I think this is a good change. Consider a program like this [one][1]: fn mutter( x : ~int ) -> ~int { let mut y = x; *y = 24; retu

I think this is a good change. Consider a program like this [one][1]: fn mutter( x : ~int ) -> ~int { let mut y = x; *y = 24; return y; } fn main() { let y = box 42; let x = mutter( y ); println!("x:{}", x); } That's currently legal, but we mutate the box with 42 in it in a function that doesn't declare itself mut, because it has the only reference to the box, it can freely start mutating it, and then give it back at the end. [1]: https://gist.github.com/tbelaire/d5d6950cb0798cf6e194 https://gist.github.com/tbelaire/d5d6950cb0798cf6e194