The EventSource type has an attribute that the trimmer will interpret to keep all members of it as well as all its descendants. A pending trimmer checkin honors this for non-public custom EventSources in library trim mode that affects the shared framework library build.

There is a sizeable difference (around 70K or so of IL) of code that is kept after this trimmer change. A quick analysis of the difference shows that there are members in custom event sources that were not noticed earlier. For example, NetEventSource is used in multiple libraries where the additional members might not be needed (we shipped without this extra code prior to the trimmer change).

Consider removing members from these custom EventSource in the source code itself if they are not needed in the libraries or add tests if they are needed.