Describe async closures

Author: cuviper

posts/2025-02-20-Rust-1.85.0.md

@@ -70,10 +70,41 @@ Note that automatic fixes via `cargo fix` are very conservative to avoid ever ch
 
 ### `async` closures
 
-**🚧 TODO 🚧**
+Rust now supports asynchronous closures like `async || {}` which return futures when called. This works like an `async fn` which can also capture values from the local environment, just like the difference between regular closures and functions. This also comes with 3 analogous traits in the standard library prelude: `AsyncFn`, `AsyncFnMut`, and `AsyncFnOnce`.
 
-- [Stabilize async closures](https://github.com/rust-lang/rust/pull/132706)
-  See [RFC 3668](https://rust-lang.github.io/rfcs/3668-async-closures.html) for more details.
+In some cases, you could already approximate this with a regular closure and an asynchronous block, like `|| async {}`. However, the future returned by such an inner block is not able to borrow from the closure captures, but this does work with `async` closures:
+
+```rust
+let mut vec: Vec<String> = vec![];
+
+let closure = async || {
+    vec.push(ready(String::from("")).await);
+};
+```
+
+It also has not been possible to properly express higher-ranked function signatures with the `Fn` traits returning a `Future`, but you can write this with the `AsyncFn` traits:
+
+```rust
+use core::future::Future;
+async fn f<Fut>(_: impl for<'a> Fn(&'a u8) -> Fut)
+where
+    Fut: Future<Output = ()>,
+{ todo!() }
+
+async fn f2(_: impl for<'a> AsyncFn(&'a u8))
+{ todo!() }
+
+async fn main() {
+    async fn g(_: &u8) { todo!() }
+    f(g).await;
+    //~^ ERROR mismatched types
+    //~| ERROR one type is more general than the other
+
+    f2(g).await; // ok!
+}
+```
+
+So `async` closures provide first-class solutions to both of these problems! See [RFC 3668](https://rust-lang.github.io/rfcs/3668-async-closures.html) and the [stabilization report](https://github.com/rust-lang/rust/pull/132706) for more details.
 
 ### Hiding trait implementations from diagnostics