Add long error explanation for E0708 #61137

Refactor code as per the suggestions

Refacotor code

provide edition support

src/librustc_error_codes/error_codes.rs

@@ -393,6 +393,7 @@ E0703: include_str!("./error_codes/E0703.md"),
 E0704: include_str!("./error_codes/E0704.md"),
 E0705: include_str!("./error_codes/E0705.md"),
 E0706: include_str!("./error_codes/E0706.md"),
+E0708: include_str!("./error_codes/E0708.md"),
 E0710: include_str!("./error_codes/E0710.md"),
 E0712: include_str!("./error_codes/E0712.md"),
 E0713: include_str!("./error_codes/E0713.md"),
@@ -605,8 +606,6 @@ E0751: include_str!("./error_codes/E0751.md"),
     E0696, // `continue` pointing to a labeled block
 //  E0702, // replaced with a generic attribute input check
 //  E0707, // multiple elided lifetimes used in arguments of `async fn`
-    E0708, // `async` non-`move` closures with parameters are not currently
-           // supported
 //  E0709, // multiple different lifetimes used in arguments of `async fn`
     E0711, // a feature has been declared with conflicting stability attributes
     E0717, // rustc_promotable without stability attribute

src/librustc_error_codes/error_codes/E0708.md

@@ -0,0 +1,26 @@
+`async` non-`move` closures with parameters are currently not supported.
+
+Erroneous code example:
+
+```compile_fail,edition2018
+#![feature(async_closure)]
+
+fn main() {
+    let add_one = async |num: u8| { // error!
+        num + 1
+    };
+}
+```
+
+`async` with non-move is currently not supported with the current
+version, you can use successfully by using move:
+
+```edition2018
+#![feature(async_closure)]
+
+fn main() {
+    let add_one = async move |num: u8| { // ok!
+        num + 1
+    };
+}
+```

src/test/ui/async-await/no-params-non-move-async-closure.stderr

@@ -8,3 +8,4 @@ LL |     let _ = async |x: u8| {};
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0708`.