What is the difference between futures::select! and tokio::select?

tokio::select! was built out of experiences with futures::select!, but improves a bit on it to make it more ergonomic. E.g. the futures-rs version of select! requires Futures to implement FusedFuture, whereas Tokio's version no longer requires this.

Instead of this, Tokio's version supports preconditions in the macro to cover the same use-cases.

The PR in the tokio repo elaborates a bit more on this.

This change was also proposed for the futures-rs version, but has not been implemented there so far.

If you already have Tokio included in your project, then using Tokio's version seems preferable. But if you have not and do not want to add an additional dependency, then the futures-rs version will cover most use-cases too in a nearly identical fashion. The main difference is that some Futures might need to be converted into FusedFutures through the FutureExt::fuse() extension method.

To complement @matthias247's answer, a related big difference is that futures::select! takes futures in branch expressions by mutable reference, so uncompleted futures can be re-used in a loop.

tokio::select!, on the other hand, consumes passed futures. To get behavior similar to futures::select! you need to explicitly pass a reference (e.g. &mut future), and pin it if necessary (e.g. if it is async fn). Tokio docs have a section on this, Resuming an async operation

This thread has an in-depth explanation of why Tokio decided not to use FusedFuture.