Fix NRT warnings in source generated code

[eiriktsarpalis]

Addresses an issue where STJ source generated code can result in warnings in cases where member types contain nullable reference type annotations. Also updates the annotations for fast-path serialize methods so that the input value type is nullable for the case of reference types.

Note that the implementation intentionally does not take Roslyn nullability metadata into account when inserting suppresions. This is due to a restriction in our current sourcegen infrastructure which abstracts the Roslyn symbols as System.Reflection types: since nullability metadata is not available in that layer it ends up being erased. We should consider changing this in .NET 7.0.0.

Should be considered for 6.0.0 servicing.

Fix#59464.

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Issue Details

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Addresses an issue where STJ source generated code can result in warnings in cases where member types contain nullable reference type annotations. Also updates the annotations for fast-path serialize methods so that the input value type is nullable for the case of reference types.

Note that the implementation intentionally does not take Roslyn nullability metadata into account when inserting suppresions. This is due to a restriction in our current sourcegen infrastructure which abstracts the Roslyn symbols as System.Reflection types: since nullability metadata is not available in that layer it ends up being erased. We should consider changing this in .NET 7.0.0.

Should be considered for 6.0.0 servicing.

Fix #59464.

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

[layomia]

LGTM

[eiriktsarpalis]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1291940989