add Placement New

[WalterBright]

This implements Placement New, described in https://github.com/WalterBright/documents/blob/master/placementnew.md

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#17057"

[WalterBright]

I got it to work for some basic cases. Next comes extending it to more complex ones.

[WalterBright]

Placement new for structs seem to be working now!

[Connor-GH]

If one desires to use classes without the GC, such as in BetterC, it's just awkward to use emplace

If i'm reading this right, this will allow classes in betterC, using a kind of "allocator argument" like in $OTHER_LANGUAGES? Once upon a time I made my own internal fork of dmd that forced betterC on all D code but of course that didn't last long because it couldn't compile Phobos.

(I should clarify, my usecase is making a kernel with betterC and so far I have had to use a hacky mixin and alias this in order to use inheritance. Also, will this unlock the door for interfaces?)

[WalterBright]

Placement works for class objects now!

[TurkeyMan]

Did you tweak it to receive pointers as arguments? I've sent several emails recently and they've all gone silent; I'm wondering if they're getting lost. I also tried commenting on the code in GitHub.

[WalterBright]

I haven't received emails about this. I will check my spam filter. I don't know why you cannot comment on github? I thought it was you who had gone silent! Glad that's not the case.

I stewed about the placement expression as pointer for a while, and figured that making it essentially a ref parameter is more the D way of eschewing pointers in favor of ref. A ref also includes bounds on the size, making that check something the language can do.

Of course, placement new will always be unsafe, but at least we can prevent memory overruns with ref parameters.