I agree that such a stronger guarantee would be much more useful. Apaarently,
though, one cannot do that without some kind of lifetime tracking. The reason
is the following: how do you prevent this?
type IntBox = ref object
i:int
proc log(box:IntBox)=
echo $box.i
var foo = box
foo.i = 2 # this is ok because `foo` is mutable
# Strong guarantee is gone :(
This may seem like a simple case, but then consider
type IntBox = ref object
i:int
proc evilModifier(box: var IntBox) =
box.i = 2
proc log(box:IntBox)=
echo $box.i
var foo = box
evilModifier(foo) # no local change in sight, and yet...
# Strong guarantee is gone :(
So, without a way to track lifetimes at compile time, as it happens in Rust,
one cannot prevent the modification of pointed objects. Of course, actually
tracking this stuff introduces a lot of complexity in the language.
