alias ExternC(T) = SetFunctionAttributes!(T, "C", functionAttributes!T);

auto assumePure(T)(T t)
if (isFunctionPointer!T || isDelegate!T)
{
    enum attrs = functionAttributes!T | FunctionAttribute.pure_;
    return cast(SetFunctionAttributes!(T, functionLinkage!T, attrs)) t;
}

int f()
{
    import core.thread : getpid;
    return getpid();
}

int g() pure @trusted
{
    auto pureF = assumePure(&f);
    return pureF();
}
assert(g() > 0);