Avoid some defensive copies on readonly structs in System.Net.Quic

Author: rzikm

src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/MsQuicConfiguration.Cache.cs

@@ -7,6 +7,7 @@
 using System.Collections.ObjectModel;
 using System.Security.Authentication;
 using System.Net.Security;
+using System.Runtime.InteropServices;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using Microsoft.Quic;
@@ -39,13 +40,13 @@ private static bool GetConfigurationCacheEnabled()
 
     private static readonly ConcurrentDictionary<CacheKey, MsQuicConfigurationSafeHandle> s_configurationCache = new();
 
-    private readonly struct CacheKey : IEquatable<CacheKey>
+    private struct CacheKey : IEquatable<CacheKey>
     {
-        public readonly List<byte[]> CertificateThumbprints;
-        public readonly QUIC_CREDENTIAL_FLAGS Flags;
-        public readonly QUIC_SETTINGS Settings;
-        public readonly List<SslApplicationProtocol> ApplicationProtocols;
-        public readonly QUIC_ALLOWED_CIPHER_SUITE_FLAGS AllowedCipherSuites;
+        private readonly List<byte[]> CertificateThumbprints;
+        private readonly QUIC_CREDENTIAL_FLAGS Flags;
+        private QUIC_SETTINGS Settings; // not readonly to be able to compare using MemoryMarshal
+        private readonly List<SslApplicationProtocol> ApplicationProtocols;
+        private readonly QUIC_ALLOWED_CIPHER_SUITE_FLAGS AllowedCipherSuites;
 
         public CacheKey(QUIC_SETTINGS settings, QUIC_CREDENTIAL_FLAGS flags, X509Certificate? certificate, ReadOnlyCollection<X509Certificate2>? intermediates, List<SslApplicationProtocol> alpnProtocols, QUIC_ALLOWED_CIPHER_SUITE_FLAGS allowedCipherSuites)
         {
@@ -98,7 +99,7 @@ public bool Equals(CacheKey other)
 
             return
                 Flags == other.Flags &&
-                Settings.Equals(other.Settings) &&
+                MemoryMarshal.AsBytes(MemoryMarshal.CreateReadOnlySpan(ref Settings, 1)).SequenceEqual(MemoryMarshal.AsBytes(MemoryMarshal.CreateReadOnlySpan(ref other.Settings, 1))) &&
                 AllowedCipherSuites == other.AllowedCipherSuites;
         }
 

src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/MsQuicSafeHandle.cs

@@ -88,7 +88,7 @@ internal sealed class MsQuicContextSafeHandle : MsQuicSafeHandle
     /// Holds a weak reference to the managed instance.
     /// It allows delegating MsQuic events to the managed object while it still can be collected and finalized.
     /// </summary>
-    private readonly GCHandle _context;
+    private GCHandle _context;
 
     /// <summary>
     /// Optional parent safe handle, used to increment/decrement reference count with the lifetime of this instance.

src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/ReceiveBuffers.cs

@@ -52,7 +52,7 @@ public int CopyFrom(ReadOnlySpan<QUIC_BUFFER> quicBuffers, int totalLength, bool
             int totalCopied = 0;
             for (int i = 0; i < quicBuffers.Length; ++i)
             {
-                Span<byte> quicBuffer = quicBuffers[i].Span;
+                ReadOnlySpan<byte> quicBuffer = quicBuffers[i].ReadOnlySpan;
                 if (totalLength < quicBuffer.Length)
                 {
                     quicBuffer = quicBuffer.Slice(0, totalLength);

src/libraries/System.Net.Quic/src/System/Net/Quic/Interop/msquic.cs

@@ -21,6 +21,7 @@ namespace Microsoft.Quic
     internal unsafe partial struct QUIC_BUFFER
     {
         public Span<byte> Span => new(Buffer, (int)Length);
+        public readonly ReadOnlySpan<byte> ReadOnlySpan => new(Buffer, (int)Length);
     }
 
     internal partial class MsQuic