Follow up on this stackoverflow question: https://stackoverflow.com/questions/53180372/fastest-way-to-concatenate-readonlyspanchar-in-c-sharp

Experiments to try to create a non-allocating StringBuilder that's as fast as possible.

The code in this repository is totally experimental and not well tested at all. Use at your own risk!

BenchmarkDotNet=v0.11.2, OS=Windows 10.0.17134.345 (1803/April2018Update/Redstone4)
Intel Core i5-2500K CPU 3.30GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET Core SDK=2.1.403
  [Host]     : .NET Core 2.1.5 (CoreCLR 4.6.26919.02, CoreFX 4.6.26919.02), 64bit RyuJIT
  DefaultJob : .NET Core 2.1.5 (CoreCLR 4.6.26919.02, CoreFX 4.6.26919.02), 64bit RyuJIT

Method	Mean	Error	StdDev	Gen 0/1k Op	Gen 1/1k Op	Gen 2/1k Op	Allocated Memory/Op
ConcatSpansStringBuilderBenchmark	142.06 ns	2.8758 ns	4.2153 ns	0.0966	-	-	304 B
ConcatSpansCopyToBenchmark	110.08 ns	2.8908 ns	8.3407 ns	0.0813	-	-	256 B
ConcatSpansFastStringBuilderBenchmark	115.91 ns	2.3242 ns	2.5834 ns	0.0813	-	-	256 B
ConcatSpansFastBufferStringBuilderBenchmark	94.12 ns	1.4318 ns	1.3393 ns	0.0407	-	-	128 B
ConcatSpansFastUnsafeStringBuilderBenchmark	67.93 ns	0.7748 ns	0.6868 ns	0.0407	-	-	128 B