Revert System.Text.Json source generator changes from PR #124077

[Copilot]

Description

Reverts System.Text.Json changes from PR #124077 per @eiriktsarpalis feedback. The change caused a performance regression by breaking the 1-1 correspondence between Compilation and KnownTypeSymbols instances, creating fresh instances for every JsonSerializerContext instead of caching per compilation.

LoggerMessageGenerator changes remain unchanged.

Changes

Restored the original incremental pipeline pattern in JsonSourceGenerator.Roslyn4.0.cs:

Before (PR #124077):

contextGenerationSpecs = context.SyntaxProvider
    .ForAttributeWithMetadataName(
        ...,
        (context, cancellationToken) =>
        {
            var knownTypeSymbols = new KnownTypeSymbols(context.SemanticModel.Compilation);
            Parser parser = new(knownTypeSymbols);
            // ... parsing
        })

After (this PR):

IncrementalValueProvider<KnownTypeSymbols> knownTypeSymbols = context.CompilationProvider
    .Select((compilation, _) => new KnownTypeSymbols(compilation));

contextGenerationSpecs = context.SyntaxProvider
    .ForAttributeWithMetadataName(
        ...,
        (context, _) => (ContextClass: context.TargetNode, context.SemanticModel))
    .Combine(knownTypeSymbols)
    .Select(static (tuple, cancellationToken) =>
    {
        Parser parser = new(tuple.Right);  // KnownTypeSymbols
        // ... parsing using tuple.Left.ContextClass, tuple.Left.SemanticModel
    })

This ensures KnownTypeSymbols is cached per compilation in the incremental pipeline, eliminating redundant type symbol resolution.

Testing

• Built clr+libs successfully
• All 154 System.Text.Json.SourceGeneration unit tests pass

Original prompt

Context

PR #124077 ("Improve LoggerMessageGenerator and JsonSourceGenerator incrementality") was merged and included changes to both the LoggerMessageGenerator and JsonSourceGenerator. However, per feedback from @eiriktsarpalis in #124077 (comment), the System.Text.Json changes should be reverted because:

1. Passing Compilation in the incremental graph does not automatically imply re-runs of the source generator every time it gets updated. This is only true in cases where such updates flow all the way to the SourceProductionContext. The STJ generator does not do this (source production is only triggered on the basis of equality of the resultant ContextGenerationSpec) and they have testing that validates that property.
2. More importantly, the change actually regresses performance: KnownTypeSymbols defines a cache of known type symbols that the generator needs to reference. By removing it from the incremental graph, there is no longer a 1-1 correspondence between Compilation instances and KnownTypeSymbols instances, instead creating a fresh instance every time a JsonSerializerContext is processed for a given compilation.

What needs to be done

Revert ONLY the changes to src/libraries/System.Text.Json/gen/JsonSourceGenerator.Roslyn4.0.cs introduced by PR #124077 (merge commit 5709f35ed344f59062042d234b9a68683ec7e7a6).

The file was changed to:

• Remove the CompilationProvider dependency and the knownTypeSymbols incremental value provider
• Move KnownTypeSymbols instantiation into the ForAttributeWithMetadataName transform callback
• Restructure the pipeline to no longer use .Combine(knownTypeSymbols) and .Select()

The revert should restore the original pattern where:

• KnownTypeSymbols is created via context.CompilationProvider.Select(...) as an IncrementalValueProvider<KnownTypeSymbols>
• The attribute provider uses .Combine(knownTypeSymbols)
• A subsequent .Select() performs the parsing using tuple.Left (for the context class + semantic model) and tuple.Right (for the known type symbols)
• The ForAttributeWithMetadataName transform only extracts (ContextClass, SemanticModel) without doing any parsing

Do NOT revert any LoggerMessageGenerator changes — those are correct and should remain as-is.

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Pull request overview

This PR reverts the System.Text.Json source generator pipeline change from PR #124077 to restore per-compilation caching of KnownTypeSymbols, avoiding redundant symbol resolution and the associated performance regression.

Changes:

• Reintroduced context.CompilationProvider.Select(...) to create an incremental KnownTypeSymbols value cached per Compilation.
• Restored the .Combine(knownTypeSymbols).Select(...) pipeline shape so parsing uses the combined (SemanticModel, ContextClass) + cached KnownTypeSymbols.
• Moved KnownTypeSymbols construction back out of the ForAttributeWithMetadataName transform.