--- app.d

import tail_unqual;

void main() @safe pure

{

    // Qualifiers on a TailUnqual variable are not transitive--that is,

    // they apply only to the variable itself, not the data it points to.

    immutable TailUnqual!(int*) p = new int(123);

    assert(*p == 123);

    // We can't mutate `p` itself, because it's immutable...

    assert(!__traits(compiles,

        p = immutable(TailUnqual!(int*))(new int(789))

    ));

​

    // ...but we can mutate the data it points to!

    *p = 456;

    assert(*p == 456);

}

​

--- tail_unqual.d

import std.traits: isPointer;

​

private union HiddenPointer

{

    // Stores a pointer, cast to an integer

    size_t address;

​

    // Prevents undefined behavior

    void* unused;

}

​

struct TailUnqual(P)

    if (isPointer!P)

{

    private HiddenPointer data;

    this(P ptr) inout

    {

        data.address = cast(size_t) ptr;

    }

    @trusted @property

    P ptr() inout

    {

        // ok because:

        //  - address is always the active field of data

        //  - data.address was originally cast from a P

        return cast(P) data.address;

    }

    alias ptr this;

}