API Proposal: Add ref field feature support to RuntimeFeature class

[AaronRobinsonMSFT]

Background and motivation

This is desired to help Roslyn perform down-level targeting for the ref field support added in #63768.

API Proposal

Implementation PR at #64167.

namespace System.Runtime.CompilerServices
{
    public static partial class RuntimeFeature
    {
+        /// <summary>
+        /// Represents a runtime feature where types can define ref fields.
+        /// </summary>
+        public const string ByRefFields = nameof(ByRefFields);
    }
}

API Usage

Used by Roslyn to detect if a runtime supports ref fields.

Alternative Designs

This is the typical way to indicate a feature to Roslyn.

Risks

None.

[dotnet-issue-labeler]

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[jkotas]

I do not think we need a feature flag for this.

[AaronRobinsonMSFT]

This was a request by the Roslyn team for down-level targeting.

/cc @jaredpar @davidwrighton

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Issue Details

---

This is desired to help Roslyn perform down-level targeting for the ref field support added in #63768.

Proposed API

namespace System.Runtime.CompilerServices
{
    public static partial class RuntimeFeature
    {
+.       public const string RefFields = nameof(RefFields);
    }
}

Author:	AaronRobinsonMSFT
Assignees:	-
Labels:	api-suggestion, area-System.Runtime.CompilerServices
Milestone:	-

[AaronRobinsonMSFT]

@jkotas I will leave this as "api-suggestion" until we get consensus on whether this is needed or not. I will however push a draft PR with the change in case it is needed.

[jkotas]

I though you meant FEATURE_... ifdef. I agree we need a flag for this under RuntimeFeatures. We should run it through API review before merging the PR.

RefFields

We have a clash of runtime and C# names here.

We have Type.IsByRef, Type.IsByRefLike and IsByRefLikeAttribute in the runtime vs. ref and ref structs in C#.

Should this flag by called ByRefFields for consistency with what we have already?

[jaredpar]

@cston, @RikkiGibson FYI

[terrajobst]

Video

• Looks good as proposed
• We should make sure that other runtimes, such as Unity, that features listed in the RuntimeFeature are required
  • Practically, if another runtime pulls in our library code they will likely find out, given that we usually use those features

namespace System.Runtime.CompilerServices
{
    public static partial class RuntimeFeature
    {
        public const string ByRefFields = nameof(ByRefFields);
    }
}