Custom DateTimeFormatter for rfc header values

[BrennanConroy]

No description provided.

[Tratcher]

Like this? https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.Net.Http.Headers/HttpRuleParser.cs#L239

[davidfowl]

But less bad 😄

[BrennanConroy]

Exactly, less allocatey

[khellang]

How much does the current method allocate today? There's no way to get this down to less than 29 bytes, right?

EDIT: Just ran a benchmark using a custom formatter and ToString("R"):

The code can be found here; https://gist.github.com/khellang/3b32c0fd453616bb2de7edabf06e4413

I can PR it if you want 😄

[aL3891]

We had some optimization fun over in the kestrel channel, here's my version:
https://gist.github.com/aL3891/2644c97e55f2f6c67b1ade15228031b8

Method	Median	StdDev	Scaled	Scaled-SD	Gen 0	Gen 1	Gen 2	Bytes Allocated/Op
ToStringR	1,130.0891 ns	56.6262 ns	1.00	0.00	2,573.00	-	-	280.17
ToRfc1123StringKristian	304.7408 ns	22.0395 ns	0.28	0.02	372.69	-	-	40.79
ToRfc1123StringTore	154.4097 ns	9.7327 ns	0.14	0.01	365.19	-	-	39.64
ToRfc1123StringAllanv1	180.8173 ns	6.2492 ns	0.16	0.01	347.00	-	-	38.02
ToRfc1123StringAllanv2	167.9066 ns	3.9408 ns	0.15	0.01	369.12	-	-	40.03
ToRfc1123StringAllanv3	133.9333 ns	2.3357 ns	0.12	0.01	376.04	-	-	40.81
ToRfc1123StringAllanv4	125.3757 ns	4.7718 ns	0.11	0.01	376.04	-	-	40.81
ToRfc1123StringAllanv1WithBuffer	98.0984 ns	7.4054 ns	0.09	0.01	-	-	-	0.01
ToRfc1123StringAllanv2WithBuffer	87.8140 ns	2.2064 ns	0.08	0.00	-	-	-	0.00
ToRfc1123StringAllanv3WithBuffer	53.3263 ns	3.7403 ns	0.05	0.00	-	-	-	0.00
ToRfc1123StringAllanv4WithBuffer	48.0761 ns	1.4900 ns	0.04	0.00	-	-	-	0.00

its a little crazy but I think its worth it for the perf (it was a fun exercise in any case) :D would you be interested in a PR?