Add `IsTrimmable` property for libraries

Two things we want to get out of this:

• Enable feature switch defaults
• Define IsTrimmable attribute in generated sources

[agocke]

I don’t really have an opinion here yet – I thought about it a bit and it just sounded like a tough problem. 😊

But I do think it’s important to think ahead to library trimming. I’d hate for us to make a choice which does the opposite of what a user would expect in those situations.

[vitek-karas]

Re feature switches: Properly designed and implemented feature switch should not cause more warnings in libraries. Let’s assume there’s an API DoSomethingWild which is annotated with RUC. And it has a feature switch which disables WildFeature in the framework. Calling DoSomethingWild is problematic regardless if the feature switch is on or off:

• If the feature switch is on - it will not work when trimmed -> produces a warning that trimming is problematic
• If the feature switch is off - the API should typically throw -> producing a warning when trimming is not ideal, but it doesn’t hurt (the API won’t do what it’s supposed to do anyway).

This is for public feature switches. For internal feature switches (like startup hooks and so on), this is even simpler. There should be no way for the library to trigger the relevant code in the framework directly, so it should not get any warnings. Once built within an app, the behavior is entirely based on the app’s config.

In the future the only place where this might cause trouble is if we provide the full library mode for linker - that is running linker on a library with framework implementation assemblies. That would require correctly setting feature switches. But we don’t have that yet.

Re IsTrimmable on apps: I just tried with PublishTrimmed on an app - it works perfectly. So I don’t have any problem at all with the proposed behavior. In the future we may decide to support IsTrimmable on the app as well, but it would effectively behave in the same way as on libs - add the attribute to the app’s assembly.