Bad diagnostic on type mismatch for associated type requirements

Code

use diesel::prelude::*; // 2.1.6 without any features

table! {
    users {
        id -> Integer,
        name -> Text,
    }
}

table! {
    posts {
        id -> Integer,
        title -> Text,
        user_id -> Integer,
    }
}

fn main() {
    let sub_query = posts::table.find(42).select(posts::user_id).single_value();

    let user_query = users::table.filter(users::id.eq(sub_query));
}

Current output

error[E0271]: type mismatch resolving `<Grouped<Subselect<SelectStatement<FromClause<table>, SelectClause<user_id>, NoDistinctClause, WhereClause<Grouped<Eq<..., ...>>>, ..., ...>, ...>> as Expression>::SqlType == Integer`
  --> src/main.rs:21:52
   |
21 |     let user_query = users::table.filter(users::id.eq(sub_query));
   |                                                    ^^ expected `Nullable<Integer>`, found `Integer`
   |
   = note: expected struct `Nullable<diesel::sql_types::Integer>`
              found struct `diesel::sql_types::Integer`
   = note: required for `Grouped<Subselect<SelectStatement<FromClause<table>, SelectClause<user_id>, ..., ..., ..., ...>, ...>>` to implement `AsExpression<diesel::sql_types::Integer>`
   = note: the full name for the type has been written to '/tmp/diesel_bad_diagnostic/target/debug/deps/diesel_bad_diagnostic-98b497cf8fd64c49.long-type-7298064120992293895.txt'
   = note: consider using `--verbose` to print the full type name to the console

Desired output

error[E0271]: type mismatch resolving `<Grouped<Subselect<SelectStatement<FromClause<table>, SelectClause<user_id>, NoDistinctClause, WhereClause<Grouped<Eq<..., ...>>>, ..., ...>, ...>> as Expression>::SqlType == Integer`
  --> src/main.rs:21:52
   |
21 |     let user_query = users::table.filter(users::id.eq(sub_query));
   |                                          ^^^^^^^^^    ^^^^^^^^
   |                                          `Integer` `Nullable<Integer>`
   |
   = note: expected struct `Nullable<diesel::sql_types::Integer>`
              found struct `diesel::sql_types::Integer`
   = note: required for `Grouped<Subselect<SelectStatement<FromClause<table>, SelectClause<user_id>, ..., ..., ..., ...>, ...>>` to implement `AsExpression<diesel::sql_types::Integer>`
   = note: the full name for the type has been written to '/tmp/diesel_bad_diagnostic/target/debug/deps/diesel_bad_diagnostic-98b497cf8fd64c49.long-type-7298064120992293895.txt'
   = note: consider using `--verbose` to print the full type name to the console

Rationale and extra context

It's currently not obviously clear which type is used for which part of the expression. For the presented case it would much easier for users to understand what's wrong if they could see which side of the expression has which type.

Other cases

No response

Rust Version

$ rustc +nightly --version --verbose 
rustc 1.80.0-nightly (6e1d94708 2024-05-10)
binary: rustc
commit-hash: 6e1d94708a0a4a35ca7e46c6cac98adf62fe800e
commit-date: 2024-05-10
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4

Anything else?

No response

Reproduce the diagnostic using the Rust code shown in src/main.rs and the stated nightly compiler version, then compare the current and desired outputs. Done means the mismatch diagnostic clearly labels the two expression sides with Integer and Nullable as shown.