[API Proposal]: Add IReadOnlySet<T> support to System.Text.Json

[sander1095]

Background and motivation

In #91875 , it's discussed that System.Text.Json doesn't support IReadOnlySet<T>, which was added in .NET 5.0. The team welcomes support, which I am working on, now.

Supporting IReadOnlySet<T> increases the amount of BCL types that System.Text.Json supports. IReadOnlySet being a pretty important interface, part of the System.Collection.Generics namespace, makes me believe that this support is quite important.

Based on the discussions in #91875, I have already started work on the PR: #120306 . There, the helpful @huoyaoyuan told me that this "formal" API proposal is needed, and @eiriktsarpalis would need to have a look at it.

API Proposal

Based on the design proposal for adding support for Memory<T>:

#86442 (comment)

namespace System.Text.Json.Serialization.Metadata;

public partial class JsonMetadataServices
{
#if NET
        public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateIReadOnlySetInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Text.Json.Serialization.Metadata.JsonCollectionInfoValues<TCollection> collectionInfo) where TCollection : System.Collections.Generic.IReadOnlySet<TElement> { throw null; }
#endif


#if NET
        /// <summary>
        /// Creates serialization metadata for types assignable to <see cref="IReadOnlySet{T}"/>.
        /// </summary>
        /// <typeparam name="TCollection">The generic definition of the type.</typeparam>
        /// <typeparam name="TElement">The generic definition of the element type.</typeparam>
        /// <param name="options"></param>
        /// <param name="collectionInfo">Provides serialization metadata about the collection type.</param>
        /// <returns>Serialization metadata for the given type.</returns>
        /// <remarks>This API is for use by the output of the System.Text.Json source generator and should not be called directly.</remarks>
        public static JsonTypeInfo<TCollection> CreateIReadOnlySetInfo<TCollection, TElement>(
            JsonSerializerOptions options,
            JsonCollectionInfoValues<TCollection> collectionInfo)
            where TCollection : IReadOnlySet<TElement>
            => CreateCore(
                options,
                collectionInfo,
                new IReadOnlySetOfTConverter<TCollection, TElement>());
#endif
}

API Usage

The average developer won't notice this, as IEnumerableConverterFactory, an internal class, will add support for IReadOnlySetConverter (another internal class).

I believe most of the API changes will be used by source generators(?)

Alternative Designs

None

Risks

None

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