add .NET 6 target framework for System.CommandLine and add support for trimming

[jonsequitur]

This is so far just a sketch of an approach to support trimming of apps using System.CommandLine. There are a large number of types that won't be bindable without a custom binder or source generator. This also removes support for using TypeConverterAttribute and default type converters.

This will fix #1465.

[eerhardt]

Typically this kind of pattern is trimming safe, but the trimmer doesn't know that you are only looking for one interface. Since you are hard-coding looking for typeof(IEnumerable), the trimmer will never trim the IEnumerable interface. It could trim other interfaces (which is why GetInterfaces is marked as unsafe). But you don't care about the other interfaces, so you can suppress this.

See the following for the same examples, and reasoning:

https://github.com/dotnet/runtime/blob/cf5b231fcbea483df3b081939b422adfb6fd486a/src/libraries/System.Linq.Queryable/src/System/Linq/EnumerableRewriter.cs#L140-L157

https://github.com/dotnet/runtime/blob/cf5b231fcbea483df3b081939b422adfb6fd486a/src/libraries/System.Linq.Queryable/src/System/Linq/EnumerableRewriter.cs#L204-L213

[eerhardt]

I don't think this is needed since you set IsTrimmable=true in the assembly.

[eerhardt]

Don't you want TrimmerRootAssembly instead?

https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming/prepare-libraries-for-trimming

[eerhardt]

Why not just a ProjectReference?

Also - don't point to ns2.0 build. Point to net6.0.

[eerhardt]

Since you aren't using a ProjectReference, all the warnings from System.CommandLine are getting collapsed into a single warning IL2014.

Here's the logic that says to not collapse warnings for ProjectReferences:

https://github.com/dotnet/sdk/blob/8f1a762aee466715a984f5757a622b85daba5387/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.ILLink.targets#L322-L325

If you can't use a ProjectReference here, you can still get full warnings for this assembly by setting the TrimmerSingleWarn=false metadata on the System.CommandLine assembly.

Alternatively, since your app only uses System.CommandLine, it would be simpler to just turn off the single warning behavior globally:

<PropertyGroup>
  <TrimmerSingleWarn>false</TrimmerSingleWarn>

fyi @sbomer

[eerhardt]

(nit) you use NET5_0_OR_GREATER elsewhere, but 6 here.