Use source generator in JSON trimming tests

[ApparentlyPlus]

Fixes #53437.

The existing collection trimming tests now use the source generator, and I rewrote the remaining 16 SerializerEntryPoint apps (one per JsonSerializer overload) along with ObjectConvertersTest. Like the collection tests, they run with reflection disabled and on the Native AOT test leg.

These were removed in #53235, the same PR that deleted the DynamicallyAccessedMembers annotations they validated (#52268), so the source generated versions verify the overloads themselves instead. The same goes for Stack, Queue, Queue<T> and ConcurrentStack, restored here and supported by the source generator since #53393.

A nice consequence is that Hashtable no longer needs its browser-wasm skip. The generated ObjectCreator roots the constructor statically, and that is what was getting trimmed.

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[Copilot]

🟡 Changes recommended

Two critical serializer tests must flush Utf8JsonWriter before reading the stream.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR migrates JSON trimming tests to source-generated metadata for reflection-disabled and Native AOT coverage.

Changes:

• Adds generated serializer-overload and collection tests.
• Restores stack, queue, and object-converter coverage.
• Updates trimming-test wiring and assertion exit codes.

File summaries

File	Summary	Status
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/System.Text.Json.TrimmingTests.proj	Registers trimming test applications and execution settings.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/SerializeAsync.ToStream.TypedObject.cs	Tests generated async stream serialization for typed objects.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/SerializeAsync.ToStream.BoxedObject.cs	Tests generated async stream serialization for boxed objects.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToString.TypedObject.WithWriter.cs	Tests typed serialization through a writer; writer must be flushed before stream inspection.	Changes required: critical flush issue
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToString.TypedObject.cs	Tests typed string serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToString.BoxedObject.WithWriter.cs	Tests boxed serialization through a writer; writer must be flushed before stream inspection.	Changes required: critical flush issue
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToString.BoxedObject.cs	Tests boxed string serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToByteArray.TypedObject.cs	Tests typed byte-array serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToByteArray.BoxedObject.cs	Tests boxed byte-array serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/DeserializeAsync.FromStream.TypedObject.cs	Tests typed async stream deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/DeserializeAsync.FromStream.BoxedObject.cs	Tests boxed async stream deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Deserialize.FromString.TypedObject.cs	Tests typed string deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Deserialize.FromString.BoxedObject.cs	Tests boxed string deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Deserialize.FromSpan.TypedObject.cs	Tests typed span deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Deserialize.FromSpan.BoxedObject.cs	Tests boxed span deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Deserialize.FromReader.TypedObject.cs	Tests typed reader deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Deserialize.FromReader.BoxedObject.cs	Tests boxed reader deserialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/ObjectConvertersTest.cs	Tests generated object converters.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Helper.cs	Provides shared generated collection-test helpers.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/StackOfT.cs	Covers generated generic stack serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/Stack.cs	Covers generated stack serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/QueueOfT.cs	Covers generated generic queue serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/Queue.cs	Covers generated queue serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ListOfT.cs	Covers generated list serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ISetOfT.cs	Covers generated set-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IReadOnlyDictionaryOfTKeyTValue.cs	Covers generated read-only dictionary serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IListOfT.cs	Covers generated generic list-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IList.cs	Covers generated list-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IEnumerableOfT.cs	Covers generated generic enumerable serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IEnumerable.cs	Covers generated enumerable serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IDictionaryOfTKeyTValue.cs	Covers generated generic dictionary-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/IDictionary.cs	Covers generated dictionary-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ICollectionOfT.cs	Covers generated generic collection-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ICollection.cs	Covers generated collection-interface serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/Hashtable.cs	Covers generated Hashtable serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/HashSetOfT.cs	Covers generated hash-set serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/DictionaryOfTKeyTValue.cs	Covers generated dictionary serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ConcurrentStack.cs	Covers generated concurrent-stack serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ConcurrentQueue.cs	Covers generated concurrent-queue serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/ConcurrentDictionary.cs	Covers generated concurrent-dictionary serialization.	Reviewed
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/Collections/Array.cs	Covers generated array serialization.	Reviewed

Review details

Suppressed comments (2)

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToString.BoxedObject.WithWriter.cs:36

• The writer still owns the serialized POCO bytes when stream.ToArray() is called; the serializer does not flush the supplied Utf8JsonWriter. This makes the assertion observe an empty or incomplete stream. Flush the writer before reading the stream.
  src/libraries/System.Text.Json/tests/System.Text.Json.Tests/TrimmingTests/SerializerEntryPoint/Serialize.ToString.TypedObject.WithWriter.cs:36
• The writer still owns the serialized POCO bytes when stream.ToArray() is called; the serializer does not flush the supplied Utf8JsonWriter. This makes the assertion observe an empty or incomplete stream. Flush the writer before reading the stream.

• Files reviewed: 41/41 changed files
• Comments generated: 2
• Review effort level: Lite

[teo-tsirpanis]

Left some comments; they also apply to the other test files.

