Latest linker is raising unexpected warnings in System.Text.Json

[eerhardt]

With change #2108, we are now getting unexpected ILLink warnings in System.Text.Json. This is blocking merging the latest linker into dotnet/runtime (dotnet/runtime#54907):

ILLink: Trim analysis warning IL2026: System.Text.Json.Nodes.JsonArray: Using method 'System.Text.Json.Nodes.JsonNode.CreateDynamicObject(Expression,JsonNode)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Using JsonNode instances as dynamic types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed.
ILLink: Trim analysis warning IL2026: System.Text.Json.Nodes.JsonArray: Using method 'System.Text.Json.Nodes.JsonArray.Add<T>(T)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Creating JsonValue instances with non-primitive types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed.
ILLink: Trim analysis warning IL2026: System.Text.Json.Nodes.JsonArray: Using method 'System.Text.Json.Nodes.JsonNode.TrySetMemberMethodInfo.get' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.
ILLink: Trim analysis warning IL2026: System.Text.Json.Serialization.Metadata.JsonTypeInfo: Using method 'System.Text.Json.Serialization.Metadata.JsonTypeInfo.JsonTypeInfo(Type,JsonSerializerOptions)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.
ILLink: Trim analysis warning IL2026: System.Text.Json.Serialization.Metadata.JsonTypeInfo: Using method 'System.Text.Json.Serialization.Metadata.JsonTypeInfo.JsonTypeInfo(Type,JsonConverter,Type,JsonSerializerOptions)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.
ILLink: Trim analysis warning IL2026: System.Text.Json.Nodes.JsonValue`1: Using method 'System.Text.Json.Nodes.JsonValue.Create<T>(T,Nullable<JsonNodeOptions>)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Creating JsonValue instances with non-primitive types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed. Use the overload that takes a JsonTypeInfo, or make sure all of the required types are preserved.
ILLink: Trim analysis warning IL2026: System.Text.Json.Nodes.JsonValue`1: Using method 'System.Text.Json.Nodes.JsonNode.CreateDynamicObject(Expression,JsonNode)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Using JsonNode instances as dynamic types is not compatible with trimming. It can result in non-primitive types being serialized, which may have their members trimmed.
ILLink: Trim analysis warning IL2026: System.Text.Json.Nodes.JsonValue`1: Using method 'System.Text.Json.Nodes.JsonNode.TrySetMemberMethodInfo.get' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.

I reverted 2f62607 locally and verified that without that change we don't get any warnings. So this feels like a bug introduced with that change.

Going through the warnings listed above, all those methods do have [RequiresUnreferencedCode] applied to them, but all calls of the unsafe methods are either suppressed, or the caller has [RequiresUnreferencedCode] itself. So I'm not seeing what is wrong with the library code.

@marek-safar @agocke @sbomer @MichalStrehovsky

[marek-safar]

I think Vitek is away so as to not block the update I think we should temporary revert the change

[sbomer]

These warnings are being produced when the linker doesn't understand DebuggerDisplayAttribute (see #1873 ) and keeps everything on the type and its base types, including RUC methods. It looks like the change intentionally makes this a warning: 2f62607#diff-01d5cdba81adac78da3b5bfc7c322f70461fe7bbf59772b62bb130ab07b7213aR692.

For example JsonTypeInfo has [DebuggerDisplay("ConverterStrategy.{ConverterStrategy}, {Type.Name}")]. The linker doesn't understand it, and keeps everything on JsonTypeInfo.

Or for JsonArray, which has [DebuggerDisplay("JsonArray[{List.Count}]")], it keeps methods on JsonArray and the base type JsonNode.