fix(linter): false positive in no-unused-vars

Author: camc314

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

@@ -281,7 +281,6 @@ impl NoUnusedVars {
                 }
                 let report = match symbol.references().rev().find(|r| r.is_write()) {
                     Some(last_write) => {
-                        // ahg
                         let span = ctx.nodes().get_node(last_write.node_id()).kind().span();
                         diagnostic::assign(symbol, span, &self.vars_ignore_pattern)
                     }

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

@@ -171,6 +171,10 @@ fn test_vars_self_use() {
         }
         foo();
         ",
+        "
+        let cancel = () => {}
+        export function close() { cancel = cancel?.() }
+        ",
     ];
     let fail = vec![
         "
@@ -183,6 +187,14 @@ fn test_vars_self_use() {
             return foo
         }
         ",
+        "
+        let cancel = () => {};
+        cancel = cancel?.();
+        ",
+        "
+        let cancel = () => {};
+        { cancel = cancel?.(); }
+        ",
     ];
 
     Tester::new(NoUnusedVars::NAME, NoUnusedVars::PLUGIN, pass, fail)

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

@@ -1,8 +1,9 @@
 //! This module contains logic for checking if any [`Reference`]s to a
 //! [`Symbol`] are considered a usage.
 
+use itertools::Itertools;
 use oxc_ast::{AstKind, ast::*};
-use oxc_semantic::{AstNode, NodeId, Reference, ScopeId, SymbolFlags, SymbolId};
+use oxc_semantic::{AstNode, NodeId, Reference, ScopeFlags, ScopeId, SymbolFlags, SymbolId};
 use oxc_span::{GetSpan, Span};
 
 use super::{NoUnusedVars, Symbol, ignored::FoundStatus};
@@ -413,9 +414,31 @@ impl<'a> Symbol<'_, 'a> {
                     match left {
                         AssignmentTarget::AssignmentTargetIdentifier(id) => {
                             if id.name == name {
+                                // Compare *variable scopes* (the nearest function / TS module / class‑static block).
+                                //
+                                // If the variable scope is the same, the the variable is still unused
+                                // ```ts
+                                // let cancel = () => {};
+                                // {                      // plain block
+                                //   cancel = cancel?.(); // `cancel` is unused
+                                // }
+                                // ```
+                                //
+                                // If the variable scope is different, the read can be observed later, so it counts as a real usage:
+                                // ```ts
+                                // let cancel = () => {};
+                                // function foo() {        // new var‑scope
+                                //   cancel = cancel?.();  // `cancel` is used
+                                // }
+                                // ```
+                                if self.get_parent_variable_scope(self.get_ref_scope(reference))
+                                    != self.get_parent_variable_scope(self.scope_id())
+                                {
+                                    return false;
+                                }
                                 is_used_by_others = false;
                             } else {
-                                return false; // we can short-circuit
+                                return false;
                             }
                         }
                         AssignmentTarget::TSAsExpression(v)
@@ -818,4 +841,18 @@ impl<'a> Symbol<'_, 'a> {
             };
         }
     }
+
+    /// Return the **variable scope** for the given `scope_id`.
+    ///
+    /// A variable scope is the closest ancestor scope (including `scope_id`
+    /// itself) whose kind can *outlive* the current execution slice:
+    ///   * function‑like scopes  
+    ///   * class static blocks  
+    ///   * TypeScript namespace/module blocks
+    fn get_parent_variable_scope(&self, scope_id: ScopeId) -> ScopeId {
+        self.scoping()
+            .scope_ancestors(scope_id)
+            .find_or_last(|scope_id| self.scoping().scope_flags(*scope_id).is_var())
+            .expect("scope iterator will always contain at least one element")
+    }
 }

crates/oxc_linter/src/snapshots/[email protected]

@@ -20,3 +20,29 @@ source: crates/oxc_linter/src/tester.rs
  3 │             return foo
    ╰────
   help: Consider removing this declaration.
+
+  ⚠ eslint(no-unused-vars): Variable 'cancel' is assigned a value but never used. Unused variables should start with a '_'.
+   ╭─[no_unused_vars.tsx:2:13]
+ 1 │ 
+ 2 │         let cancel = () => {};
+   ·             ───┬──
+   ·                ╰── 'cancel' is declared here
+ 3 │         cancel = cancel?.();
+   ·         ───┬──
+   ·            ╰── it was last assigned here
+ 4 │         
+   ╰────
+  help: Did you mean to use this variable?
+
+  ⚠ eslint(no-unused-vars): Variable 'cancel' is assigned a value but never used. Unused variables should start with a '_'.
+   ╭─[no_unused_vars.tsx:2:13]
+ 1 │ 
+ 2 │         let cancel = () => {};
+   ·             ───┬──
+   ·                ╰── 'cancel' is declared here
+ 3 │         { cancel = cancel?.(); }
+   ·           ───┬──
+   ·              ╰── it was last assigned here
+ 4 │         
+   ╰────
+  help: Did you mean to use this variable?