Implement druntime side of DIP1014

[lesderid]

This PR implements the druntime side of DIP1014.

The accepted DIP is a little ambiguous about whether nothrow should be required on opPostMove (i.e. applied to __move_post_blt). Quoting:

opPostMove, if implemented, MUST be nothrow.

[...]
This proposal does require that nothrow be defined on opPostMove, because throwing would mean a change in the program flow from a place that appears to execute no code. There is a danger that this will be too confusing for the programmer to properly take into account.

[...]

The DIP author suggested the requirement could be eased to "a very hearty recommendation" because D moves "everywhere" and it is "impossible" to prevent it from doing so.

cc @Shachar

[Shachar]

I don't think I'm authorize to make a definite decision on this point, but here's my hearty recommendation. I'll give my rational.

Why it should require nothrow:

• The rational given in the DIP still stands, and it is only through people actually using the functionality that we will find out how restrictive it is.
• Changing behavior in the language is easier from code disallowing something to code allowing something, as that direction creates no backwards compatibility problems.

Why it should not require nothrow:

• I fear that the implementation might become sacred, with the above rational lost to memory. This is especially since I am personally no longer involved in D.

So my suggestion is to require nothrow, but to document that this is an interim decision in both commit message and in the code itself, with reference to this discussion.

[12345swordy]

cc @thewilsonator @TurkeyMan

[thewilsonator]

Needs a rebase though.

[TurkeyMan]

Yeah it'd be really good if this got merged!

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Testing this PR locally

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[lesderid]

@thewilsonator Rebased it.

What about the nothrow though?

[lesderid]

What about the nothrow though?

Actually it's probably fine like this for now.

[JinShil]

What about the nothrow though?

I suggest making choosing the most restrictive option for now; it can always be relaxed later after there's more experience with it.

[codecov]

Codecov Report

Merging #2638 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2638      +/-   ##
==========================================
+ Coverage   74.27%   74.29%   +0.02%     
==========================================
  Files         149      149              
  Lines       16808    16819      +11     
==========================================
+ Hits        12484    12496      +12     
+ Misses       4324     4323       -1

Impacted Files	Coverage Δ
src/core/internal/traits.d	60.86% <ø> (ø)	⬆️
src/object.d	67.44% <100%> (+0.3%)	⬆️
src/rt/aaA.d	88.01% <0%> (-0.28%)	⬇️
src/rt/util/container/treap.d	80.59% <0%> (ø)	⬆️
src/core/thread.d	82.97% <0%> (+0.1%)	⬆️
src/core/internal/spinlock.d	77.77% <0%> (+5.55%)	⬆️

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