Library trimming steps should support feature switch substitutions

[agocke]

Our recommended way to get all warnings from a library is to use a sample app that references the library with the 'copy' action, as described in https://learn.microsoft.com/en-us/dotnet/core/deploying/trimming/prepare-libraries-for-trimming

Unfortunately, it doesn't seem like feature switches work as expected in this scenario. Specifically, the properties don't get replaced and the warnings don't get suppressed.

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Issue Details

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Our recommended way to get all warnings from a library is to use a sample app that references the library with the 'copy' action, as described in https://learn.microsoft.com/en-us/dotnet/core/deploying/trimming/prepare-libraries-for-trimming

Unfortunately, it doesn't seem like feature switches work as expected in this scenario. Specifically, the properties don't get replaced and the warnings don't get suppressed.

Author:	agocke
Assignees:	sbomer
Labels:	area-Tools-ILLink, needs-area-label
Milestone:	9.0.0

[sbomer]

TrimmerRootAsembly maps to -a, which has long used the copy action under the hood, and copy is designed to make no modifications to the assembly, but instead copy it as-is to the output.

To fix this I think -a should change behavior so that it roots every method/member in the assembly, but allows modifications. This is a potentially risky change if people were relying on TrimmerRootAssembly for behavior other than rooting. I suggest we change the default, but add an escape hatch that goes back to the "copy" behavior. Any opinion on this @vitek-karas @MichalStrehovsky?

[MichalStrehovsky]

The current behavior causes us to unnecessarily root facade assemblies (System.Runtime, netstandard, etc.) because we're not allowed to rewrite typeforwards to their definition in assemblies rooted with TrimmerRootAssembly. I think making this change would be overall goodness. I assume our C++/CLI logic doesn't depend on TrimmerRootAssembly.

This is a potentially risky change if people were relying on TrimmerRootAssembly for behavior other than rooting

Trying to think about where this would break, the only scenario I can think about is if this was an assembly full of type forwards - we should make sure type forwards are also kept. Looks like people like to TrimmerRootAssembly mscorlib 🤦.

but add an escape hatch that goes back to the "copy" behavior.

Would setting the TrimMode metadata on ManagedAssemblyToLink work as the escape hatch?

[vitek-karas]

If I understand this correctly - we will still root everything in the assembly, so the only changes are:

• Method body edits (like branch removal, constant prop and so on)
• Metadata edits (probably just type ref resolution/rewrite due to type forwarders)
• Metadata optimizations (removal of parameter names on non-reflectable methods)
• Type forwards (see Michal's comment)
• Global removals - like removal of some attributes as dictated by substitution XML

These all sound relatively minor. I agree we should give this a try - if we merge this into 9 relatively soon, we should have a long window of validation.

[MichalStrehovsky]

Metadata optimizations (removal of parameter names on non-reflectable methods)

I think TrimmerRootAssembly should assume everything is a reflection target (otherwise why root it?) so I'd not expect that one to happen.

[vitek-karas]

That's a good point - and it should work that way already.