// Test: Mutability errors
// Expected: Should show "Cannot assign to immutable variable"
fn main() {
// Test 1: Assign to immutable variable
let mut x = 10
x = 20 // Error: cannot assign twice to immutable variable
// Test 2: Mutate immutable reference
let mut y = vec![1, 2, 3]
y.push(4) // Error: cannot borrow as mutable
println!("{} {:?}", x, y)
}