DataContractSerialization is not linker friendly

[eerhardt]

See this comment:

runtime/src/libraries/System.Private.DataContractSerialization/src/System.Private.DataContractSerialization.csproj

Lines 10 to 11 in 890fab0

	<!-- Too much private reflection. Do not bother with trimming -->
	<ILLinkTrimAssembly>false</ILLinkTrimAssembly>

This setting only applies to running the linker during the dotnet/runtime build. It doesn't affect when a user's application uses this library and is trimmed.

If there is too much private reflection for this assembly to be trimmed, it isn't going to work well when an application using it is trimmed. We should ensure DataContractSerialization can be used safely by linked applications.

[HongGit]

The code is too intricate to be trimmed safely.

[eerhardt]

We should annotate the API with [RequiresUnreferencedCode] attributes then, so someone who is trimming their app gets warnings when they call this code.

[HongGit]

@eerhardt are you going to do this work? Can you please assign a milestone to this issue?

[eerhardt]

@eerhardt are you going to do this work? Can you please assign a milestone to this issue?

I'm not sure who will actually do the work. But there will be an effort to get the libraries annotated in 6.0, so I moved it to there.