[stephentoub]

Rather than using a manual loop to copy data from a stream to a buffer, use the Stream.CopyTo{Async} method. Downside is there's an extra 32 byte allocation for the temporary destination passthrough Stream object. Upside is it allows the source Stream's to optimize via its CopyTo{Async} override, e.g. MemoryStream.CopyTo just makes a single Write call with its buffer. The base Stream.CopyTo also uses a larger rented buffer size than these APIs were using.

Related to #103539

Before:

Method	Kind	Mean	Allocated
Hash	MemoryStream	79.76 us	568 B
Hash	FileStream	837.08 us	568 B

After:

Method	Kind	Mean	Allocated
Hash	MemoryStream	59.76 us	600 B
Hash	FileStream	131.51 us	600 B

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.IO.Hashing;
using System.Security.Cryptography;

BenchmarkSwitcher.FromAssembly(typeof(Tests).Assembly).Run(args);

[MemoryDiagnoser]
[HideColumns("Job", "Error", "StdDev", "Median", "RatioSD")]
public class Tests
{
    private Stream _stream;
    private byte[] _bytes = RandomNumberGenerator.GetBytes(1024 * 1024);

    [Params("MemoryStream", "FileStream")]
    public string Kind { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        switch (Kind)
        {
            case "MemoryStream":
                _stream = new MemoryStream(_bytes);
                break;

            case "FileStream":
                string path = Path.GetRandomFileName();
                File.WriteAllBytes(path, _bytes);
                _stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, 0, FileOptions.DeleteOnClose);
                break;
        }
    }

    [GlobalCleanup]
    public void Cleanup() => _stream.Dispose();

    [Benchmark]
    public ulong Hash()
    {
        _stream.Position = 0;
        var hash = new XxHash3();
        hash.Append(_stream);
        return hash.GetCurrentHashAsUInt64();
    }
}

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