Provide trimming annotations for self-contained apps

[yaakov-h]

.NET 5 previewed the ability to trim unused code from applications and from the runtime, and .NET 6 now makes that generally available.

Relevant docs are here: https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming/trim-self-contained

Considering how many protobuf messages we ship that most apps won't use, this is quite valuable. DepotDownloader can trim about 4.5-5 MB away from SteamKit2.dll and a fully self-contained trimmed build of DD is about 15-20MB IIRC.

Related issues:

• .NET 6: Trimmer trims collection item objects away dotnet/linker#2365 (blocking)
• .NET 6: Trimmer does not follow async flows dotnet/linker#2366 (non-blocking)
• Add NestedTypes to DynamicallyAccessedMember use protobuf-net/protobuf-net#817 (probably non-blocking)

Some initial prototyping has already been done in this pull request:

• .NET 6.0 experimentation #1002

[yaakov-h]

We may have to switch to the Google.Protobuf library which isn't reflection-based in order to (a) get this working properly, and (b) be able to run in an AOT environment such as what is being planned at the moment for .NET 7.

🤔

[xPaw]

Unified messages are no longer a blocker for this, since they are fully generated without reflection.

[xPaw]

With updated protobuf-net, there are still issues with properties being trimmed away:

LoggedOnCallback has this: this.ParentalSettings = Serializer.Deserialize<ParentalSettings>( ms );

ParentalSettings has public ParentalPlaytimeRestrictions playtime_restrictions { get; set; }, but after trimming only an empty class ParentalPlaytimeRestrictions is produced so it fails to deserialize it.

I tried putting the trimming attribute on messages and fields, that doesn't fix the issue: (not the exact same place, i generated it everywhere)

[yaakov-h]

I did have fun problems previously with trimming and nested classes.

That said, what does "fails to deserialise" look like?

[xPaw]

It's trying to call an empty constructor which does not exist, it probably would have worked if it trimmed away the entire field if no one uses it.

[mizantrop777]

trimming can be disabled via

<TrimMode>link</TrimMode>
<JsonSerializerIsReflectionEnabledByDefault>true</JsonSerializerIsReflectionEnabledByDefault>

...

<ItemGroup>
	<TrimmerRootDescriptor Include="ILLink.xml" />
</ItemGroup>

ILLink.xml:

<?xml version="1.0" encoding="utf-8" ?>
<linker>
  <!-- Пример сохранения всех типов в сборке -->
  <assembly fullname="Updater" preserve="all" />
  <assembly fullname="SteamKit2" preserve="all" />
  <assembly fullname="protobuf-net" preserve="all" />
  <assembly fullname="protobuf-net.Core" preserve="all" />
</linker>