add const mut check

compiler/src/constraints/statement.rs

@@ -276,8 +276,13 @@ impl<F: Field + PrimeField, G: GroupType<F>> ConstrainedProgram<F, G> {
             expression,
         )?;
 
-        if let Declare::Let = declare {
-            value.allocate_value(cs, span)?;
+        match declare {
+            Declare::Let => value.allocate_value(cs, span)?,
+            Declare::Const => {
+                if variable.mutable {
+                    return Err(StatementError::immutable_assign(variable.to_string(), span));
+                }
+            }
         }
 
         self.store_definition(function_scope, variable, value)

compiler/tests/mutability/const.leo

@@ -0,0 +1,5 @@
+// Constant variables are immutable by default.
+function main() {
+  const a = 1u32;
+  a = 0;
+}

compiler/tests/mutability/const_mut.leo

@@ -0,0 +1,4 @@
+// Adding the `mut` keyword to a constant variable is illegal
+function main() {
+  const mut a = 1u32;
+}

compiler/tests/mutability/mod.rs

@@ -47,6 +47,22 @@ fn test_let_mut() {
     mut_success(program);
 }
 
+#[test]
+fn test_const_fail() {
+    let bytes = include_bytes!("const.leo");
+    let program = parse_program(bytes).unwrap();
+
+    mut_fail(program);
+}
+
+#[test]
+fn test_const_mut_fail() {
+    let bytes = include_bytes!("const_mut.leo");
+    let program = parse_program(bytes).unwrap();
+
+    mut_fail(program);
+}
+
 #[test]
 fn test_array() {
     let bytes = include_bytes!("array.leo");