Fix 1037 (rebased against master)

[timotheecour]

just a rebase of #1154 from @WebFreak001 against master

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[s-ludwig]

Finally looked through this line by line. Looks good apart from the added GC allocation and ConfigEntry should be called PackageConfigs or similar. I can make those changes separately, though, as to not further delay this PR unnecessarily.

[s-ludwig]

Produces a GC allocation in the inner loop.

[timotheecour]

did you measure whether it actually had a performance impact?
also, if it does, StaticArray(Config.invalid) could be used

[WebFreak001]

oh wow that actually uses the GC... who would have thought if you could simply put that array in a separate line as static immutable to not do that.

[s-ludwig]

Yes, in the sense that I got huge speedups with all allocations removed from that loop. Pretty stupid that this is not optimized away by the compiler automatically.

[timotheecour]

dlang/druntime#2093 [WIP] core.array.staticArray static array litteral: staticArray(1, 2, 3) #2093

[s-ludwig]

Definitely an improvement, although I still think that the compiler should be smarter about array literals. It could for example simply infer the possible constness of the literal, and if it is not required to be mutable, it can safely allocate a static array instead of issuing a GC allocation (marking it as @nogc, too).