fix: invalid folded mutable constant value

src/negative_tests.rs

@@ -472,6 +472,29 @@ fn test_generic_custom_type_mismatched() {
     ));
 }
 
+#[test]
+fn test_generic_mutated_cst_var_in_loop() {
+    let code = r#"
+        fn gen(const LEN: Field) -> [Field; LEN] {
+            return [0; LEN];
+        }
+
+        fn main(pub xx: Field) {
+            let mut loopvar = 1;
+            for ii in 0..3 {
+                loopvar = loopvar + 1;
+            }
+            let arr = gen(loopvar);
+        }
+        "#;
+
+    let res = tast_pass(code).0;
+    assert!(matches!(
+        res.unwrap_err().kind,
+        ErrorKind::ArgumentTypeMismatch(..)
+    ));
+}
+
 #[test]
 fn test_array_bounds() {
     let code = r#"

src/type_checker/checker.rs

@@ -261,6 +261,10 @@ impl<B: Backend> TypeChecker<B> {
                     .compute_type(lhs, typed_fn_env)?
                     .expect("type-checker bug: lhs access on an empty var");
 
+                if let Some(var_name) = &lhs_node.var_name {
+                    typed_fn_env.invalidate_cst_var(var_name);
+                }
+
                 // todo: check and update the const field type for other cases
                 // lhs can be a local variable or a path to an array
                 let lhs_name = match &lhs.kind {

src/type_checker/fn_env.rs

@@ -110,6 +110,28 @@ impl TypedFnEnv {
         self.current_scope >= prefix_scope
     }
 
+    /// Because we don't support forloop unrolling,
+    /// we should invalidate the constant value behind a mutable variable, which is used in a forloop.
+    /// ```ignore
+    /// let mut pow2 = 1;
+    /// for ii in 0..LEN {
+    ///     pow2 = pow2 + pow2;
+    /// }
+    /// ```
+    /// Instead of folding the constant value to the mutable variable in this case,
+    /// the actual value will be calculated during synthesizer phase.
+    pub fn invalidate_cst_var(&mut self, ident: &str) {
+        // only applies to the variables in the parent scopes
+        // remove the constant value
+        if let Some((scope, info)) = self.vars.get_mut(ident) {
+            if scope < &mut self.current_scope
+                && matches!(info.typ, TyKind::Field { constant: true })
+            {
+                info.typ = TyKind::Field { constant: false };
+            }
+        }
+    }
+
     /// Since currently we don't support unrolling, the generic function calls are assumed to target a same instance.
     /// Each loop iteration should instantiate generic function calls with the same parameters.
     /// This assumption requires a few type checking rules to forbid the cases that needs unrolling.