Rust: What is the difference between async fn and fn returning Pin<Box<dyn Future>>?

async fn — compiler-generated. Pin<Box<dyn Future>> — explicit.

async fn:

• Cleaner syntax.
• Type-erased by compiler.
• Cannot return different types.

Pin<Box<dyn Future>>:

• Explicit control.
• Can return different types.
• Heap allocation.

1// Async fn
2async fn get_data() -> String {
3    String::from("data")
4}
5
6// Explicit Future
7use std::future::Future;
8use std::pin::Pin;
9
10fn get_data() -> Pin<Box<dyn Future<Output = String> + Send>> {
11    Box::pin(async {
12        String::from("data")
13    })
14}

Key: async fn for simplicity, explicit Future for flexibility.