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 (JsonPropertyInfo property in typeInfo.Properties)
{
    property.Name = property.Name.ToUpperInvariant();
}

string json = 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); 
Person person = 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.