Compilation error when using pipelining

[stormshield-kg]

Versions

• Rust:: 1.88.0
• Diesel:: commit cdf7c51c35da7ebc6fb12e71857bfba8487c20cd
• Diesel_async:: commit 0e703ba
• Operating System: Ubuntu 22

Problem Description

The following code doesn't compile:

fn f(conn: &mut crate::AsyncPgConnection) {
    use diesel::sql_types::Integer;
    use diesel::IntoSql;
    use futures_util::future::try_join;
    use futures_util::FutureExt;

    use crate::RunQueryDsl;

    let f1 = diesel::select(1_i32.into_sql::<Integer>()).get_result::<i32>(conn);
    let f2 = diesel::select(2_i32.into_sql::<Integer>()).get_result::<i32>(conn);

    // This doesn't work
    let _ = async { try_join(f1, f2).await }.boxed();
}

Error

error[E0308]: mismatched types
    --> src/pg/mod.rs:1145:17
     |
1145 |         let _ = async { try_join(f1, f2).await }.boxed();
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
     |
     = note: expected struct `Map<StreamFuture<Pin<Box<Map<Pin<Box<dyn Stream<Item = ...> + Send>>, ...>>>>, ...>`
                found struct `Map<StreamFuture<Pin<Box<Map<Pin<Box<dyn Stream<Item = ...> + Send>>, ...>>>>, ...>`
note: the lifetime requirement is introduced here
    --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/futures-util-0.3.31/src/future/future/mod.rs:516:23
     |
516  |         Self: Sized + Send + 'a,
     |                       ^^^^
     = note: the full name for the type has been written to '/home/runner/Desktop/diesel_async/target/debug/deps/diesel_async-0e1fc68988d593f9.long-type-4500313906557700460.txt'
     = note: consider using `--verbose` to print the full type name to the console

error[E0308]: mismatched types
    --> src/pg/mod.rs:1145:17
     |
1145 |         let _ = async { try_join(f1, f2).await }.boxed();
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
     |
     = note: expected struct `Map<StreamFuture<Pin<Box<Map<Pin<Box<dyn Stream<Item = ...> + Send>>, ...>>>>, ...>`
                found struct `Map<StreamFuture<Pin<Box<Map<Pin<Box<dyn Stream<Item = ...> + Send>>, ...>>>>, ...>`
note: the lifetime requirement is introduced here
    --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/futures-util-0.3.31/src/future/future/mod.rs:516:23
     |
516  |         Self: Sized + Send + 'a,
     |                       ^^^^
     = note: the full name for the type has been written to '/home/runner/Desktop/diesel_async/target/debug/deps/diesel_async-0e1fc68988d593f9.long-type-9446752501490231457.txt'
     = note: consider using `--verbose` to print the full type name to the console

error: implementation of `FnOnce` is not general enough
    --> src/pg/mod.rs:1145:17
     |
1145 |         let _ = async { try_join(f1, f2).await }.boxed();
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
     |
     = note: `fn(Pin<Box<(dyn Stream<Item = Result<PgRow, diesel::result::Error>> + std::marker::Send + '0)>>) -> futures_util::stream::Map<Pin<Box<dyn Stream<Item = Result<PgRow, diesel::result::Error>> + std::marker::Send>>, fn(Result<PgRow, diesel::result::Error>) -> Result<i32, diesel::result::Error>>` must implement `FnOnce<(Pin<Box<(dyn Stream<Item = Result<PgRow, diesel::result::Error>> + std::marker::Send + '1)>>,)>`, for any two lifetimes `'0` and `'1`...
     = note: ...but it actually implements `FnOnce<(Pin<Box<dyn Stream<Item = Result<PgRow, diesel::result::Error>> + std::marker::Send>>,)>`

error[E0308]: mismatched types
    --> src/pg/mod.rs:1145:17
     |
1145 |         let _ = async { try_join(f1, f2).await }.boxed();
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
     |
     = note: expected struct `futures_util::stream::Map<Pin<Box<dyn Stream<Item = Result<PgRow, diesel::result::Error>> + std::marker::Send>>, _>`
                found struct `futures_util::stream::Map<Pin<Box<dyn Stream<Item = Result<PgRow, diesel::result::Error>> + std::marker::Send>>, _>`
note: the lifetime requirement is introduced here
    --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/futures-util-0.3.31/src/future/future/mod.rs:516:23
     |
516  |         Self: Sized + Send + 'a,
     |                       ^^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `diesel-async` (lib test) due to 4 previous errors

This seems to be related to the GAT lifetimes of the returned futures.

The workaround is to erase the lifetimes of the futures:

// This works
let _ = async { try_join(f1, f2).boxed().await }.boxed();

// This also works
fn erase<'a, F: Future + Send + 'a>(fut: F) -> impl Future<Output = F::Output> + Send + 'a {
    fut
}
let _ = async { erase(try_join(f1, f2)).await }.boxed();

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]

Thanks for opening this issue. Could you explain what you expect diesel-async to do about that? For me the "one type is more general than the other" error is almost always an indication that you managed to hit a case where the compiler is incomplete, so this would be more a rustc than a diesel-async issue.

That written: It might be that you can fix that by using impl Future<> + use<> (so impl trait + the exact capture syntax) instead of the GAT in the connect trait to make the compile and working around the rustc bugs. I personally won't have the capacity to investigate that soon so either someone else looks into this or that issue stays open. Strictly speaking it's a feature request to allow to use the future in more places instead of a bug report from my point of view.