fix(useAwait): treat yield* as async operation in async generators

.changeset/use-await-yield-star.md

@@ -0,0 +1,6 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#8645](https://github.com/biomejs/biome/issues/8645).
+[useAwait](https://biomejs.dev/linter/rules/use-await/) no longer reports `async` generator functions that use `yield*`, since `yield*` in an async generator delegates to an `AsyncIterable` and requires the `async` modifier.

crates/biome_js_analyze/src/lint/suspicious/use_await.rs

@@ -5,7 +5,8 @@ use biome_analyze::{
 use biome_console::markup;
 use biome_diagnostics::Severity;
 use biome_js_syntax::{
-    AnyFunctionLike, JsAwaitExpression, JsForOfStatement, JsLanguage, TextRange, WalkEvent,
+    AnyFunctionLike, JsAwaitExpression, JsForOfStatement, JsLanguage, JsYieldExpression,
+    TextRange, WalkEvent,
 };
 use biome_rowan::{AstNode, AstNodeList, Language, SyntaxNode, TextSize};
 use biome_rule_options::use_await::UseAwaitOptions;
@@ -46,6 +47,11 @@ declare_lint_rule! {
     ///
     /// // Nor does it warn about empty `async` functions
     /// async function noop() { }
+    ///
+    /// // Async generators that use `yield*` with an async iterable
+    /// async function* delegateToAsyncIterable() {
+    ///   yield* otherAsyncIterable();
+    /// }
     /// ```
     pub UseAwait {
         version: "1.4.0",
@@ -87,6 +93,11 @@ impl Visitor for MissingAwaitVisitor {
                         *has_await = true;
                     } else if let Some(for_of) = JsForOfStatement::cast_ref(node) {
                         *has_await = *has_await || for_of.await_token().is_some();
+                    } else if let Some(yield_expr) = JsYieldExpression::cast_ref(node) {
+                        *has_await = *has_await
+                            || yield_expr
+                                .argument()
+                                .is_some_and(|arg| arg.star_token().is_some());
                     }
                 }
             }

crates/biome_js_analyze/tests/specs/suspicious/useAwait/invalid.js

@@ -88,3 +88,8 @@ class Sample {
 		return true;
 	}
 }
+
+async function* yieldWithoutStar() {
+	yield 1;
+	yield 2;
+}

crates/biome_js_analyze/tests/specs/suspicious/useAwait/invalid.js.snap

@@ -95,6 +95,11 @@ class Sample {
 	}
 }
 
+async function* yieldWithoutStar() {
+	yield 1;
+	yield 2;
+}
+
 ```
 
 # Diagnostics
@@ -589,3 +594,35 @@ invalid.js:86:2 lint/suspicious/useAwait ━━━━━━━━━━━━━
 
 
 ```
+
+```
+invalid.js:92:1 lint/suspicious/useAwait ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! This async function lacks an await expression.
+
+    90 │ }
+    91 │ 
+  > 92 │ async function* yieldWithoutStar() {
+       │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  > 93 │ 	yield 1;
+  > 94 │ 	yield 2;
+  > 95 │ }
+       │ ^
+    96 │ 
+
+  i Remove this async modifier, or add an await expression in the function.
+
+    90 │ }
+    91 │ 
+  > 92 │ async function* yieldWithoutStar() {
+       │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  > 93 │ 	yield 1;
+  > 94 │ 	yield 2;
+  > 95 │ }
+       │ ^
+    96 │ 
+
+  i Async functions without await expressions may not need to be declared async.
+
+
+```

crates/biome_js_analyze/tests/specs/suspicious/useAwait/valid.js

@@ -67,3 +67,12 @@ async function awaitExpressionWithForOf () {
 	return sum;
 };
 
+async function* yieldStarAsyncIterable() {
+	yield* otherAsyncGenerator();
+}
+
+async function* yieldStarWithAwait() {
+	const data = await fetch('/data');
+	yield* processData(data);
+}
+

crates/biome_js_analyze/tests/specs/suspicious/useAwait/valid.js.snap

@@ -73,5 +73,14 @@ async function awaitExpressionWithForOf () {
 	return sum;
 };
 
+async function* yieldStarAsyncIterable() {
+	yield* otherAsyncGenerator();
+}
+
+async function* yieldStarWithAwait() {
+	const data = await fetch('/data');
+	yield* processData(data);
+}
+
 
 ```