Add tests for customizing source-gen contracts

[layomia]

Attempting to understand/codify behavior expectations when users attempt to modify contracts generated by the source generator.

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Issue Details

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Attempting to understand/codify behavior expectations when users attempt to modify contracts generated by the source generator.

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

[eiriktsarpalis]

If the user modifies source gen metadata, then they're necessarily using a contract resolver other than source generated JsonSerializerContext instances. In such cases I would expect that CanUseSerializeHandler is always flipped back to false:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs

Line 537 in bce23ab

CanUseSerializeHandler&=Options.SerializerContext?.CanUseSerializationLogic==true;

Is there any particular case that was missed out and there's a failing test without this check?

[layomia]

That would be this:

JsonTypeInfo<Person>typeInfo=SourceGenContext.Default.Person;foreach(JsonPropertyInfopropertyintypeInfo.Properties){property.Name=property.Name.ToUpperInvariant();}stringjson=JsonSerializer.Serialize(person,typeInfo);// Fail: regular-case is serialized instead (because fast-path is taken; metadata should be used instead since we have it).JsonTestHelper.AssertJsonEqual(@"{""FIRSTNAME"":""Jane"",""LASTNAME"":""Doe""}",json);Personperson=JsonSerializer.Deserialize(json,typeInfo);Assert.Equal("Jane",person.FirstName);Assert.Equal("Doe",person.LastName);

[eiriktsarpalis]

Oh wow. Fundamentally I think the problem here is that static metadata is mutable even though these should be locked for modification. We should try to service this.

cc @krwq

[layomia]

Closing this PR - the most succinct way to customize src-gen'd contracts IMO is by compisition, where a JsonSerializerContext instance is nested within a custom user's IJsonTypeInfoResolver and probed for metadata which can then be modified. However, there are integral issues with modifying src-gen contracts (#76535). Further testing for src-gen contract modification should come with fixes for these issues.