Our HTTP3 support depends on having an msquic assembly around to load. In many cases, this assembly isn't available (for example in a Docker container where the developer didn't explicitly add the msquic package). When this happens, all the code in System.Net.Quic.dll is basically dead code that will never execute. However, the code can't be trimmed because we take a hard-dependency on it from System.Net.Http.dll.

We already have a couple of places that allow this feature to be shut off or trimmed away on platforms that don't support msquic:

We should consider making these switches a full-blown feature switch. Basically we should:

1. Add an entry to System.Net.Http's ILLink.Substitutions.xml file which would change IsHttp3Supported() to always return false when the System.Net.SocketsHttpHandler.Http3Support runtime switch is set to false.
2. Add an MSBuild property and RuntimeHostConfigurationOption entry to https://github.com/dotnet/sdk/blob/main/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets#L386-L399 that allows a developer to trim away HTTP3 support.

Note that Kestrel also has a hard-dependency on System.Net.Quic in UseKestrel(). So Kestrel would need to be updated as well to respect this feature switch, if we wanted to trim this code away in a web app.

Doing some quick prototyping on my machine, this can save about 250KB of IL from a trimmed ASP.NET Linux server app. When publishing with -p:PublishAot=true, the savings drops to about 43KB in the native executable.

cc @halter73 @wfurt