Investigate new nightly warning about a soundness hole in the type system

[weiznich]

Setup

Versions

• Rust: 1.80 (nightly)
• Diesel: master

Problem Description

We get the following new warning for builds of diesel-cli on rust nightly:

warning[E0210]: type parameter `Col` must be covered by another type when it appears before the first local type (`database::multi_connection_impl::backend::MultiBackend`)
   --> diesel_cli/src/database.rs:109:10
    |
109 | #[derive(diesel::MultiConnection)]
    |          ^^^^^^^^^^^^^^^^^^^^^^^ type parameter `Col` must be covered by another type when it appears before the first local type (`database::multi_connection_impl::backend::MultiBackend`)
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #124559 <https://github.com/rust-lang/rust/issues/124559>
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
    = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
    = note: `#[warn(uncovered_param_in_projection)]` on by default
    = note: this warning originates in the derive macro `diesel::MultiConnection` (in Nightly builds, run with -Z macro-backtrace for more info)

What is the expected output?

Builds without warning

Related: rust-lang/rust#124559

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate