New APIs to support "Ref Struct Interfaces" feature

[AlekseyTs]

Background and Motivation

https://github.com/dotnet/csharplang/blob/main/proposals/ref-struct-interfaces.md

Proposed API

namespace Microsoft.CodeAnalysis
{
    /// <summary>
    /// Specifies capabilities that may or may not be supported by the common language runtime the compilation is
    /// targeting.
    /// </summary>
    public enum RuntimeCapability
    {
+       /// <summary>
+       /// Indicates that this version of runtime supports generic type parameters allowing substitution with a ref struct.
+       /// </summary>
+       ByRefLikeGenerics = 8,
    }
}

The name of the new enum item matches the name of the field from BCL - System.Runtime.CompilerServices.RuntimeFeature.ByRefLikeGenerics (see https://github.com/dotnet/csharplang/blob/main/proposals/ref-struct-interfaces.md#representation-in-metadata).

namespace Microsoft.CodeAnalysis
{
    /// <summary>
    /// Represents a type parameter in a generic type or generic method.
    /// </summary>
    /// <remarks>
    /// This interface is reserved for implementation by its associated APIs. We reserve the right to
    /// change it in the future.
    /// </remarks>
    public interface ITypeParameterSymbol : ITypeSymbol
    {
+       /// <summary>
+       /// True if the 'allows ref struct' constraint was specified for the type parameter.
+       /// </summary>
+       bool AllowsByRefLike { get; }
    }
}

The new property is named after a flag in BCL System.Reflection.GenericParameterAttributes.AllowByRefLike (see https://github.com/dotnet/csharplang/blob/main/proposals/ref-struct-interfaces.md#representation-in-metadata).

[333fred]

API Review

• Naming of Allows vs Allow?
  • English prefers allows here
• Shakespeare's famous question: "To By or not to By?"
  • For consistency with ITypeSymbol.IsRefLikeType, we'll name it ITypeParameterSymbol.AllowsRefLikeType

Conclusion: Approved, with the rename of to ITypeParameterSymbol.AllowsRefLikeType. The RuntimeCapability name is approved as-is.