Add a STJ feature flag treating non-optional constructor parameters as required

[eiriktsarpalis]

Background and motivation

System.Text.Json constructor-based deserialization currently treats all constructor parameters as optional, as can be highlighted in the following example:

var result = JsonSerializer.Deserialize<Person>("{}");
Console.WriteLine(result); // Person { Name = , Age = 0 }

public record Person(string Name, int Age);

This can create unexpected validation problems since users typically think of non-optional parameters as required values in the object contract. It typically also results in deserialized properties violating their own nullability annotations. Another problem is that it could result either in discrepancies or overly verbose outputs in components that are attempting to derive a JSON schema from the STJ contracts.

Changing this behavior would most likely constitute a serious breaking change, so we would instead like to expose a feature flag that opts in to new behavior, marking non-optional constructor parameters as required. Rather than making this an application-wide feature switch, this should instead be a per-JsonSerializerOptions setting to avoid breaking third-party components making use of STJ.

Related to #1256

API Proposal

namespace System.Text.Json;

public partial class JsonSerializerOptions
{
    public bool RequireConstructorParameters { get; set; } = false;
}

namespace System.Text.Json.Serialization;

public partial class JsonSourceGenerationOptionsAttribute
{
    public bool RequireConstructorParameters { get; set; } = false;
}

API Usage

var options = new JsonSerializerOptions {  RequireNonOptionalConstructorParameters = true };

JsonSerializer.Deserialize<Person>("""{"Name" : "John", "Age" : 27 }""", options); // Success
JsonSerializer.Deserialize<Person>("""{"Name" : "John", "Address" : "known"}""", options);
//  JSON deserialization for type 'Person' was missing required properties, including the following: Age

public record Person(string Name, int Age, string Address = "unknown");

Alternative Designs

This proposal is very closely related to #100144. We should consider a design that unifies both behaviors under a single feature switch.

Risks

No response

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

[bartonjs]

Video

• RequireConstructorParameters => RespectRequiredConstructorParameters
• Should use an AppContext value to determine the default, but absent one it should be on (breaking change).
  • System.Text.Json.JsonSerializer.RespectRequiredConstructorParametersDefault

namespace System.Text.Json;

public partial class JsonSerializerOptions
{
    public bool RespectRequiredConstructorParameters { get; set; } = AppContext(true);
}

namespace System.Text.Json.Serialization;

public partial class JsonSourceGenerationOptionsAttribute
{
    public bool RespectRequiredConstructorParameters { get; set; } = AppContext(true);
}

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this issue has the breaking-change label.

1. Create and link to this issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.

Tagging @dotnet/compat for awareness of the breaking change.

[eiriktsarpalis]

Similar to nullability annotations, we're reverting this to be an opt-in feature hence it no longer constitutes a breaking change.