how do I activate contracts for phobos functions in dmd

[Richard Palme]

On Friday, 2 November 2018 at 16:41:32 UTC, Stefan Koch wrote:
> Which phobos functions are used in dmd?
> there _should_ be none!

I think I didn't phrase the title correctly:

I'm using dmd as compiler and want to activate the contracts of 
phobos functions. For example there's a phobos function 
std.bitmanip.opIndex implemented like this:

bool opIndex(size_t i) const @nogc pure nothrow
     in {
         assert(i < _len);
     }
     do {
         return cast(bool) bt(_ptr, i);
     }

and when I call this function from my code I want the pre 
contract to be checked (assert(i < _len)