add SafeObject to object.d

[WalterBright]

Since Object has resisted all attempts to add attributes, I created a derived class SafeObject that adds in all the attributes. By deriving from SafeObject instead of Object, one can create objects that are pure, nothrow, const, etc.

Note that derived classes will not be able to remove the pure, nothrow, @nogc, etc. attributes. For classes that cannot abide by this, they can still derive from Object.

Having SafeObject enables Phobos code to have separate overloads for SafeObject, so that they can operate safely.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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[jacob-carlborg]

I recommend putting the PR description in the commit message. Next time, if you have only one commit, GitHub will use the commit message as the PR title and description. Just separate the title and description with an empty newline.

[wilzbach]

I thought @andralex wanted to go the other way and define an RootObject without opEquals and friends, s.t. it's easier to overwrite it.

See: #1439 (comment) (the PR itself was a solution to this problem too)

[WalterBright]

It's nice to have multiple ideas put forth to solving it. I have been thinking about this for a while, and decided to just put it out there to push some progress.

[12345swordy]

Couple of questions regarding this:
What if I want only some attributes and not all of them? Can I create a safe only class that allows gc to run?
Does calling the SafeObject finalize member function have the attributes pure, nothrow, nogc, etc? If it doesn't have a finalize member function what is the equivalent for this?
Wouldn't implementing system default metaclasses(I.E. SafeClass, NoGCClass, PureClass) better suited for this?

[andralex]

@WalterBright and I discussed and agreed we'll pursue the ProtoObject angle instead of this. This is a good catalyst nevertheless to get the DIP started!

[WalterBright]

Actually what we're going to do is write a DIP and see if we can't think of all the angles.