Make it possible to use RefCounted from @safe code

[atilaneves]

Reviews definitely required here.

I thought it through and I think this is memory safe. I tried to do nasty things with -preview=dip1000 and the compiler prevented me, so that's nice.

This is currently preventing std.file.dirEntries from being @safe.

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[Geod24]

@safe:

import std.stdio;
import std.typecons;

struct Container
{
    int[] data;
}

void main ()
{
    auto ptr = getPtr();
    writeln(ptr);
    const save = ptr.dup;
    auto other = getPtr();
    assert(save == ptr);
}

int[] getPtr ()
{
    int[42] local;
    return getPtr2(Container(local));
}

int[] getPtr2 (scope Container local)
{
    RefCounted!Container rc = local;
    return rc.refCountedPayload().data;
}

Compiles, print garbage, fails the assert.

Compiled with ./generated/osx/release/64/dmd -preview=dip1000 -run atila.d and this PR.

[atilaneves]

@safe:

import std.stdio;
import std.typecons;

struct Container
{
    int[] data;
}

void main ()
{
    auto ptr = getPtr();
    writeln(ptr);
    const save = ptr.dup;
    auto other = getPtr();
    assert(save == ptr);
}

int[] getPtr ()
{
    int[42] local;
    return getPtr2(Container(local));
}

int[] getPtr2 (scope Container local)
{
    RefCounted!Container rc = local;
    return rc.refCountedPayload().data;
}

Compiles, print garbage, fails the assert.

Compiled with ./generated/osx/release/64/dmd -preview=dip1000 -run atila.d and this PR.

This is what reviews are for. Good work and thanks! Now to try and fix...

[Geod24]

Now to try and fix...

As I mentioned in our previous discussion, given the nature of DIP1000, I have little hope that we can end up with a (truly) @safe container that is usable. I would love nothing more than to be proven wrong though.

[pbackus]

Now that SafeRefCounted (#8368) has been merged, we can probably close this.