Rust: What is the difference between async and sync code?

Async — non-blocking, cooperative multitasking. Sync — blocking.

Async:

• Non-blocking.
• Cooperative.
• Requires runtime.

Sync:

• Blocking.
• OS-managed.
• Simple.

1// Sync
2fn read_file_sync() -> String {
3    std::fs::read_to_string("file.txt").unwrap()
4}
5
6// Async
7async fn read_file_async() -> String {
8    tokio::fs::read_to_string("file.txt").await.unwrap()
9}

When to use:

• Async: I/O-bound, high concurrency.
• Sync: CPU-bound, simple apps.

Key: Async for I/O, sync for CPU-bound.