(Please Review Before 2.0 releases!) Remove need to use Box::pin for transactions.

[landonpoch]

PR Info

With rust >= 1.85 we shouldn't need to Box::pin async closures anymore. This fixes that. I'm hoping you'll accept this because it is a breaking change and 2.0 is in RC right now. This would be timely to get it in prior to a major upgrade.

• Closes Feature Request: Support for async closures #2749

New Features

• Just uses the new async closures built in to the language now.

Bug Fixes

• Resolves Feature Request: Support for async closures #2749

Breaking Changes

• You'll need to remove those nasty Box::pin from your transaction closures. <- this is a good thing.

Note!!! If you don't want this as a breaking change consider modifying this PR to just create new methods for this and leave the old stuff in tact. However, I think it's much better just to rip the bandaid off in this release.

[landonpoch]

@tyt2y3 - I know you guys are pretty far along in the RC process at this point. I just want to make sure you get some eyes on this to see if it could make the cut. This would be a really nice thing to include as it makes using the api a lot easier.

[ProbstDJakob]

The problem with this PR is that, as @Huliiiiii wrote, you can not specify the Sync requirement for the returned Future. Since those are only AsyncFnOnce traits, one could add <F as AsyncFnOnce>::CallOnceFuture: Send but this, as well as the RTN, is nightly only at the moment. Thus there is currently no way to add this without nightly.

[landonpoch]

@ProbstDJakob - Got it. Waiting for CallOnceFuture is probably the move then. Once that is in, I can update this to add the Send contraint and then it should be ready to go. Oh well.

[tyt2y3]

would be nice if this works on stable!

[landonpoch]

Realized I needed to update the tests to remove the Box::pin syntax to get them to pass. This makes sense because this is the breaking change called out earlier.

[Huliiiiii]

#2749 (comment)

I think we could follow diesel's approach here. I don't really have time to work on this recently. Anyone interested in experimenting with it?

https://blog.weiznich.de/blog/diesel-async-0-9/

[tyt2y3]

I've tried the approach Hulii suggested, but it couldn't work because of the associated type TransactionTrait::Transaction.

I've added an experimental API in

sea-orm/src/database/executor.rs

Lines 205 to 209 in 59af61e

	pub async fn transaction_async<F, T, E>(&self, callback: F) -> Result<T, TransactionError<E>>
	where
	F: for<'c> AsyncFnOnce(&'c DatabaseTransaction) -> Result<T, E> + Send,
	T: Send,
	E: std::fmt::Display + std::fmt::Debug + Send,