Can't implement InternalSaveChangesDsl for another backend

[pgab]

Setup

Versions

• Rust: nightly-2018-03-07-x86_64-unknown-linux-gnu
• Diesel: 1.2.2
• Database: Oracle 12g
• Operating System Ubuntu 18.04

Problem Description

I am trying to get another backend for diesel working. Namely oracle/oci. Therefore I need to implement e.g. InternalSaveChangesDsl. Since all oracle/oci related code is currently outside of diesel, it is impossible to implement this. Please note that I made save_changes_dsl public.

What is the actual output?

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g. `MyStruct<T>`); only traits defined in the current crate can be implemented for a type parameter
  --> src/oracle/query_dsl/save_changes_dsl.rs:15:1
   |
15 | / impl<T, U> InternalSaveChangesDsl<OciConnection, U> for T
16 | |     where
17 | |         T: Copy + Identifiable,
18 | |         T: AsChangeset<Target = <T as HasTable>::Table> + IntoUpdateTarget,
...  |
26 | |     }
27 | | }
   | |_^

Steps to reproduce

There is some code at https://github.com/GiGainfosystems/diesel-oci/tree/feature/save_changes_dsl available. I also had to change some diesel code: https://github.com/GiGainfosystems/diesel/tree/feature/save_changes_dsl
Within diesel/diesel_tests I run

cargo +nightly test --features oracle

Checklist

• I have already looked over the issue tracker for similar issues.
• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)

[sgrif]

This isn't really a bug in Diesel, but more a limitation of the language. To fix this we would need disjointness based on associated types, and probably specialization as well. Without those in the language, there's nothing we can do about this. The orphan rules exist for a reason.

[sgrif]

@weiznich Has an idea about how to fix this.