Moving Between Sync and Async Rust

2024-01-04

Here's a handy cheat sheet for moving between sync and async Rust when using the tokio and/or smol runtimes.

tokio ecosystem

• async to sync: tokio::runtime::Runtime::block_on
  • This is what the tokio::main procedural macro uses
  • There's also tokio::runtime::Handle::block_on if you already have a runtime
• sync to async: tokio::task::spawn_blocking
• Async{Read,Write,Seek} to {Read,Write,Seek}: tokio_util::io::SyncIoBridge
• {Read,Write,Seek} to Async{Read,Write,Seek}: I couldn't find anything (tokio has a solution, but it's private)
• Stream to Iterator: futures_lite::stream::block_on
• Iterator to Stream: tokio_stream::iter or futures_lite::stream::iter

smol ecosystem

The smol crate just re-exports other smaller crates.

• async to sync: smol::block_on / async_io::block_on
• sync to async: smol::unblock / blocking::unblock
• Async{Read,Write,Seek} to {Read,Write,Seek}: smol::io::BlockOn / futures_lite::io::BlockOn
• {Read,Write,Seek} to Async{Read,Write,Seek}: smol::Unblock / blocking::Unblock
• Stream to Iterator: smol::stream::block_on / futures_lite::stream::block_on
• Iterator to Stream: smol::stream::iter / futures_lite::stream::iter

Compat

You can use things from the smol ecosystem with tokio, or vice versa, using one of these:

• tokio_util::compat
• async-compat

smol and tokio both use the same Stream trait from futures-core.