Support feature conditions on XML descriptors

[sbomer]

This would allow us to use the embedded ILLink.Descriptors.xml to root "global" dependencies for which we don't have a great place to put DynamicDependencyAttribute, while allowing them to be linked out using feature settings. From discussion with @vitek-karas, it sounds like this will be useful for removing debugger support, COM, etc.

@marek-safar what do you think of this?

[vitek-karas]

I'm all for doing this 😄

[vitek-karas]

/cc @eerhardt

[marek-safar]

I agree that this would be a useful extension of XML descriptors.

[eerhardt]

If we take this approach, we should be able to close dotnet/runtime#37352, right?

The motivation behind dotnet/runtime#37352 was being able to "root" types globally, but also conditionally. See the discussion here: dotnet/runtime#37288 (comment).

So if there are scenarios where we need to preserve a type globally under certain conditions (Debug, COM, JS, etc), we can use the XML descriptor + feature switch mechanism, instead of putting a [DynamicDependency("foo", Condition = "feature switch")].

Note that the advantage of having [DynamicDependency] on the Type vs having a global XML descriptor, is that the global XML descriptor will keep the dependency even if the Type isn't being used. With the [DynamicDependency] on the Type approach, if the whole Type could be trimmed, so could its dependencies.

[marek-safar]

So if there are scenarios where we need to preserve a type globally under certain conditions (Debug, COM, JS, etc), we can use the XML descriptor + feature switch mechanism, instead of putting a [DynamicDependency("foo", Condition = "feature switch")].

This scenario should be used only for cases where the source of the dependency is outside of src/libraries (e.g. external code, different runtime, etc) for anything else DynamicDependency is the preference

global XML descriptor will keep the dependency even if the Type isn't being used.

That's not true. See https://github.com/mono/linker/blob/master/docs/data-formats.md#preserve-a-type