JSON src-gen fast-path does not consider writer Encoder option

[layomia]

When writing via the serializer e.g. JsonSerializer.Serialize(foo, context.Foo), fast-path logic does not consider the encoder on the writer (via Utf8JsonWriter.Options.Encoder). This means property names would be written with default encoding and values could be written with a custom encoder. This mismatch can be considered invalid - we should disallow the use of a custom encoder on the writer to align with supported features for fast-path.

We should also verify the behavior of both metadata and reflection-based serializers if/when JsonSerializerOptions.Encoder and Utf8JsonWriter.Options.Encoder differ.

Calling fast-path logic directly e.g. context.Foo.SerializeHandler(writer, foo) with writer encoder info that differs from options specified ahead of time via [JsonSourceGenerationOptions] is considered a user error. The raw serialize logic will not perform any options validation.

cc @eiriktsarpalis @steveharter for any thoughts.

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Issue Details

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When writing via the serializer e.g. JsonSerializer.Serialize(foo, context.Foo), fast-path logic does not consider the encoder on the writer (via Utf8JsonWriter.Options.Encoder). This means property names would be written with default encoding and values could be written with a custom encoder. This mismatch can be considered invalid - we should disallow the use of a custom encoder on the writer to align with supported features for fast-path.

We should also verify the behavior of both metadata and reflection-based serializers if/when JsonSerializerOptions.Encoder and Utf8JsonWriter.Options.Encoder differ.

Calling fast-path logic directly e.g. context.Foo.SerializeHandler(writer, foo) with writer encoder info that differs from options specified ahead of time via [JsonSourceGenerationOptions] is considered a user error. The raw serialize logic will not perform any options validation.

cc @eiriktsarpalis @steveharter for any thoughts.

Author:	layomia
Assignees:	layomia
Labels:	area-System.Text.Json
Milestone:	7.0.0

[eiriktsarpalis]

Moving to Future, as we won't have time to work on this in the .NET 7 timeframe.