DAM on extension properties is silently ignored

[sbomer]

With #119017, the trimming tools ignore DAM on extension properties (and don't produce any warnings about it).

An extension property lowers to a property on a compiler-generated type, a stub method on the generated type, and an implementation static method on the user-declared type. To avoid having to reverse this lowering in ILLink and ILC, annotations on extension properties are not automatically propagated to the parameter/return of the static method that plays the role of the set/get accessor.

Options to address this:

1. Leave it as-is (annotations on extension properties have no effect)
2. Warn about the unused annotation
3. Reverse the extension property lowering to propagate the annotation

Since property annotations don't normally flow to the accessor methods (that's an ILLink rule, not a Roslyn behavior, to my knowledge), I don't think this should be handled by Roslyn via CompilerLoweringPreserve.

Personally, I think 2. makes the most sense, but 1. is fine too since I expect this to be rare.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/illink
See info in area-owners.md if you want to be subscribed.

[agocke]

Isn't this the same as dotnet/roslyn#80017?

[sbomer]

No, this is specifically about the automatic propagation of property annotations to the accessors. Roslyn doesn't do that for normal properties, but ILLink does.

In dotnet/roslyn#80017, even if you explicitly annotate the return: of the extension property, it doesn't end up in the implementation.

[agocke]

I think (2) makes sense.