Regression in distinct_on: cannot use .nullable() on column

[Ploppz]

Setup

Versions

• Rust: rustc 1.71.1
• Diesel: 2.1.1
• Database: Postgres
• Operating System Linux

Problem Description

Code using fn distinct_on works in Diesel v2.0.1 but not in v2.1.1.

Minimal reproducible example: https://gist.github.com/Ploppz/f8235d83f287bb7926655cd91dfba8fa
Cargo.toml: diesel = { version = "=2.0.1", features = ["postgres", "r2d2"] }

It compiles in v2.0.1 but in v2.1.1 there is the compiler error:

error[E0277]: the trait bound `NullableExpression<measurement::columns::id>: Column` is not satisfied
  --> src/main.rs:74:10
   |
74 |         .distinct_on(m::id.nullable())
   |          ^^^^^^^^^^^ the trait `Column` is not implemented for `NullableExpression<measurement::columns::id>`
   |

So I tried to remove the nullable() so the two lines become:

        .order_by((m::id, wj::submit_date.nullable().desc()))
        .distinct_on(m::id)

But then I get the error:

error[E0271]: type mismatch resolving `<Only<table> as AppearsInFromClause<table>>::Count == Never`
  --> src/main.rs:74:10
   |
74 |         .distinct_on(m::id)
   |          ^^^^^^^^^^^ expected `Once`, found `Never`
   |

Which makes sense I guess, because the measurement table is left-joined so m::id is not always present.

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