System.Text.Json fails to build on .NET Native due to unconstrained raw pointer uses

[Sergio0694]

Description

I'm dogfooding .NET 8 builds of System.Text.Json in the Microsoft Store, now that we've fully migrated to it, and noticed that since the .NET 8 Preview 1 release, .NET Native has starting failing to build. We've investigated this and @AntonLapounov managed to narrow this down to some uses of unconstrained raw pointers, which is a known issue with .NET Native.

Specifically, the build failure is caused by some changes in 4deccc02588990626180eaa812e63603d6c2790e:

https://github.com/dotnet/runtime/blob/8cb3bf89e4b28b66bf3b4e2957fd015bf925a787/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs#L135-L182

https://github.com/dotnet/runtime/blob/8cb3bf89e4b28b66bf3b4e2957fd015bf925a787/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs#L236-L262

This is a complete blocker for us, as we cannot update to System.Text.Json 8.0 at all.

Could we revert these changes in EnumConverter<T> and just keep using Unsafe.As<TFrom, TTo>? Since it's an intrinsic it wouldn't really have any performance impacts on .NET 8, and it'd at least fix the compile issues on .NET Native we're hitting.

cc. @eiriktsarpalis, also could we mark this as partner impact? We're completely stuck on 7.0 as of right now 🥲

Reproduction Steps

• Create a blank UWP app
• Reference System.Text.Json 8.0.0 Preview 1 or greater
• Add this code

[JsonConverter(typeof(JsonStringEnumConverter))]
public enum MyEnum
{
    A,
    B,
    C
}

public sealed class MyClass
{
    public MyEnum Value { get; set; }
}

[JsonSerializable(typeof(MyClass))]
public sealed partial class MyContext : JsonSerializerContext
{
}

Compile in Release.

Expected behavior

The app should build correctly.

Actual behavior

The build fails with an error like this:

"Error ILT0005: 'C:\Users\sergiopedri.nuget\packages\runtime.win10-x64.microsoft.net.native.compiler\2.2.12-rel-31116-00\tools\x64\ilc\Tools\nutc_driver.exe @"C:\Users\sergiopedri\source\repos\SystemTextJsonILCRepro\obj\x64\Release\ilc\intermediate\MDIL\SystemTextJsonILCRepro.rsp"' returned exit code -1073741819"

Specifically, the real issue is here:

"System.Text.Json.Serialization.Converters.EnumConverter`1[[System.UniversalCanon,System.Private.CoreLib]],System.Text.Json" "Read" /singleMethodParam "System.UniversalCanon,System.Private.CoreLib" /singleMethodParam "System.Text.Json.Utf8JsonReader&,System.Text.Json" /singleMethodParam "System.Type,System.Private.CoreLib" /singleMethodParam "System.Text.Json.JsonSerializerOptions,System.Text.Json""

Regression?

Yes, it works just fine with System.Text.Json 7.0.2.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis See info in area-owners.md if you want to be subscribed.

Issue Details

---

### Description I'm dogfooding .NET 8 builds of System.Text.Json in the Microsoft Store, now that we've fully migrated to it, and noticed that since the .NET 8 Preview 1 release, .NET Native has starting failing to build. We've investigated this and @AntonLapounov managed to narrow this down to some uses of unconstrained raw pointers, which is a known issue with .NET Native. Specifically, the build failure is caused by some changes in [4deccc02588990626180eaa812e63603d6c2790e](https://github.com/dotnet/runtime/commit/4deccc02588990626180eaa812e63603d6c2790e#diff-2fdb28f63c50c262d9ccdd1bea7e7ba9affaa934491b07c59f3ddc40a235ee4bL140): https://github.com/dotnet/runtime/blob/8cb3bf89e4b28b66bf3b4e2957fd015bf925a787/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs#L135-L182 https://github.com/dotnet/runtime/blob/8cb3bf89e4b28b66bf3b4e2957fd015bf925a787/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs#L236-L262 This is a **complete blocker** for us, as we cannot update to System.Text.Json 8.0 at all. Could we revert these changes in `EnumConverter` and just keep using `Unsafe.As`? Since it's an intrinsic it wouldn't really have any performance impacts on .NET 8, and it'd at least fix the compile issues on .NET Native we're hitting. cc. @eiriktsarpalis, also could we mark this as partner impact? We're completely stuck on 7.0 as of right now 🥲 ### Reproduction Steps - Create a blank UWP app - Reference System.Text.Json 8.0.0 Preview 1 or greater - Add this code ```csharp [JsonConverter(typeof(JsonStringEnumConverter))] public enum MyEnum { A, B, C } public sealed class MyClass { public MyEnum Value { get; set; } } [JsonSerializable(typeof(MyClass))] public sealed partial class MyContext : JsonSerializerContext { } ``` Compile in Release. ### Expected behavior The app should build correctly. ### Actual behavior The build fails with an error like this: > "Error ILT0005: 'C:\Users\sergiopedri\.nuget\packages\runtime.win10-x64.microsoft.net.native.compiler\2.2.12-rel-31116-00\tools\x64\ilc\Tools\nutc_driver.exe @"C:\Users\sergiopedri\source\repos\SystemTextJsonILCRepro\obj\x64\Release\ilc\intermediate\MDIL\SystemTextJsonILCRepro.rsp"' returned exit code -1073741819" Specifically, the real issue is here: > "System.Text.Json.Serialization.Converters.EnumConverter`1[[System.__UniversalCanon,System.Private.CoreLib]],System.Text.Json" "Read" /singleMethodParam "System.__UniversalCanon,System.Private.CoreLib" /singleMethodParam "System.Text.Json.Utf8JsonReader&,System.Text.Json" /singleMethodParam "System.Type,System.Private.CoreLib" /singleMethodParam "System.Text.Json.JsonSerializerOptions,System.Text.Json"" ### Regression? Yes, it works just fine with System.Text.Json 7.0.2. ### Known Workarounds _No response_ ### Configuration _No response_ ### Other information _No response_

