scalars::EmptyMutation<store::Context>>`, the trait `std::marker::Sync` is not implemented for `std::cell::Cell<i32>`
   = note: required because it appears within the type `diesel::connection::AnsiTransactionManager`
   = note: required because it appears within the type `diesel::SqliteConnection`
   = note: required because it appears within the type `rocket_contrib::databases::r2d2::Conn<diesel::SqliteConnection>`
   = note: required because it appears within the type `std::option::Option<rocket_contrib::databases::r2d2::Conn<diesel::SqliteConnection>>`
   = note: required because it appears within the type `diesel::r2d2::PooledConnection<diesel::r2d2::ConnectionManager<diesel::SqliteConnection>>`
   = note: required because it appears within the type `store::db::Database`
   = note: required because it appears within the type `store::Context`
   = note: required because it appears within the type `std::marker::PhantomData<store::Context>`
   = note: required because it appears within the type `juniper::types::scalars::EmptyMutation<store::Context>`
   = note: required because it appears within the type `juniper::schema::model::RootNode<'static, store::Query, juniper::types::scalars::EmptyMutation<store::Context>>`
   = note: required by `rocket::State`

This is surprising, because the store::db::Database type actually implements Sync. I found that I could resolve the issue by not using juniper::EmptyMutation but instead doing

pub struct Mutation;

#[juniper::object(Context=Store)]
impl Mutation {}

Therefore, I assume that there is a bug in the implementation of EmptyMutation that prevents correct inference of the Sync trait.

While it is not a self-contained example, the error is present on this commit and is fixed in this commit.

[LegNeato]

I think we can just do:

unsafe impl<T> Send for EmptyMutation<T> {}

Because EmptyMutation never references T...it only uses it for PhantomData. That being said, I am unsure as I have never used unsafe in Rust before. @theduke ?

[j-maas]

Wow, you guys are awesome! Thanks a lot!

[LegNeato]

I pushed a point release with this fix in it.