@safe, scope, and RC related question marks

safety_dance.d

//==============================================================================
//

struct String
{
    /// clone string, doesn't escape, but return value must have independent lifetime
    String clone() scope
    {
        return String();
    }
}

unittest
{
    static String bug(scope String s)
    {
        return s.clone;
    }
    
    String s;
    String s2 = bug(s);
}

//==============================================================================
// https://forum.dlang.org/post/vgazhxutaxbvqgmqtkbt@forum.dlang.org

struct IOPipe
{
    ubyte[] window()
    {
        return ptr[beg .. end];
    }
    
    void extend(size_t n)
    {
        ptr = .realloc(ptr, end - beg - n);
    }
}

void use()
{
    IOPipe pipe;
    auto w = pipe.window;
    pipe.extend(100); // window invalidates (similar to iterator invalidation)
    w[0];
}

//==============================================================================

void assign(scope ref int* a, scope ref int* b)
{
    b = a; // scope variable a assigned to b with longer lifetime
}

void copyScopePtr() @safe
{
    int var;
    scope int* p1 = &var;
    int* p2;
    // p2 = p1; // will correctly infer p2 as scope
    assign(p1, p2); // function itself doesn't compile
    return p2;
}