Emit AOT/trim analyzer warning when reference doesn't have IsTrimmable/IsAotCompatible

[MichalStrehovsky]

The Roslyn trim analyzer doesn't have a whole program view and doesn't know about trim/AOT unsafety of dependencies if they're not annotated. One can easily get into a situation where an unannotated dependency is not detected as problematic and an IsTrimmable assembly that can't actually be used with trimming is produced by the build.

It happened to us in #102341 where we have an IsTrimmable assembly that depends on another assembly that we didn't annotate yet.

Our guidance is to have a test app exercising everything in the library that gets PublishTrimmed/PublishAot with WarnAsError true to catch whatever the analyzer missed.

We might want to consider emitting a warning from the analyzer when a reference doesn't have the IsTrimmable attribute. This warning would only be generated by the analyzer, not by illink/ilc. It would detect cases where the dependency isn't contractually trimmable. The contractual aspect (the author of the library guarantees trimability) is interesting - the test app approach doesn't ensure that - a dependency that isn't contractually trimmable, just happens to be fine for the test app doesn't guarantee it won't be broken in next version.

Cc @dotnet/illink @eerhardt

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[Sergio0694]

This would be really nice to have. FYI @jevansaks I know this is something you also wanted for a while 😄

[jevansaks]

+1, many devs on our team have been burned by this. If they got a warning right away they wouldn't do so much throw-away work before they found out some dependency wasn't actually AOT compatible.

[Sergio0694]

To clarify, this would also need some .NET SDK support to emit some assembly attribute to indicate AOT compatibility, right? As far as I know, right now there's only the "IsTrimmable" metadata, which doesn't tell you whether a .dll is actually AOT compatible 🤔

[jevansaks]

There's a couple open nuget issues asking for this:
NuGet/NuGetGallery#9743
NuGet/NuGetGallery#9820

[sbomer]

Talked about this with @agocke and we think it would make sense for .NET 10 to support this behavior for trimming only - which would catch most cases even if we don't have "IsAotCompatible" assembly metadata. We think it should be opt-in to prevent breaking projects that use trimming.

[jevansaks]

Would IsAotCompatible set the "supports trimming" bit in this case for nuget packages?