casting between structs

[Mariusz `shd` Gliwiński" <alienballance at gmail.com>]

I'd like to reinterpret data as different type without copying.

extern(C) struct A{ /* */ }
struct B
{
   A a;
   alias a this;
}

extern(C) A* f1(){ /* */ }

B* f2()
{
   return cast(B*) f1();
}

Assuming that i cast() correct state, i'd like to know if:
* it's currently safe
* it's guaranteed to be safe in the future
* there any precautions in using this pattern

Most important thing for me is a type-check, but defining 
operators ( since they can't be external ) would be nice too.

I'm asking because i'd like to use this pattern a lot in my code, 
and with my limited knowledge it's good to get some feedback of 
more experienced.

Ps.
   In situations when i return by value instead of pointer - will 
memory copying be optimized away ?