Stabilize async closures

Author: compiler-errors

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -511,11 +511,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
     );
     gate_all!(let_chains, "`let` expressions in this position are unstable");
-    gate_all!(
-        async_closure,
-        "async closures are unstable",
-        "to use an async block, remove the `||`: `async {`"
-    );
     gate_all!(
         async_trait_bounds,
         "`async` trait bounds are unstable",

compiler/rustc_error_codes/src/error_codes/E0708.md

@@ -5,8 +5,6 @@
 Erroneous code example:
 
 ```edition2018
-#![feature(async_closure)]
-
 fn main() {
     let add_one = async |num: u8| {
         num + 1
@@ -18,8 +16,6 @@ fn main() {
 version, you can use successfully by using move:
 
 ```edition2018
-#![feature(async_closure)]
-
 fn main() {
     let add_one = async move |num: u8| { // ok!
         num + 1