Author:	Sergio0694
Assignees:	-
Labels:	`area-System.Text.Json`, `untriaged`
Milestone:	-

[Sergio0694]

Thinking about this more, and since @tannergooding mentioned that the previous use of Unsafe.As<TFrom, TTo> had some concerns around additional generic instantiations, could we not just stop using pointers altogether and just box/unbox? The JIT/AOT compilers should recognize things anyway and elide the boxing anyway. That is, in both directions:

// Read
if (reader.TryGetInt32(out int int32))
{
    return (T)(object)int32;
}

// Write
writer.WriteNumberValue((int)(object)value);

This would both solve the issue on .NET Native and also potentially gives even better codegen due to no address taken?

[EgorBo]

(TEnum)(object)integer pattern won't work for .NET Framework and likely won't for Mono too, e.g.

https://sharplab.io/#v2:EYLgHgbALANALiAhgZwLYB8ACAmAjAWAChMAGAAk1wDoAlAVwDs4BLVAUyoGEB7VAB2YAbNgCcAyqIBuzAMZtkAbiJFMAZgrYynMgG8iZA2X2G1ZNgzqoyAWQCeAUQupjBvYUMeyAQRgAhGJwuZAC+QUGUEDYOTmQAKvJwABQAlEFuniYA7HHcjpYAPHZ5qAB8iVCqyUruhqGEQQDa1mxwABbcACYAkvyCic1tnT18ggDyfCzcDMhUXgDmcyLyyMySbF0MgswM23PJALrhuJGxOcX5sWXbcGSIyWQA7q2ibHFkIGTIcCJ0MnAwZGKaSCHkw2USsWSiW4wAAVmw/slENUPHUiMEgA

CoreCLR learned to fold it fairly recently (in net7)

[Sergio0694]

"won't work for .NET Framework"

Just to clarify, do you mean it'll work slowly, and not that it won't work at all? Because ai tried this in the NETFX C# interactive window and it seems to work fine on NETFX too.

If performance on NETFX and Mono is an issue, it seems we could:

• Revert the Unsafe.As use assuming that's fine with respect to additional generics.
• Use a few #ifdef-s to keep raw pointers on .NET 8+, and revert only on downlevel targets? If it's just this couple methods it seems manageable?

[EgorBo]

it'll work slowly

Yes, it boxes (allocates)

[jkotas]

it's an intrinsic it wouldn't really have any performance impacts on .NET 8

The generic intrinsic references contribute to static footprint that is also a performance metric. Reducing the static footprint was my motivation for the change.

Revert the Unsafe.As use assuming that's fine with respect to additional generics

I think it is fine to revert the change for System.Text.Json. System.Text.Json EnumConverter has large static footprint (e.g. it instantiates ConcurrentDictionary<string, T> - that is 10's kB of static footprint right there for each serialized enum type). A few extra generic instantiations from Unsafe.As are not a big deal.

[Sergio0694]

"The generic intrinsic references contribute to static footprint that is also a performance metric. Reducing the static footprint was my motivation for the change."

Yup, no that makes perfect sense. I should've clarified, I only meant to revert those changes in EnumConverter<T> specifically, I do understand the motivation for the change in all other places in general to reduce generic instantiations 🙂

---

Was about to say feel free to assign me to this but I see you already have a PR up ahah awesome! 😄