fix(lint/useExpect): reject asymmetric matchers and utilities as assertions

.changeset/fix-use-expect-false-negatives.md

@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#9174](https://github.com/biomejs/biome/issues/9174): [`useExpect`](https://biomejs.dev/linter/rules/use-expect/) now correctly rejects Vitest [asymmetric matchers](https://vitest.dev/api/expect.html#expect-stringcontaining) like `expect.stringContaining()`, `expect.objectContaining()`, and utilities like `expect.extend()` that are not valid assertions. Previously these constructs caused false negatives, allowing tests without real assertions to pass the lint rule.

crates/biome_js_analyze/src/frameworks/playwright.rs

@@ -219,12 +219,29 @@ fn is_expect_expression(expr: &AnyJsExpression) -> bool {
             false
         }
         AnyJsExpression::JsStaticMemberExpression(member) => {
-            // expect.soft(), expect.poll(), expect(...).method(), expect(...).not.method()
             if let Ok(object) = member.object() {
-                // Recursively check the object - this handles chained member expressions
-                // like expect(page).not where the object is itself a member expression
-                // NB: This is overly permissive for certain Vitest constructs (ex: `expect.stringContaining()`)
-                // that do not assert anything in and of themselves (see issue #9174)
+                // When the object is a bare `expect` identifier, only allow known
+                // assertion-qualifying members. This prevents asymmetric matchers
+                // (https://vitest.dev/api/expect.html#expect-stringcontaining)
+                // and utilities like `expect.extend()` from being counted as
+                // assertions.
+                if let AnyJsExpression::JsIdentifierExpression(id) = &object {
+                    if let Ok(name) = id.name()
+                        && let Ok(token) = name.value_token()
+                        && token.text_trimmed() == "expect"
+                    {
+                        if let Ok(member_name) = member.member()
+                            && let Some(js_name) = member_name.as_js_name()
+                            && let Ok(member_token) = js_name.value_token()
+                        {
+                            return matches!(
+                                member_token.text_trimmed(),
+                                "soft" | "poll" | "assertions" | "hasAssertions"
+                            );
+                        }
+                        return false;
+                    }
+                }
                 return is_expect_expression(&object);
             }
             false

crates/biome_js_analyze/tests/specs/nursery/useExpect/invalid/asymmetric-matchers.js

@@ -0,0 +1,17 @@
+/* should generate diagnostics */
+
+test("only asymmetric matcher, not an assertion", () => {
+    const matcher = expect.stringContaining("foo");
+});
+
+test("expect.extend is not an assertion", () => {
+    expect.extend({
+        toBeWithinRange(received, floor, ceiling) {
+            return { pass: received >= floor && received <= ceiling };
+        },
+    });
+});
+
+it("negated asymmetric matcher is not an assertion", () => {
+    const matcher = expect.not.objectContaining({ secret: expect.any(String) });
+});

crates/biome_js_analyze/tests/specs/nursery/useExpect/invalid/asymmetric-matchers.js.snap

@@ -0,0 +1,99 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: asymmetric-matchers.js
+---
+# Input
+```js
+/* should generate diagnostics */
+
+test("only asymmetric matcher, not an assertion", () => {
+    const matcher = expect.stringContaining("foo");
+});
+
+test("expect.extend is not an assertion", () => {
+    expect.extend({
+        toBeWithinRange(received, floor, ceiling) {
+            return { pass: received >= floor && received <= ceiling };
+        },
+    });
+});
+
+it("negated asymmetric matcher is not an assertion", () => {
+    const matcher = expect.not.objectContaining({ secret: expect.any(String) });
+});
+
+```
+
+# Diagnostics
+```
+asymmetric-matchers.js:3:1 lint/nursery/useExpect ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i Test callback is missing an expect() assertion.
+
+    1 │ /* should generate diagnostics */
+    2 │ 
+  > 3 │ test("only asymmetric matcher, not an assertion", () => {
+      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  > 4 │     const matcher = expect.stringContaining("foo");
+  > 5 │ });
+      │ ^^
+    6 │ 
+    7 │ test("expect.extend is not an assertion", () => {
+
+  i Tests without assertions may pass even when the behavior is broken.
+
+  i Add an assertion using expect() to verify the expected behavior.
+
+  i This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
+
+
+```
+
+```
+asymmetric-matchers.js:7:1 lint/nursery/useExpect ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i Test callback is missing an expect() assertion.
+
+     5 │ });
+     6 │ 
+   > 7 │ test("expect.extend is not an assertion", () => {
+       │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   > 8 │     expect.extend({
+        ...
+  > 12 │     });
+  > 13 │ });
+       │ ^^
+    14 │ 
+    15 │ it("negated asymmetric matcher is not an assertion", () => {
+
+  i Tests without assertions may pass even when the behavior is broken.
+
+  i Add an assertion using expect() to verify the expected behavior.
+
+  i This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
+
+
+```
+
+```
+asymmetric-matchers.js:15:1 lint/nursery/useExpect ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i Test callback is missing an expect() assertion.
+
+    13 │ });
+    14 │ 
+  > 15 │ it("negated asymmetric matcher is not an assertion", () => {
+       │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  > 16 │     const matcher = expect.not.objectContaining({ secret: expect.any(String) });
+  > 17 │ });
+       │ ^^
+    18 │ 
+
+  i Tests without assertions may pass even when the behavior is broken.
+
+  i Add an assertion using expect() to verify the expected behavior.
+
+  i This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
+
+
+```

crates/biome_js_analyze/tests/specs/nursery/useExpect/valid/assertion-counts.js

@@ -0,0 +1,23 @@
+/* should not generate diagnostics */
+
+test("expect.assertions is a valid assertion check", () => {
+    expect.assertions(1);
+    expect(true).toBe(true);
+});
+
+test("expect.hasAssertions is a valid assertion check", () => {
+    expect.hasAssertions();
+    expect(1).toBe(1);
+});
+
+test("expect.assertions alone counts as assertion-aware", () => {
+    expect.assertions(0);
+});
+
+test("expect.soft is a valid assertion", async ({ page }) => {
+    await expect.soft(page.locator("h1")).toBeVisible();
+});
+
+test("expect.poll is a valid assertion", async ({ page }) => {
+    await expect.poll(() => page.title()).toBe("Title");
+});

crates/biome_js_analyze/tests/specs/nursery/useExpect/valid/assertion-counts.js.snap

@@ -0,0 +1,31 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: assertion-counts.js
+---
+# Input
+```js
+/* should not generate diagnostics */
+
+test("expect.assertions is a valid assertion check", () => {
+    expect.assertions(1);
+    expect(true).toBe(true);
+});
+
+test("expect.hasAssertions is a valid assertion check", () => {
+    expect.hasAssertions();
+    expect(1).toBe(1);
+});
+
+test("expect.assertions alone counts as assertion-aware", () => {
+    expect.assertions(0);
+});
+
+test("expect.soft is a valid assertion", async ({ page }) => {
+    await expect.soft(page.locator("h1")).toBeVisible();
+});
+
+test("expect.poll is a valid assertion", async ({ page }) => {
+    await expect.poll(() => page.title()).toBe("Title");
+});
+
+```