Unable to specify the type for sqlite pool

[pickfire]

Setup

Versions

• Rust: rustc 1.80.0 (051478957 2024-07-21)
• Diesel: 2.2.0
• Diesel_async: 0.5
• Database: sqlite
• Operating System Linux mi 6.10.3-zen1-2-zen #1 ZEN SMP PREEMPT_DYNAMIC Tue, 06 Aug 2024 07:47:21 +0000 x86_64 GNU/Linux (arch)

Feature Flags

• diesel: chrono, returning_clauses_for_sqlite_3_35
• diesel_async: sync-connection-wrapper, async-connection-wrapper, sqlite, deadpool

Problem Description

Unable to write out the type for sqlite pooled connection.

What are you trying to accomplish?

Using pooled connection for teloxide, it does dependency injection based on type, however I am unable to explicitly put out the type Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>>. The inferred type from rust-analyzer

What is the expected output?

The inferred type should be usable when explicitly typed down.

let _pool: Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>> = Pool::builder(config).build().unwrap();

What is the actual output?

Are you seeing any additional errors?

errors

error[E0277]: the trait bound `AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>: PoolableConnection` is not satisfied
   --> src/main.rs:14:16
    |
14  |     let _pool: Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>> = Pool::builder(co...
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `PoolableConnection` is not implemented for `AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>`, which is required by `AsyncDieselConnectionManager<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>>: deadpool::managed::Manager`
    |
    = help: the trait `PoolableConnection` is implemented for `SyncConnectionWrapper<C>`
    = note: required for `AsyncDieselConnectionManager<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>>` to implement `deadpool::managed::Manager`
note: required by a bound in `deadpool::managed::Pool`
   --> /home/ivan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-0.12.1/src/managed/mod.rs:249:20
    |
249 | pub struct Pool<M: Manager, W: From<Object<M>> = Object<M>> {
    |                    ^^^^^^^ required by this bound in `Pool`

error[E0277]: the trait bound `AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>: DerefMut` is not satisfied
   --> src/main.rs:14:16
    |
14  |     let _pool: Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>> = Pool::builder(co...
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `DerefMut` is not implemented for `AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>`, which is required by `AsyncDieselConnectionManager<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>>: deadpool::managed::Manager`
    |
    = help: the trait `deadpool::managed::Manager` is implemented for `AsyncDieselConnectionManager<C>`
    = note: required for `AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>` to implement `AsyncConnection`
    = note: required for `AsyncDieselConnectionManager<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>>` to implement `deadpool::managed::Manager`
note: required by a bound in `deadpool::managed::Pool`
   --> /home/ivan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-0.12.1/src/managed/mod.rs:249:20
    |
249 | pub struct Pool<M: Manager, W: From<Object<M>> = Object<M>> {
    |                    ^^^^^^^ required by this bound in `Pool`

error[E0308]: mismatched types
  --> src/main.rs:14:94
   |
14 | ...l: Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>> = Pool::builder(config).build().unwrap...
   |       ---------------------------------------------------------------------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `AsyncDieselConnectionManager<SyncConnectionWrapper<...>>`, found `SyncConnectionWrapper<SqliteConnection>`
   |       |
   |       expected due to this
   |
   = note: expected struct `Pool<AsyncDieselConnectionManager<AsyncDieselConnectionManager<...>>>`
              found struct `deadpool::managed::Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>>`
   = note: the full type name has been written to '/home/ivan/src/pickfire/rs/diesel-async-pool/target/debug/deps/diesel_async_pool-d703ecf4a14c8b60.long-type-12503167683362586601.txt'
   = note: consider using `--verbose` to print the full type name to the console

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `diesel-async-pool` (bin "diesel-async-pool") due to 3 previous errors

Steps to reproduce

1. cargo new diesel-async-pool
2. Add the following code

src/main.rs

use diesel::sqlite::SqliteConnection;
use diesel_async::pooled_connection::deadpool::Pool;
use diesel_async::pooled_connection::AsyncDieselConnectionManager;
use diesel_async::sync_connection_wrapper::SyncConnectionWrapper;

#[tokio::main]
async fn main() {
    let db_url = std::env::var("DATABASE_URL").expect("Env var `DATABASE_URL` not set");

    let config =
        AsyncDieselConnectionManager::<SyncConnectionWrapper<SqliteConnection>>::new(db_url);
    let _pool = Pool::builder(config).build().unwrap();
    // putting the type fails
    let _pool: Pool<AsyncDieselConnectionManager<SyncConnectionWrapper<SqliteConnection>>> = Pool::builder(config).build().unwrap();
}

Cargo.toml

[package]
name = "diesel-async-pool"
version = "0.1.0"
edition = "2021"

[dependencies]
tokio = { version = "1.28.2", default-features = false, features = ["macros", "rt-multi-thread", "sync"] }
diesel = { version = "2.2.0", default-features = false, features = ["chrono", "returning_clauses_for_sqlite_3_35"] }
diesel-async = { version = "0.5", features = ["sync-connection-wrapper", "async-connection-wrapper", "sqlite", "deadpool"] }

3. Run cargo check

Checklist

• I have already looked over the issue tracker 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

[weiznich]

This is expected behaviour as you mixed up the pool types. deadpool advises that crates implementing the necessary pool traits provide an reexported pool type that already contains the right connection manager. That makes diesel_async::pooled_connection::deadpool::Pool<SyncConnectionWrapper<SqliteConnection>> the correct type in this location.

This can be seen in the error message that mentions double nested AsyncConnectionManager constructs and in the documentation of the Pool type.

I know that quite a few people seem to struggle with this behavior, but given that this is what upstream documents (by recommending using this macro) that downstream providers should do I'm not sure how to address that in a better way.

(Leaving this open for now, as we certainly need to find some way to address this in the documentation)

[pickfire]

Not sure if we should create an issue in deadpool too but cc @bikeshedder for visibility.

[bikeshedder]

What's the issue with the managed_reexports macro?

The reason for it to exist were constant mixups in the deadpool-* crates where I made a "small" type errors. With the macros it's basically impossible to mess up the reexports and they're guaranteed to be consistent for all deadpool-* crates.

[bikeshedder]

Just wanted to add. You're of course free to not use this macro. It's merely a convenience tool and part of the public API which makes writing deadpool powered pool implementations easier and more consistent.

[weiznich]

What's the issue with the managed_reexports macro?

The "problem" here is that the managed_reexports macro encourages reexporting a Pool type with the manager type already filled in. For diesel-async I followed that pattern, but as the manager itself is generic over the connection type users still end up with a Pool<T> type, and some of the users seem to miss the difference between deadpool::Pool and diesel_async::pooled_connection::deadpool::Pool, which results in a duplicated AsyncConnectionManager in the type, which in turn is problematic. Maybe better documentation can solve this, maybe it requires some additional changes.

Just wanted to add. You're of course free to not use this macro. It's merely a convenience tool and part of the public API which makes writing deadpool powered pool implementations easier and more consistent.

I could, but it would make the API inconsistent with the other deadpool crates.

[bikeshedder]

How about creating separate modules that contain the reexports for each connection type? That's exactly what I did in deadpool-diesel. e.g. https://github.com/bikeshedder/deadpool/blob/master/diesel/src/postgres.rs

It's rare that one wants to target multiple databases in a single code base and those could still use the generic Pool<T>.

[weiznich]

That would be a possibility for the built-in connection types, but I would prefer to keep things in such a way that third party connection types can get the same level of integration as the first party ones.