[Feature Request] Add ability to limit types compiled into reference assemblies

[rladuca]

WPF makes extensive use of InternalsVisibleTo in its own builds. However, we don't want these items to show up in reference assemblies. This is because we don't ship a reference assembly for every runtime assembly and the internal surface area generates dangling references to these intentionally excluded assemblies.

We'd like some way of configuring the output of types into a reference assembly, perhaps akin to what GenAPI does with exclusion/inclusion files. At the very least, we'd like to be able to turn off the inclusion of internal surface area via some compiler switch/msbuild property.

Related to dotnet/wpf#932.

[tannergooding]

CC. @ericstj

[rladuca]

CC. @vatsan-madhavan

[vatsan-madhavan]

@jcouv pointed to these related links in an internal thread:

https://blog.paranoidcoding.com/2016/02/15/are-private-members-api-surface.html
https://github.com/dotnet/roslyn/blob/master/docs/features/refout.md#definition-of-ref-assemblies

[ericstj]

I did some initial investigation and found that the IL Linker can actually handle reference assemblies and does a decent job at trimming. I used the same settings for the linker we use in dotnet/runtime to trim our runtime assemblies. I was able to compare the handcrafted reference assembly for System.Text.Json (25 KB), the roslyn generated one (75 KB), and the trimmed version (25 KB). Using the Linker we were able to get size similar to our custom tools, and Roslyn+Linker did a better job at preserving relevant internals (fields in structs, usage of generic parameters, etc).