E0275 when ToSql and FromSql are undefined for a custom type

[revsuine]

Rust Version

1.94.0

Diesel Version

2.3.7

Diesel Features

postgres uuid

Database Version

PostgreSQL 18.3

Operating System Version

Artix Linux x86_64, up to date

What happened?

Minimal example that can be used to reproduce this:

Schema:

pub mod sql_types {
    #[derive(diesel::query_builder::QueryId, Clone, diesel::sql_types::SqlType)]
    #[diesel(postgres_type(name = "user"))]
    pub struct User;
}

diesel::table! {
    use diesel::sql_types::*;
    use super::sql_types::User;

    foo (id) {
        id -> Uuid,
        users -> Array<Nullable<User>>,
        content -> Text,
    }
}

Models:

#![allow(unused)]
#![allow(clippy::all)]

use crate::schema::{
    self,
    foo,
};
use uuid::Uuid;
use diesel::prelude::*;

#[derive(Debug, Clone, diesel::FromSqlRow, diesel::AsExpression)]
#[diesel(sql_type = schema::sql_types::User)]
pub struct User {
    pub username: String,
    pub password: String,
}

#[derive(Queryable, Debug, Identifiable, Selectable, Insertable)]
#[diesel(table_name = foo)]
pub struct FooEntry {
    pub id: Uuid,
    pub users: Vec<Option<User>>,
    pub content: String,
}

lib.rs:

mod models;
mod schema;

use diesel::prelude::*;

fn my_func(conn: &mut PgConnection, foo: models::FooEntry) {
    diesel::insert_into(schema::foo::table)
        .values(&foo)
        .returning(models::FooEntry::as_returning())
        .get_result(conn)
        .unwrap();
}

Cargo.toml for this example:

[package]
name = "diesel_issue"
version = "0.1.0"
edition = "2024"

[dependencies]
diesel = { version = "2.3", features = ["postgres", "uuid"] }
uuid = "1.22"

ToSql and FromSql need to be manually implemented for the User model. They have not been manually implemented in this example, so an error is expected, but this causes the compiler error:

   Compiling diesel_issue v0.1.0 (/home/laura/git/mine_temp/diesel_issue)
error[E0275]: overflow evaluating the requirement `_: Sized`
    --> src/lib.rs:10:10
     |
  10 |         .get_result(conn)
     |          ^^^^^^^^^^
     |
     = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`diesel_issue`)
     = note: required for `DefaultableColumnInsertValue<ColumnInsertValue<users, Bound<Array<...>, ...>>>` to implement `QueryFragment<_>`
     = note: 123 redundant requirements hidden
     = note: required for `DefaultableColumnInsertValue<ColumnInsertValue<users, Bound<Array<...>, ...>>>` to implement `QueryFragment<_>`
     = note: required for `DefaultableColumnInsertValue<ColumnInsertValue<users, Bound<Array<...>, ...>>>` to implement `InsertValues<_, table>`
     = note: 3 redundant requirements hidden
     = note: required for `InsertStatement<table, ValuesClause<(..., ..., ...), ...>, ..., ...>` to implement `QueryFragment<_>`
     = note: required for `InsertStatement<table, ValuesClause<(..., ..., ...), ...>, ..., ...>` to implement `LoadQuery<'_, _, _>`
note: required by a bound in `get_result`
    --> /home/laura/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/diesel-2.3.7/src/query_dsl/mod.rs:1776:15
     |
1774 |     fn get_result<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ---------- required by a bound in this associated function
1775 |     where
1776 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::get_result`
     = note: the full name for the type has been written to '/home/laura/git/mine_temp/diesel_issue/target/debug/deps/diesel_issue-ba21453b93f9833b.long-type-1854442447241316390.txt'
     = note: consider using `--verbose` to print the full type name to the console

Adding the following impls to the models leads the code to be able to compile:

impl ToSql<schema::sql_types::User, diesel::pg::Pg> for User {
    fn to_sql<'b>(
        &'b self,
        out: &mut diesel::serialize::Output<'b, '_, diesel::pg::Pg>,
    ) -> diesel::serialize::Result {
        todo!()
    }
}

impl FromSql<schema::sql_types::User, diesel::pg::Pg> for User {
    fn from_sql(
        bytes: <diesel::pg::Pg as diesel::backend::Backend>::RawValue<'_>,
    ) -> diesel::deserialize::Result<Self> {
        todo!()
    }
}

What did you expect to happen?

