fix: async trait method for unnecessary_async

crates/ide-assists/src/handlers/unnecessary_async.rs

@@ -1,12 +1,14 @@
+use hir::AssocItem;
 use ide_db::{
     assists::{AssistId, AssistKind},
     base_db::FileId,
     defs::Definition,
     search::FileReference,
     syntax_helpers::node_ext::full_path_of_name_ref,
+    traits::resolve_target_trait,
 };
 use syntax::{
-    ast::{self, NameLike, NameRef},
+    ast::{self, HasName, NameLike, NameRef},
     AstNode, SyntaxKind, TextRange,
 };
 
@@ -44,7 +46,16 @@ pub(crate) fn unnecessary_async(acc: &mut Assists, ctx: &AssistContext<'_>) -> O
     if function.body()?.syntax().descendants().find_map(ast::AwaitExpr::cast).is_some() {
         return None;
     }
-
+    // Do nothing if the method is an async member of trait.
+    if let Some(fname) = function.name() {
+        if let Some(trait_item) = find_corresponding_trait_member(ctx, fname.to_string()) {
+            if let AssocItem::Function(method) = trait_item {
+                if method.is_async(ctx.db()) {
+                    return None;
+                }
+            }
+        }
+    }
     // Remove the `async` keyword plus whitespace after it, if any.
     let async_range = {
         let async_token = function.async_token()?;
@@ -88,6 +99,23 @@ pub(crate) fn unnecessary_async(acc: &mut Assists, ctx: &AssistContext<'_>) -> O
     )
 }
 
+fn find_corresponding_trait_member(
+    ctx: &AssistContext<'_>,
+    function_name: String,
+) -> Option<AssocItem> {
+    let impl_ = ctx.find_node_at_offset::<ast::Impl>()?;
+    let trait_ = resolve_target_trait(&ctx.sema, &impl_)?;
+
+    trait_
+        .items(ctx.db())
+        .iter()
+        .find(|item| match item.name(ctx.db()) {
+            Some(method_name) => method_name.to_string() == function_name,
+            _ => false,
+        })
+        .cloned()
+}
+
 fn find_all_references(
     ctx: &AssistContext<'_>,
     def: &Definition,
@@ -254,4 +282,39 @@ pub async fn f(s: &S) { s.f2() }"#,
     fn does_not_apply_when_not_on_prototype() {
         check_assist_not_applicable(unnecessary_async, "pub async fn f() { $0f2() }")
     }
+
+    #[test]
+    fn applies_on_unnecessary_async_on_trait_method() {
+        check_assist(
+            unnecessary_async,
+            r#"
+trait Trait {
+    fn foo();
+}
+impl Trait for () {
+    $0async fn foo() {}
+}"#,
+            r#"
+trait Trait {
+    fn foo();
+}
+impl Trait for () {
+    fn foo() {}
+}"#,
+        );
+    }
+
+    #[test]
+    fn does_not_apply_on_async_trait_method() {
+        check_assist_not_applicable(
+            unnecessary_async,
+            r#"
+trait Trait {
+    async fn foo();
+}
+impl Trait for () {
+    $0async fn foo() {}
+}"#,
+        );
+    }
 }