[NativeAOT] Add support for [Preserve] attributes #18666
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…ationAwareSubStep This improves error reporting.
…cDependency attributes. Add support for mapping Preserve attributes to DynamicDependency attributes, and use for NativeAOT. This fixes a problem where NativeAOT would trim away all our tests in our test suites.
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Make 'Full' the default link mode for all platforms for NativeAOT because: * It will achieve the best app size. * NativeAOT is not a drop-in replacement that's guaranteed to work - there are already known features NativeAOT don't support (and may never support), so this may be an opportunity for app developers (and component vendors) to make their products trimmer safe. * NativeAOT will be in preview in .NET 8, so we can change our mind later if it turns out to cause too many problems. * In some cases NativeAOT might require trimming, because we may optimize/rewrite assemblies to be NativeAOT-compatible to support existing libraries. This requires #18666 to be merged first, so that ests aren't linked away.
This is a follow-up to #18666 The idea is to transform `[Preserve(Conditional = true)]` into `[DynamicDependency(signature, type)]` on the static constructor of the given type: ```c# class MyClass { [Preserve (Conditional = true)] public void MyMethod () { } // transformed into ... [DynamicDependency ("MyMethod", typeof (MyClass))] static MyClass() { /* either the existing cctor or a new empty cctor */ } public void MyMethod () { } } ```
This is a follow-up to #18666 This PR resolves the following warnings that I've seen in build logs when running MySingleView with NativeAOT: ``` ILLink : warning IL2034: <Module>..cctor(): The 'DynamicDependencyAttribute' could not be analyzed. [/..../xamarin/xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj] ILLink : warning IL2034: <Module>..cctor(): The 'DynamicDependencyAttribute' could not be analyzed. [/..../xamarin/xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj] ... ``` The generated module cctor code looks like this: ```c# internal class <Module> { [DynamicDependency("InvokeConformsToProtocol(ObjCRuntime.NativeHandle)", typeof(NSObject))] [DynamicDependency("Foundation.NSObject", null)] [DynamicDependency("ConformsToProtocol(ObjCRuntime.NativeHandle)", typeof(NSObject))] [DynamicDependency("Foundation.NSObject", null)] // ... static <Module>() { // ... } } ``` The `[DynamicDependency("T", null)]` attributes are invalid. We could change them to `[DynamicDependency(DynamicallyAccessedMemberTypes.None, typeof(T))]` but I think that attribute is redundant because the type itself is always already preserved through the attribute that precedes it with a selector of one of its methods/fields.
Add partial support for the
[Preserve]attribute for NativeAOT. This is done by injecting an equivalent[DynamicDependency]attribute. The partial support comes from the fact that there's no way to map a conditional preserve attribute ([Preserve (Conditional = true)]) to a[DynamicDependency]attribute, so we report a warning instead.For non-conditional
[Preserve]attributes, we'll now add a[DynamicDependency]attribute to the assembly's module constructor for the type/member in question, effectively rooting it.This makes it possible to fully link all our test suites when NativeAOT (otherwise NativeAOT would just link out all the tests, leaving the test suites empty - and unfortunately green, so this was a rather accidental discovery).