To have an error other than an overflow/recursion limit error. To have an error reflecting what the programmer needs to implement to fix the compiler error.

Additional details

No response

Steps to reproduce

Code is above. I've also uploaded my example to a repo here: https://git.revsuine.xyz/revsuine/diesel_issue

Although I can't guarantee that repo won't be deleted in future, the relevant code is also in this issue.

Compile time error

cargo build:

   Compiling diesel_issue v0.1.0 (/home/laura/git/mine_temp/diesel_issue)
error[E0275]: overflow evaluating the requirement `_: Sized`
    --> src/lib.rs:10:10
     |
  10 |         .get_result(conn)
     |          ^^^^^^^^^^
     |
     = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`diesel_issue`)
     = note: required for `DefaultableColumnInsertValue<ColumnInsertValue<users, Bound<Array<...>, ...>>>` to implement `QueryFragment<_>`
     = note: 123 redundant requirements hidden
     = note: required for `DefaultableColumnInsertValue<ColumnInsertValue<users, Bound<Array<...>, ...>>>` to implement `QueryFragment<_>`
     = note: required for `DefaultableColumnInsertValue<ColumnInsertValue<users, Bound<Array<...>, ...>>>` to implement `InsertValues<_, table>`
     = note: 3 redundant requirements hidden
     = note: required for `InsertStatement<table, ValuesClause<(..., ..., ...), ...>, ..., ...>` to implement `QueryFragment<_>`
     = note: required for `InsertStatement<table, ValuesClause<(..., ..., ...), ...>, ..., ...>` to implement `LoadQuery<'_, _, _>`
note: required by a bound in `get_result`
    --> /home/laura/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/diesel-2.3.7/src/query_dsl/mod.rs:1776:15
     |
1774 |     fn get_result<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ---------- required by a bound in this associated function
1775 |     where
1776 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::get_result`
     = note: the full name for the type has been written to '/home/laura/git/mine_temp/diesel_issue/target/debug/deps/diesel_issue-ba21453b93f9833b.long-type-1854442447241316390.txt'
     = note: consider using `--verbose` to print the full type name to the console

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. (Please submit the issue in the Rust issue tracker instead if that's not the case)

• This issue can be reproduced without requiring a third party crate

[ThunderComplex]

So I've taken a look at it and I'm obviously not that well versed in the diesel internals yet so keep that in mind.
From what I can tell the problem boils down to how diesel/src/query_dsl/load_dsl.rs loads the query. In line 49 the U is only required to be FromSqlRow and 'static: https://github.com/diesel-rs/diesel/blob/main/diesel/src/query_dsl/load_dsl.rs#L49

I changed that line to this:

U: FromSqlRow<<T::SqlType as CompatibleType<U, DB>>::SqlType, DB>
        + ToSql<U, DB>
        + FromSql<U, DB>
        + 'static,

And I don't know if I'm happy with the result or not. As you can see in a moment, the error will point towards ToSql and FromSql being unsatisfied, but the error log itself becomes much longer, and harder to parse. I think the new output looks even scarier than the original error you reported, but judge for yourself:

