What are (dis)advantages of using pure and immutable by default?

[Bahman Movaqar via Digitalmars-d-learn]

On Monday, 7 September 2015 at 11:49:32 UTC, anonymous wrote:
> void f(int a) {}
> void g(int* a) {}
>
> void main()
> {
>     int xm;
>     immutable int xi;
>     f(xm); /* ok, obviously */
>     f(xi); /* ok */
>
>     int* ym = &xm;
>     immutable int* yi = ξ
>     g(ym); /* ok, obviously */
>     g(yi); /* doesn't compile */
> }
> ----
>
> f(xi) is ok because when passing an int or an immutable(int), 
> it's copied completely. So the f cannot possibly mutate the 
> original immutable variable xi.
>
> g(yi) is not ok, because when passing an int* or an 
> immutable(int*), only the pointer is copied. So g could 
> dereference yi and mutate the referenced immutable variable xi.
>
> The same applies to other forms of references: classes, ref 
> parameters, arrays.

Thanks for the explanation.
So from what I can gather, `immutable`, apart from possible 
compiler-level optimisations, is mainly there to help the 
programmer catch otherwise runtime errors at compile time.

This is nice.  Indeed very helpful in a language when one deals 
with pointers and references.