Idea

immutable.d

//fun(T)(ref immutable T a){
// //If T is a struct, and I only use immutable fields of T
// //then it should be ok to call fun with a partially mutable type
//}


void f(T: class)(immutable T a) {
    return a.x+1;
}

void main() {
    class A {
        immutable int x = 10;
        double f;
    }
    auto x = new A;
    f(x); // <- fine
}