Move doesPointTo

[TurkeyMan]

As a bonus on top of #2442, this re-enables the message that complains if you try and move a type with an interior pointer.

I'm not even convinced the assert should exist; if I want to move something, I want to bloody move it! If I was stupid enough to create a thing with an interior pointer; then shame on me, but that shouldn't cause a compile error when I move it... perhaps the interior pointer is a tiny subset of the thing, which I intend to handle manually. If I need to move it, there's no escape hatch here. It feels like a higher-level concern.

Anyway, aside from the question of whether that assert should even be there at all, this PR pulls ~2000 new lines of code... to emit that error message. I don't know if I'm comfortable with the idea of introducing 2000 lines to druntime that do nothing except emit an error message when the user made a possible mistake. I'm not convinced the value justifies the cost...

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Testing this PR locally

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[thewilsonator]

Is the second commit dependent on the first one? i.e. does the first one need to be there if its already in another PR?

[TurkeyMan]

Yes, the first PR should be merged, this one should not be merged... unless someone can convince me otherwise.

[TurkeyMan]

Another thought; opMove, which has been accepted, will make that check incorrect. It will be the case that with opMove, you will be able to move objects with interior pointers.

[thewilsonator]

Yeah, for compile generated moves you need DIP1014 implemented, but for explicit moves there's no reason why we can't start supporting it now.

[wilzbach]

Note that without this PR we can't get rid of the now duplicate implementation of move,{Emplace} in Phobos as there are tests for the doesPointTo check. See also: https://github.com/dlang/phobos/pull/6848

[TurkeyMan]

Yeah see, I didn't move that function, because it's obscene. The fact that function is like 2 thousand lines suggests at some serious deficiency somewhere. Whatever the hell it's for, it should be worked-around ;)