Analyzer: W0103 also fires on unused state-local variables

Previously the unused-variable warning only ran on globals. Now it
walks \`state.locals\` too, so state-scoped variables that are
declared and never read also emit W0103. Factored out to
\`check_unused_var\` to share the \`_\`-prefix exemption and help text.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/analyzer/mod.rs

@@ -211,25 +211,15 @@ impl Analyzer {
         // Compute max call depths from entry points (state handlers)
         self.compute_max_depths(program);
 
-        // Check for unused global variables (W0103). Variables whose names
-        // start with '_' are exempt by convention. State-local variables are
-        // left out for now to avoid noise during early development.
+        // Check for unused variables (W0103). Variables whose names
+        // start with '_' are exempt by convention. Both globals and
+        // state-local variables are checked.
         for var in &program.globals {
-            if var.name.starts_with('_') {
-                continue;
-            }
-            if !self.used_vars.contains(&var.name) {
-                self.diagnostics.push(Diagnostic {
-                    level: Level::Warning,
-                    code: ErrorCode::W0103,
-                    message: format!("unused variable '{}'", var.name),
-                    span: var.span,
-                    labels: Vec::<Label>::new(),
-                    help: Some(
-                        "prefix with '_' to silence this warning, or remove the declaration".into(),
-                    ),
-                    note: None,
-                });
+            self.check_unused_var(var);
+        }
+        for state in &program.states {
+            for var in &state.locals {
+                self.check_unused_var(var);
             }
         }
 
@@ -242,6 +232,26 @@ impl Analyzer {
         self.used_vars.insert(name.to_string());
     }
 
+    /// Emit W0103 if `var` is never read anywhere. Variables named
+    /// with a leading `_` are exempt by convention.
+    fn check_unused_var(&mut self, var: &VarDecl) {
+        if var.name.starts_with('_') {
+            return;
+        }
+        if self.used_vars.contains(&var.name) {
+            return;
+        }
+        self.diagnostics.push(Diagnostic {
+            level: Level::Warning,
+            code: ErrorCode::W0103,
+            message: format!("unused variable '{}'", var.name),
+            span: var.span,
+            labels: Vec::<Label>::new(),
+            help: Some("prefix with '_' to silence this warning, or remove the declaration".into()),
+            note: None,
+        });
+    }
+
     /// Recursively walk an expression tree and mark every identifier that
     /// appears as an `Expr::Ident` (or as an `Expr::ArrayIndex` base) as
     /// "read". Used by the W0103 unused-variable analysis. Also emits

src/analyzer/tests.rs

@@ -562,6 +562,31 @@ fn analyze_unused_variable_warning() {
     );
 }
 
+#[test]
+fn analyze_unused_state_local_warning() {
+    // State-local `bonus` is declared but never read — W0103 should fire.
+    let (prog, diags) = parser::parse(
+        r#"
+        game "Test" { mapper: NROM }
+        state Main {
+            var bonus: u8 = 0
+            on frame { wait_frame }
+        }
+        start Main
+    "#,
+    );
+    assert!(diags.is_empty(), "parse errors: {diags:?}");
+    let result = analyze(&prog.unwrap());
+    assert!(
+        result
+            .diagnostics
+            .iter()
+            .any(|d| d.code == ErrorCode::W0103 && d.message.contains("bonus")),
+        "expected W0103 for unused state-local 'bonus', got: {:?}",
+        result.diagnostics
+    );
+}
+
 #[test]
 fn analyze_unused_variable_no_warning_when_read() {
     // `counter` is both written and read (in the `if` condition),