Remove capture allocation from WriteArray

[davkean]

This allocation was causing 1.4% of all allocations in the trace in dotnet/project-system#2576.

[sharwell]

@davkean Can you check the other locations from #20171 and see if they are also affected?

[davkean]

I checked all callers of StartNew, and verified that they were already using this overload, was there anything else?

[CyrusNajmabadi]

wait... doesn't this still need to capture 'this'?

[davkean]

@CyrusNajmabadi and I spoke offline. There's still an allocation of a delegate - it doesn't appear to be showing at all on the radar. I looked at the code gen and figured out why:

• The first thing that the compiler does in the method is instantiate the capture class and "capture" array.
• It then replaces all reads/writes of the parameter to the field in the capture class

Basically, in summary it looks like this method is always paying for the capture class regardless of the branch. That's why the delegate creation itself isn't showing - the branch is never hit.

[davkean]

I filed #20775 #20777 to track the unexpected code gen here - but I suspect based on the fact that the compiler reuses the capture class's field as if it was the parameter that this was deliberate.