fix(linter/no-unused-vars): false positive for functions and classes …

…in arrays (#6394)

Fixes a false positive when a function or class expression is an array element.
Found while linting Jest tests in an internal repo.

```js
import { stringify } from './custom-stringify';

test.each(['a', 1, function foo() {}])('When stringifying %p, matches the snapshot', input => {
    expect(stringify(input)).toMatchSnapshot();
})
```

crates/oxc_linter/src/rules/eslint/no_unused_vars/allowed.rs

@@ -26,7 +26,13 @@ impl<'s, 'a> Symbol<'s, 'a> {
 
         for parent in self.iter_parents() {
             match parent.kind() {
-                AstKind::MemberExpression(_) | AstKind::ParenthesizedExpression(_) => {
+                AstKind::MemberExpression(_) | AstKind::ParenthesizedExpression(_)
+                // e.g. `const x = [function foo() {}]`
+                // Only considered used if the array containing the symbol is used.
+                | AstKind::ArrayExpressionElement(_)
+                | AstKind::ExpressionArrayElement(_)
+                | AstKind::ArrayExpression(_)
+                => {
                     continue;
                 }
                 // Returned from another function. Definitely won't be the same
@@ -37,6 +43,9 @@ impl<'s, 'a> Symbol<'s, 'a> {
                 // Function declaration is passed as an argument to another function.
                 | AstKind::CallExpression(_) | AstKind::Argument(_)
                 // e.g. `const x = { foo: function foo() {} }`
+                // Allowed off-the-bat since objects being the only child of an
+                // ExpressionStatement is rare, since you would need to wrap the
+                // object in parentheses to avoid creating a block statement.
                 | AstKind::ObjectProperty(_)
                 // e.g. var foo = function bar() { }
                 // we don't want to check for violations on `bar`, just `foo`

crates/oxc_linter/src/rules/eslint/no_unused_vars/tests/oxc.rs

@@ -689,6 +689,37 @@ fn test_imports() {
         .test_and_snapshot();
 }
 
+#[test]
+fn test_used_declarations() {
+    let pass = vec![
+        // function declarations passed as arguments, used in assignments, etc. are used, even if they are
+        // first put into an intermediate (e.g. an object or array)
+        "arr.reduce(function reducer (acc, el) { return acc + el }, 0)",
+        "console.log({ foo: function foo() {} })",
+        "test.each([ function foo() {} ])('test some function', (fn) => { expect(fn(1)).toBe(1) })",
+        "export default { foo() {}  }",
+        "const arr = [function foo() {}, function bar() {}]; console.log(arr[0]())",
+        "const foo = function foo() {}; console.log(foo())",
+        "const foo = function bar() {}; console.log(foo())",
+        // Class expressions behave similarly
+        "console.log([class Foo {}])",
+        "export default { foo: class Foo {} }",
+        "export const Foo = class Foo {}",
+        "export const Foo = class Bar {}",
+        "export const Foo = @SomeDecorator() class Foo {}",
+    ];
+    let fail = vec![
+        // array is not used, so the function is not used
+        ";[function foo() {}]",
+        ";[class Foo {}]",
+    ];
+
+    Tester::new(NoUnusedVars::NAME, pass, fail)
+        .intentionally_allow_no_fix_tests()
+        .with_snapshot_suffix("oxc-used-declarations")
+        .test_and_snapshot();
+}
+
 #[test]
 fn test_exports() {
     let pass = vec![

crates/oxc_linter/src/snapshots/no_unused_vars@oxc-used-declarations.snap

@@ -0,0 +1,18 @@
+---
+source: crates/oxc_linter/src/tester.rs
+---
+  ⚠ eslint(no-unused-vars): Function 'foo' is declared but never used.
+   ╭─[no_unused_vars.tsx:1:12]
+ 1 │ ;[function foo() {}]
+   ·            ─┬─
+   ·             ╰── 'foo' is declared here
+   ╰────
+  help: Consider removing this declaration.
+
+  ⚠ eslint(no-unused-vars): Class 'Foo' is declared but never used.
+   ╭─[no_unused_vars.tsx:1:9]
+ 1 │ ;[class Foo {}]
+   ·         ─┬─
+   ·          ╰── 'Foo' is declared here
+   ╰────
+  help: Consider removing this declaration.