Fix NETStandard library using JSON source gen (#63520)

NETStandard libraries using JSON source gen would fail to load on
.NETCore due to missing IsExternalInit in the assembly.

On .NETCore this is defined, whereas on .NETStandard JSON carries an
internal copy.  The compiler emits references to the internal type when
a NETStandard library calls init-only setters in JSON types, but these
references are not satisfied when the library runs on .NETCore.

Fix this by adding a type forward to JSON for the IsExternalInit type.

src/libraries/System.Text.Json/ref/System.Text.Json.Typeforwards.netcoreapp.cs

@@ -0,0 +1,6 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// The compiler emits a reference to the internal copy of this type in our non-NETCoreApp assembly
+// so we must include a forward to be compatible with libraries compiled against non-NETCoreApp System.Text.Json
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]

src/libraries/System.Text.Json/ref/System.Text.Json.csproj

@@ -7,6 +7,11 @@
   <ItemGroup>
     <Compile Include="System.Text.Json.cs" />
   </ItemGroup>
+
+  <ItemGroup Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'">
+    <Compile Include="System.Text.Json.Typeforwards.netcoreapp.cs" />
+  </ItemGroup>
+
   <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net5.0'))">
     <Compile Include="$(CommonPath)System\Runtime\CompilerServices\IsExternalInit.cs" Link="Common\System\Runtime\CompilerServices\IsExternalInit.cs" />
     <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\DynamicallyAccessedMembersAttribute.cs" />

src/libraries/System.Text.Json/src/System.Text.Json.Typeforwards.netcoreapp.cs

@@ -0,0 +1,6 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// The compiler emits a reference to the internal copy of this type in our non-NETCoreApp assembly
+// so we must include a forward to be compatible with libraries compiled against non-NETCoreApp System.Text.Json
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -312,6 +312,7 @@ System.Text.Json.Utf8JsonReader</PackageDescription>
     <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\UnconditionalSuppressMessageAttribute.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'">
+    <Compile Include="System.Text.Json.Typeforwards.netcoreapp.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializerOptionsUpdateHandler.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/System.Text.Json.TestLibrary.Roslyn3.11.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <Import Project="System.Text.Json.TestLibrary.targets" />
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn3.11.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+  </ItemGroup>
+</Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/System.Text.Json.TestLibrary.Roslyn4.0.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <Import Project="System.Text.Json.TestLibrary.targets" />
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+  </ItemGroup>
+</Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/System.Text.Json.TestLibrary.targets

@@ -0,0 +1,17 @@
+<Project>
+  <PropertyGroup>
+    <TargetFrameworks>netstandard2.0</TargetFrameworks>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\System.Text.Json.csproj" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="TestClasses.cs" />
+  </ItemGroup>
+
+  <Target Name="FixIncrementalCoreCompileWithAnalyzers" BeforeTargets="CoreCompile">
+    <ItemGroup>
+      <CustomAdditionalCompileInputs Include="@(Analyzer)" />
+    </ItemGroup>
+  </Target>
+</Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/TestClasses.cs

@@ -0,0 +1,17 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Text.Json.Serialization;
+
+namespace System.Text.Json.SourceGeneration.Tests.NETStandard
+{
+    public class MyPoco
+    {
+        public string Value { get; set; }
+    }
+
+    [JsonSerializable(typeof(MyPoco))]
+    public partial class NETStandardSerializerContext : JsonSerializerContext
+    {
+    }
+}

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/NETStandardContextTests.cs

@@ -0,0 +1,31 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
+using Xunit;
+
+namespace System.Text.Json.SourceGeneration.Tests.NETStandard
+{
+    public class NETStandardContextTests
+    {
+        /// <summary>
+        /// Tests that we can serialize and deserialize a type defined in a NETStandard assembly.
+        /// This tests an issue where we were emitting source-gen logic that caused the compiler
+        /// to emit a reference to an internal definition of IsExternalInit that was missing
+        /// on later versions of .NET (since it was defined by the framework).
+        /// </summary>
+        [Fact]
+        public void RoundTripNETStandardDefinedSourceGenType()
+        {
+            MyPoco expected = new MyPoco() { Value = "Hello from NETStandard type."};
+
+            string json = JsonSerializer.Serialize(expected, NETStandardSerializerContext.Default.MyPoco);
+            MyPoco actual = JsonSerializer.Deserialize(json, NETStandardSerializerContext.Default.MyPoco);
+            Assert.Equal(expected.Value, actual.Value);
+        }
+    }
+}

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Roslyn3.11.Tests.csproj

@@ -4,5 +4,6 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn3.11.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+    <ProjectReference Include="..\System.Text.Json.SourceGeneration.TestLibrary\System.Text.Json.TestLibrary.Roslyn3.11.csproj" />
   </ItemGroup>
 </Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Roslyn4.0.Tests.csproj

@@ -4,5 +4,6 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+    <ProjectReference Include="..\System.Text.Json.SourceGeneration.TestLibrary\System.Text.Json.TestLibrary.Roslyn4.0.csproj" />
   </ItemGroup>
 </Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Tests.targets

@@ -74,6 +74,7 @@
     <Compile Include="MetadataAndSerializationContextTests.cs" />
     <Compile Include="MetadataContextTests.cs" />
     <Compile Include="MixedModeContextTests.cs" />
+    <Compile Include="NETStandardContextTests.cs" />
     <Compile Include="RealWorldContextTests.cs" />
     <Compile Include="SerializationContextTests.cs" />
     <Compile Include="SerializationLogicTests.cs" />