[Copilot]

🟡 Changes recommended

Restore the helper’s type-compatibility assertion and address the repeated exit-code issue.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 41/41 changed files
• Comments generated: 2
• Review effort level: Lite

[eiriktsarpalis]

Hi, an important thing to note about the reflection-based trimming tests is that they were introduced intentionally to support such use cases (most prominently in the case of blazor). Even though that arrangement has its problems, I wouldn't want to see coverage gone at the moment.

[teo-tsirpanis]

@eiriktsarpalis the problem is that these tests can break by seemingly unrelated changes, like in #132115. If there are scenarios where reflection-based serialization is guaranteed to be supported with trimming (the only thing I can imagine is POD types with primitive fields/properties and arrays thereof, after we root the assembly in question), maybe the trimming tests can be restricted down to these.

[ApparentlyPlus]

Everything is green besides the known failures.

[eiriktsarpalis]

the problem is that these tests can break by seemingly unrelated changes, like in #132115.

Repurposing test suites is generally not good practice, since it may result in unintended regressions down the line. I would prefer it if the improvements were incremental-only.

[teo-tsirpanis]

I'm not sure I understand. The existing purpose of the tests is to exercise a flaky and unsupported scenario that gives trim warnings. Isn't this repurposing what #53437 is for?

[eiriktsarpalis]

Isn't this repurposing what #53437 is for?

I'm sceptical because of the blazor issue. Unless @dotnet/dotnet-maui-blazor-eng can confirm that they have moved their core types to the source generator I don't think we should action this.

[ApparentlyPlus]

I largely agree with @teo-tsirpanis. My case is that this adds coverage rather than removing it.

Hashtable's browser-wasm skip is gone here and the wasm linker-tests leg is green. That skip existed because the trimmer removed Hashtable's constructor and reflection had no way to recover, which is the exact same failure @teo-tsirpanis hit in #132115.

Dropping Hashtable from TypeBuilderInstantiation trimmed the type and broke an unrelated PR. Same cause, five years apart, and both times the test was disabled instead of being fixed. Faulty tests blocking unrelated PRs from progressing should never happen, imho.

The proposed fix was the same both times too. The 2021 skip in #53235 says "These tests should be converted to use the source generator. See #53437". Teo's commit says "Disable it until it gets updated to use the source generator. It's not the first; see also StackOfT.cs". This PR does exactly that.

Reflection is still exercised, too. EnumConverterTest, JsonConverterAttributeTest and StackOrQueueNotRootedTest all run with JsonSerializerIsReflectionEnabledByDefault on.

Blazor coverage today is StackOfT commented out since 2021, most of the suite disabled on Native AOT by #101418, and Hashtable.cs about to return 100 without asserting anything if Teo's PR is merged. That protects nobody.

Still, if you wish @eiriktsarpalis, we could restrict the reflection tests to the subset expected to survive.

[javiercn]

I don't think we should switch the trimming tests to exclusively target the source-generated serializer at this point. Blazor still relies on reflection-based STJ serialization in trimmed apps today (at least for .NET 11), and moving the tests entirely to source generation could allow regressions in that scenario to go undetected.

[teo-tsirpanis]

regressions in that scenario to go undetected

My impression has been that reflection-based serialization is not supported with trimming whatsoever. What are the actual supported scenarios? Is it "POD types with primitive fields/properties and arrays thereof, after we root the assembly in question" that I mentioned before, or are there more? Can we scope the trimming tests down to a supported subset that is not prone to breaks from unrelated changes to the BCL (like my aforementioned PR to Reflection.Emit)?

[eiriktsarpalis]

My impression has been that reflection-based serialization is not supported with trimming whatsoever.

Yes and no. It isn't guaranteed to work, but nevertheless blazor runs on the fact that certain scenaria it relies on do not break.

[ApparentlyPlus]

@javiercn I had a look at where Blazor depends on this today, to work out what a test here could usefully pin.

Most of it is user types the framework never sees at build time. JS interop only chains in a reflection resolver when the switch is on:

if (JsonSerializer.IsReflectionEnabledByDefault)
{
    JsonSerializerOptions.TypeInfoResolverChain.Add(CreateReflectionResolver());
}

and WebAssemblyComponentParameterDeserializer deserializes prerendered parameters against a Type resolved from an assembly qualified name at runtime. Apps root those themselves, which is what NativeAotTestApp.csproj does with JsonSerializerIsReflectionEnabledByDefault=true alongside TrimmerRootAssembly.

The framework's own types mostly go through the source generator, between WebAssemblyJsonSerializerContext, JSInteropJsonTypeInfoResolver and the contexts in Endpoints and the Server circuits. The one exception I found is AuthenticationStateSerializer, which persists AuthenticationStateData through PersistentComponentState.PersistAsJson, and that is [RequiresUnreferencedCode] and reflection based.

That last one is the only case I found worth keeping reflection tests for, a framework POCO with primitive and collection members serialized reflectively with the assembly rooted, and it's very close to the subset @teo-tsirpanis suggested. A Hashtable holding a single int doesn't model it. I'd be happy to add a test for that shape here and keep the source-generated ones alongside if that would settle the disagreement.

Let me know if I've missed a path, otherwise I don't quite understand why this can't move forward.