Simplify File.WriteAllTextAsync

[stephentoub]

Avoid the unnecessary ArrayPool usage and extra layer of buffering. The code ends up being much simpler, and also a bit faster.

Method	Toolchain	Mean	Ratio	Allocated
WriteAllTextAsync	\main\CoreRun.exe	1.560 ms	1.00	22 KB
WriteAllTextAsync	\pr\CoreRun.exe	1.486 ms	0.97	19 KB

usingSystem.IO;usingSystem.Linq;usingSystem.Threading;usingSystem.Threading.Tasks;usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;[MemoryDiagnoser]publicclassProgram{publicstaticvoidMain(string[]args)=>BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);privatestaticstrings_contents=string.Concat(Enumerable.Range(0,100_000).Select(i =>(char)('a'+(i%26))));privatestaticstrings_path=Path.GetRandomFileName();privatestaticCancellationTokenSources_cts=new();[Benchmark]publicTaskWriteAllTextAsync()=>File.WriteAllTextAsync(s_path,s_contents,s_cts.Token);}

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

Issue Details

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Avoid the unnecessary ArrayPool usage and extra layer of buffering. The code ends up being much simpler, and also a bit faster.

Method	Toolchain	Mean	Ratio	Allocated
WriteAllTextAsync	\main\CoreRun.exe	1.560 ms	1.00	22 KB
WriteAllTextAsync	\pr\CoreRun.exe	1.486 ms	0.97	19 KB

usingSystem.IO;usingSystem.Linq;usingSystem.Threading;usingSystem.Threading.Tasks;usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;[MemoryDiagnoser]publicclassProgram{publicstaticvoidMain(string[]args)=>BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);privatestaticstrings_contents=string.Concat(Enumerable.Range(0,100_000).Select(i =>(char)('a'+(i%26))));privatestaticstrings_path=Path.GetRandomFileName();privatestaticCancellationTokenSources_cts=new();[Benchmark]publicTaskWriteAllTextAsync()=>File.WriteAllTextAsync(s_path,s_contents,s_cts.Token);}

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO
Milestone:	-