C++ has const_cast to add or remove const modifier from type. Currently in D casting to/from const is unsafe as it doesn't protect from type conversions. Safe variants of const casts can be added with the following syntax:

cast(const)obj; //to const-qualified
cast(~const)obj; //to mutable
cast(immutable)obj; //instead of assumeUnique
cast(~immutable)obj;
cast(shared)obj;
cast(~shared)obj;

I'm not sure whether cast(~const) should behave exactly as cast(~immutable).

P.S. This doesn't impose any additional requirement on the runtime library.