Fix error message

Author: hyf0

src/tools/rust-analyzer/crates/ide-diagnostics/src/handlers/trait_impl_missing_assoc_item.rs

@@ -13,6 +13,7 @@ pub(crate) fn trait_impl_missing_assoc_item(
 ) -> Diagnostic {
     let missing = d.missing.iter().format_with(", ", |(name, item), f| {
         f(&match *item {
+            hir::AssocItem::Function(func) if func.is_async(ctx.sema.db)  => "`async fn ",
             hir::AssocItem::Function(_) => "`fn ",
             hir::AssocItem::Const(_) => "`const ",
             hir::AssocItem::TypeAlias(_) => "`type ",
@@ -56,22 +57,25 @@ trait Trait {
     const C: ();
     type T;
     fn f();
+    async fn async_f();
 }
 
 impl Trait for () {
     const C: () = ();
     type T = ();
     fn f() {}
+    async fn async_f() {}
 }
 
 impl Trait for () {
    //^^^^^ error: not all trait items implemented, missing: `const C`
     type T = ();
     fn f() {}
+    async fn async_f() {}
 }
 
 impl Trait for () {
-   //^^^^^ error: not all trait items implemented, missing: `const C`, `type T`, `fn f`
+   //^^^^^ error: not all trait items implemented, missing: `const C`, `type T`, `fn f`, `async fn async_f`
 }
 
 "#,