Allow DecompressionHandler to be trimmed when the application isn't using AutomaticDecompression in HttpClient.

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/SocketsHttpHandler.cs

@@ -18,6 +18,7 @@ public sealed class SocketsHttpHandler : HttpMessageHandler
     {
         private readonly HttpConnectionSettings _settings = new HttpConnectionSettings();
         private HttpMessageHandlerStage? _handler;
+        private Func<HttpConnectionSettings, HttpMessageHandlerStage, HttpMessageHandlerStage>? _decompressionHandlerFactory;
         private bool _disposed;
 
         private void CheckDisposedOrStarted()
@@ -62,6 +63,7 @@ public DecompressionMethods AutomaticDecompression
             set
             {
                 CheckDisposedOrStarted();
+                EnsureDecompressionHandlerFactory();
                 _settings._automaticDecompression = value;
             }
         }
@@ -511,7 +513,8 @@ private HttpMessageHandlerStage SetupHandlerChain()
 
             if (settings._automaticDecompression != DecompressionMethods.None)
             {
-                handler = new DecompressionHandler(settings._automaticDecompression, handler);
+                Debug.Assert(_decompressionHandlerFactory is not null);
+                handler = _decompressionHandlerFactory(settings, handler);
             }
 
             // Ensure a single handler is used for all requests.
@@ -523,6 +526,13 @@ private HttpMessageHandlerStage SetupHandlerChain()
             return _handler;
         }
 
+        // Allows for DecompressionHandler (and its compression dependencies) to be trimmed when
+        // AutomaticDecompression is not being used.
+        private void EnsureDecompressionHandlerFactory()
+        {
+            _decompressionHandlerFactory ??= (settings, handler) => new DecompressionHandler(settings._automaticDecompression, handler);
+        }
+
         protected internal override HttpResponseMessage Send(HttpRequestMessage request,
             CancellationToken cancellationToken)
         {

src/libraries/System.Net.Http/tests/TrimmingTests/DecompressionHandlerTrimmedTest.cs

@@ -0,0 +1,37 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.IO;
+using System.Net.Http;
+using System.Runtime.Versioning;
+using System.Threading.Tasks;
+
+class Program
+{
+    static async Task<int> Main(string[] args)
+    {
+        using HttpClient client = new();
+
+        // send a request, but ignore its result
+        try
+        {
+            await client.GetAsync("https://www.microsoft.com");
+        }
+        catch { }
+
+        Type decompressionHandler = GetHttpType("System.Net.Http.DecompressionHandler");
+
+        // DecompressionHandler should have been trimmed since AutomaticDecompression was not used
+        if (decompressionHandler is not null)
+        {
+            return -1;
+        }
+
+        return 100;
+    }
+
+    // The intention of this method is to ensure the trimmer doesn't preserve the Type.
+    private static Type GetHttpType(string name) =>
+        typeof(HttpClient).Assembly.GetType(name, throwOnError: false);
+}

src/libraries/System.Net.Http/tests/TrimmingTests/System.Net.Http.TrimmingTests.proj

@@ -2,6 +2,7 @@
   <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.props))" />
 
   <ItemGroup>
+    <TestConsoleAppSourceFiles Include="DecompressionHandlerTrimmedTest.cs" />
     <TestConsoleAppSourceFiles Include="HttpClientTest.cs">
       <SkipOnTestRuntimes>browser-wasm</SkipOnTestRuntimes>
     </TestConsoleAppSourceFiles>