Auto merge of rust-lang#17736 - hyf0:hyf_09234908234, r=Veykril

feat(ide-completion): explictly show `async` keyword on `impl trait` methods

OLD:

<img width="676" alt="image" src="https://github.com/user-attachments/assets/f6fa626f-6b6d-4c22-af27-b0755e7a6bf8">

Now:

<img width="684" alt="image" src="https://github.com/user-attachments/assets/efbaac0e-c805-4dd2-859d-3e44b2886dbb">

---

This is an preparation for rust-lang/rust-analyzer#17719.

```rust
use std::future::Future;

trait DesugaredAsyncTrait {
    fn foo(&self) -> impl Future<Output = usize> + Send;
    fn bar(&self) -> impl Future<Output = usize> + Send;
}

struct Foo;

impl DesugaredAsyncTrait for Foo {
    fn foo(&self) -> impl Future<Output = usize> + Send {
        async { 1 }
    }

    //
    async fn bar(&self) -> usize {
        1
    }
}

fn main() {
    let fut = Foo.bar();
    fn _assert_send<T: Send>(_: T) {}
    _assert_send(fut);
}
```

If we don't distinguish `async` or not. It would be confusing to generate sugared version `async fn foo ....` and original form `fn foo`  for `async fn in trait` that is defined in desugar form.

Author: bors

src/tools/rust-analyzer/crates/ide-completion/src/completions/item_list/trait_impl.rs

@@ -180,8 +180,10 @@ fn add_function_impl(
 ) {
     let fn_name = func.name(ctx.db);
 
+    let is_async = func.is_async(ctx.db);
     let label = format_smolstr!(
-        "fn {}({})",
+        "{}fn {}({})",
+        if is_async { "async " } else { "" },
         fn_name.display(ctx.db),
         if func.assoc_fn_params(ctx.db).is_empty() { "" } else { ".." }
     );
@@ -193,9 +195,13 @@ fn add_function_impl(
     });
 
     let mut item = CompletionItem::new(completion_kind, replacement_range, label);
-    item.lookup_by(format!("fn {}", fn_name.display(ctx.db)))
-        .set_documentation(func.docs(ctx.db))
-        .set_relevance(CompletionRelevance { is_item_from_trait: true, ..Default::default() });
+    item.lookup_by(format!(
+        "{}fn {}",
+        if is_async { "async " } else { "" },
+        fn_name.display(ctx.db)
+    ))
+    .set_documentation(func.docs(ctx.db))
+    .set_relevance(CompletionRelevance { is_item_from_trait: true, ..Default::default() });
 
     if let Some(source) = ctx.sema.source(func) {
         let assoc_item = ast::AssocItem::Fn(source.value);

src/tools/rust-analyzer/crates/ide-completion/src/tests/item_list.rs

@@ -299,6 +299,7 @@ trait Test {
     const CONST1: ();
     fn function0();
     fn function1();
+    async fn function2();
 }
 
 impl Test for () {
@@ -310,8 +311,9 @@ impl Test for () {
 "#,
         expect![[r#"
             ct const CONST1: () =
+            fn async fn function2()
             fn fn function1()
-            ma makro!(…)          macro_rules! makro
+            ma makro!(…)            macro_rules! makro
             md module
             ta type Type1 =
             kw crate::