   Compiling diesel v2.3.7 (/home/home/Documents/dev/diesel/diesel)
error[E0277]: the trait bound `InnerPgTypeMetadata: ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not satisfied
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:127:20
     |
 127 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = help: the following other types implement trait `ToSql<A, DB>`:
               `&T` implements `ToSql<A, DB>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<sql_types::Nullable<types::sql_types::Range<ST>>, pg::backend::Pg>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<types::sql_types::Range<ST>, pg::backend::Pg>`
               `Cow<'a, T>` implements `ToSql<ST, DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Nullable<sql_types::Text>, __DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Text, DB>`
               `NdArray<T>` implements `ToSql<sql_types::Nullable<types::sql_types::Array<T>>, __DB>`
               `Option<T>` implements `ToSql<sql_types::Nullable<ST>, DB>`
             and 96 others
note: required for `SelectStatement<FromClause<table>, SelectClause<(oid, typarray)>, NoDistinctClause, ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  51 |         + ToSql<U, DB>
     |           ------------ unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-3922193057477187084.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0277]: cannot deserialize a value of the database type `InnerPgTypeMetadata` as `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:127:20
     |
 127 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `deserialize::FromSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: double check your type mappings via the documentation of `InnerPgTypeMetadata`
     = help: the following other types implement trait `deserialize::FromSql<A, DB>`:
               `(T, T1)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1)>, pg::backend::Pg>`
               `(T, T1, T2)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2)>, pg::backend::Pg>`
               `(T, T1, T2, T3)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7, T8)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8)>, pg::backend::Pg>`
             and 55 others
note: required for `SelectStatement<FromClause<table>, SelectClause<(oid, typarray)>, NoDistinctClause, ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  52 |         + FromSql<U, DB>
     |           -------------- unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-15067107993868939163.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0277]: the trait bound `InnerPgTypeMetadata: ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not satisfied
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:114:20
     |
 114 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = help: the following other types implement trait `ToSql<A, DB>`:
               `&T` implements `ToSql<A, DB>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<sql_types::Nullable<types::sql_types::Range<ST>>, pg::backend::Pg>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<types::sql_types::Range<ST>, pg::backend::Pg>`
               `Cow<'a, T>` implements `ToSql<ST, DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Nullable<sql_types::Text>, __DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Text, DB>`
               `NdArray<T>` implements `ToSql<sql_types::Nullable<types::sql_types::Array<T>>, __DB>`
               `Option<T>` implements `ToSql<sql_types::Nullable<ST>, DB>`
             and 96 others
note: required for `SelectStatement<FromClause<JoinOn<Join<table, table, Inner>, Grouped<...>>>, ..., ..., ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  51 |         + ToSql<U, DB>
     |           ------------ unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-3109548803002054322.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0277]: cannot deserialize a value of the database type `InnerPgTypeMetadata` as `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:114:20
     |
 114 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `deserialize::FromSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: double check your type mappings via the documentation of `InnerPgTypeMetadata`
     = help: the following other types implement trait `deserialize::FromSql<A, DB>`:
               `(T, T1)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1)>, pg::backend::Pg>`
               `(T, T1, T2)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2)>, pg::backend::Pg>`
               `(T, T1, T2, T3)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7, T8)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8)>, pg::backend::Pg>`
             and 55 others
note: required for `SelectStatement<FromClause<JoinOn<Join<table, table, Inner>, Grouped<...>>>, ..., ..., ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  52 |         + FromSql<U, DB>
     |           -------------- unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-15067107993868939163.txt'
     = note: consider using `--verbose` to print the full type name to the console

