Leverage C# 13 to simplify and optimize async serialization

[AArnott]

Summary

• async serialize dropped from a 25-66% async tax to roughly 0.
• async deserialize dropped from a 1100% async tax to 400-800%.

DeserializeAsync obviously needs more work, but there are plenty opportunities for improvement.

In the following metrics, pay attention to the improved ratios instead of looking at the Mean column, which shows significant regressions for unchanged code paths and it didn't repro later.

Before

Method	Categories	Mean	Error	StdDev	Ratio	RatioSD
DeserializeAsArrayInit	array-init,Deserialize	54.43 ns	0.289 ns	0.256 ns	1.00	0.01
DeserializeAsyncAsArrayInit	array-init,Deserialize	637.12 ns	5.798 ns	5.423 ns	11.70	0.11
SerializeAsArrayInit	array-init,Serialize	86.64 ns	1.101 ns	1.030 ns	1.00	0.02
SerializeAsyncAsArrayInit	array-init,Serialize	107.98 ns	0.843 ns	0.704 ns	1.25	0.02
DeserializeAsArray	array,Deserialize	52.79 ns	0.608 ns	0.569 ns	1.00	0.01
DeserializeAsyncAsArray	array,Deserialize	622.12 ns	7.645 ns	7.151 ns	11.79	0.18
SerializeAsArray	array,Serialize	80.59 ns	1.099 ns	1.028 ns	1.00	0.02
SerializeAsyncAsArray	array,Serialize	100.56 ns	1.927 ns	1.802 ns	1.25	0.03
DeserializeMapInit	map-init,Deserialize	103.90 ns	1.047 ns	0.979 ns	1.00	0.01
DeserializeAsyncMapInit	map-init,Deserialize	1,168.60 ns	23.107 ns	54.466 ns	11.25	0.53
SerializeMapInit	map-init,Serialize	97.32 ns	1.033 ns	0.967 ns	1.00	0.01
SerializeAsyncMapInit	map-init,Serialize	161.89 ns	2.195 ns	2.054 ns	1.66	0.03
DeserializeMap	map,Deserialize	100.33 ns	0.610 ns	0.541 ns	1.00	0.01
DeserializeAsyncMap	map,Deserialize	1,040.28 ns	20.102 ns	19.743 ns	10.37	0.20
SerializeMap	map,Serialize	99.47 ns	0.667 ns	0.624 ns	1.00	0.01
SerializeAsyncMap	map,Serialize	159.49 ns	1.741 ns	1.628 ns	1.60	0.02

After

Method	Categories	Mean	Error	StdDev	Ratio	RatioSD
DeserializeAsArrayInit	array-init,Deserialize	94.40 ns	1.886 ns	4.372 ns	1.00	0.06
DeserializeAsyncAsArrayInit	array-init,Deserialize	758.08 ns	14.929 ns	24.529 ns	8.05	0.45
SerializeAsArrayInit	array-init,Serialize	145.46 ns	2.902 ns	6.058 ns	1.00	0.06
SerializeAsyncAsArrayInit	array-init,Serialize	136.35 ns	2.720 ns	3.023 ns	0.94	0.04
DeserializeAsArray	array,Deserialize	92.38 ns	1.820 ns	4.071 ns	1.00	0.06
DeserializeAsyncAsArray	array,Deserialize	749.10 ns	14.508 ns	19.859 ns	8.12	0.41
SerializeAsArray	array,Serialize	136.23 ns	2.293 ns	2.144 ns	1.00	0.02
SerializeAsyncAsArray	array,Serialize	140.85 ns	2.784 ns	5.750 ns	1.03	0.04
DeserializeMapInit	map-init,Deserialize	188.56 ns	3.727 ns	9.281 ns	1.00	0.07
DeserializeAsyncMapInit	map-init,Deserialize	801.47 ns	15.379 ns	15.794 ns	4.26	0.23
SerializeMapInit	map-init,Serialize	170.40 ns	3.343 ns	4.463 ns	1.00	0.04
SerializeAsyncMapInit	map-init,Serialize	162.65 ns	3.241 ns	5.761 ns	0.96	0.04
DeserializeMap	map,Deserialize	181.09 ns	3.359 ns	7.444 ns	1.00	0.06
DeserializeAsyncMap	map,Deserialize	800.18 ns	15.741 ns	30.327 ns	4.43	0.24
SerializeMap	map,Serialize	165.76 ns	2.583 ns	2.416 ns	1.00	0.02
SerializeAsyncMap	map,Serialize	166.74 ns	2.860 ns	2.676 ns	1.01	0.02

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 56.59341% with 79 lines in your changes missing coverage. Please review.

Project coverage is 75.00%. Comparing base (e3a73d5) to head (6c2fdb1). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ank.MessagePack/Converters/ObjectMapConverter`1.cs	52.77%	13 Missing and 4 partials :warning:
...k.MessagePack/Converters/ObjectArrayConverter`1.cs	70.00%	6 Missing and 6 partials :warning:
...nverters/ObjectMapWithNonDefaultCtorConverter`2.cs	47.82%	10 Missing and 2 partials :warning:
src/Nerdbank.MessagePack/MessagePackAsyncReader.cs	57.69%	10 Missing and 1 partial :warning:
...erdbank.MessagePack/Converters/ArrayConverter`1.cs	36.36%	7 Missing :warning:
...erters/ObjectArrayWithNonDefaultCtorConverter`2.cs	66.66%	4 Missing and 3 partials :warning:
...nk.MessagePack/Converters/DictionaryConverter`3.cs	0.00%	6 Missing :warning:
...nk.MessagePack/Converters/EnumerableConverter`2.cs	0.00%	5 Missing :warning:
src/Nerdbank.MessagePack/MessagePackAsyncWriter.cs	0.00%	0 Missing and 1 partial :warning:
src/Nerdbank.MessagePack/StandardVisitor.cs	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #81 +/- ## ========================================== - Coverage 76.37% 75.00% -1.37% ========================================== Files 115 115 Lines 6746 6837 +91 Branches 924 960 +36 ========================================== - Hits 5152 5128 -24 - Misses 1304 1415 +111 - Partials 290 294 +4 ``` | [Flag](https://app.codecov.io/gh/AArnott/Nerdbank.MessagePack/pull/81/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Arnott) | Coverage Δ | | |---|---|---| | [Linux](https://app.codecov.io/gh/AArnott/Nerdbank.MessagePack/pull/81/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Arnott) | `75.00% <56.59%> (-1.37%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Arnott#carryforward-flags-in-the-pull-request-comment) to find out more.

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[AArnott]

Code coverage for the async paths isn't where it should be by the time we stabilize. But I'm holding out writing tests to specifically cover these lines until I'm sure those lines will stick around.