For more information about this error, try `rustc --explain E0277`.
error: could not compile `diesel` (lib) due to 4 previous errors   Compiling diesel v2.3.7 (/home/home/Documents/dev/diesel/diesel)
error[E0277]: the trait bound `InnerPgTypeMetadata: ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not satisfied
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:127:20
     |
 127 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = help: the following other types implement trait `ToSql<A, DB>`:
               `&T` implements `ToSql<A, DB>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<sql_types::Nullable<types::sql_types::Range<ST>>, pg::backend::Pg>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<types::sql_types::Range<ST>, pg::backend::Pg>`
               `Cow<'a, T>` implements `ToSql<ST, DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Nullable<sql_types::Text>, __DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Text, DB>`
               `NdArray<T>` implements `ToSql<sql_types::Nullable<types::sql_types::Array<T>>, __DB>`
               `Option<T>` implements `ToSql<sql_types::Nullable<ST>, DB>`
             and 96 others
note: required for `SelectStatement<FromClause<table>, SelectClause<(oid, typarray)>, NoDistinctClause, ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  51 |         + ToSql<U, DB>
     |           ------------ unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-3922193057477187084.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0277]: cannot deserialize a value of the database type `InnerPgTypeMetadata` as `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:127:20
     |
 127 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `deserialize::FromSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: double check your type mappings via the documentation of `InnerPgTypeMetadata`
     = help: the following other types implement trait `deserialize::FromSql<A, DB>`:
               `(T, T1)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1)>, pg::backend::Pg>`
               `(T, T1, T2)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2)>, pg::backend::Pg>`
               `(T, T1, T2, T3)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7, T8)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8)>, pg::backend::Pg>`
             and 55 others
note: required for `SelectStatement<FromClause<table>, SelectClause<(oid, typarray)>, NoDistinctClause, ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  52 |         + FromSql<U, DB>
     |           -------------- unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-15067107993868939163.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0277]: the trait bound `InnerPgTypeMetadata: ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not satisfied
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:114:20
     |
 114 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `ToSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = help: the following other types implement trait `ToSql<A, DB>`:
               `&T` implements `ToSql<A, DB>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<sql_types::Nullable<types::sql_types::Range<ST>>, pg::backend::Pg>`
               `(std::collections::Bound<T>, std::collections::Bound<T>)` implements `ToSql<types::sql_types::Range<ST>, pg::backend::Pg>`
               `Cow<'a, T>` implements `ToSql<ST, DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Nullable<sql_types::Text>, __DB>`
               `MigrationVersion<'a>` implements `ToSql<sql_types::Text, DB>`
               `NdArray<T>` implements `ToSql<sql_types::Nullable<types::sql_types::Array<T>>, __DB>`
               `Option<T>` implements `ToSql<sql_types::Nullable<ST>, DB>`
             and 96 others
note: required for `SelectStatement<FromClause<JoinOn<Join<table, table, Inner>, Grouped<...>>>, ..., ..., ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  51 |         + ToSql<U, DB>
     |           ------------ unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-3109548803002054322.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0277]: cannot deserialize a value of the database type `InnerPgTypeMetadata` as `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/metadata_lookup.rs:114:20
     |
 114 |             .first(conn)?
     |              ----- ^^^^ unsatisfied trait bound
     |              |
     |              required by a bound introduced by this call
     |
help: the trait `deserialize::FromSql<InnerPgTypeMetadata, pg::backend::Pg>` is not implemented for `InnerPgTypeMetadata`
    --> /home/home/Documents/dev/diesel/diesel/src/pg/backend.rs:17:1
     |
  17 | pub struct InnerPgTypeMetadata {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: double check your type mappings via the documentation of `InnerPgTypeMetadata`
     = help: the following other types implement trait `deserialize::FromSql<A, DB>`:
               `(T, T1)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1)>, pg::backend::Pg>`
               `(T, T1, T2)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2)>, pg::backend::Pg>`
               `(T, T1, T2, T3)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7)>, pg::backend::Pg>`
               `(T, T1, T2, T3, T4, T5, T6, T7, T8)` implements `deserialize::FromSql<types::sql_types::Record<(ST, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8)>, pg::backend::Pg>`
             and 55 others
note: required for `SelectStatement<FromClause<JoinOn<Join<table, table, Inner>, Grouped<...>>>, ..., ..., ..., ..., ...>` to implement `query_dsl::load_dsl::LoadQuery<'_, T, InnerPgTypeMetadata>`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/load_dsl.rs:43:33
     |
  43 | impl<'query, Conn, T, U, DB, B> LoadQuery<'query, Conn, U, B> for T
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^
...
  52 |         + FromSql<U, DB>
     |           -------------- unsatisfied trait bound introduced here
note: required by a bound in `query_dsl::RunQueryDsl::first`
    --> /home/home/Documents/dev/diesel/diesel/src/query_dsl/mod.rs:1834:22
     |
1831 |     fn first<'query, U>(self, conn: &mut Conn) -> QueryResult<U>
     |        ----- required by a bound in this associated function
...
1834 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`
     = note: the full name for the type has been written to '/home/home/Documents/dev/dieseltest/target/debug/deps/diesel-386a19369d36cbd2.long-type-15067107993868939163.txt'
     = note: consider using `--verbose` to print the full type name to the console

For more information about this error, try `rustc --explain E0277`.
error: could not compile `diesel` (lib) due to 4 previous errors

I see two possibilities: either a) I'm wrong and this isn't how you'd go about fixing this (most likely)
or b) the proper "fix" would just be better documentation. Though it would be nice if the compiler could shout at you to implement the unsatisfied traits in a concise matter.

[revsuine]

I'm also unfamiliar with Diesel internals so can't comment on whether or not this is the right way to fix it, however in my personal opinion, whilst the length of the error is not ideal, an error that mentions that a trait bound is not satisfied points the programmer in the right direction. The current error message doesn't really give you any clues.

[weiznich]

Just adding ToSql bounds there doesn't really help as far as I can say, as that will break other things. Specifically FromSql is not implemented for tuples (and shouldn't be implemented for tuples). ToSql shouldn't even be needed at that place.

I need to sit down and check a few more things to be able to say more about this.