chore: import upstream snapshot with attribution

dotnet/src/LegacySupport/CompilerFeatureRequiredAttribute/CompilerFeatureRequiredAttribute.cs

@@ -0,0 +1,37 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+#pragma warning disable SA1623 // Property summary documentation should match accessors
+
+namespace System.Runtime.CompilerServices;
+
+/// <summary>
+/// Indicates that compiler support for a particular feature is required for the location where this attribute is applied.
+/// </summary>
+[AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+internal sealed class CompilerFeatureRequiredAttribute : Attribute
+{
+    public CompilerFeatureRequiredAttribute(string featureName)
+    {
+        this.FeatureName = featureName;
+    }
+
+    /// <summary>
+    /// The name of the compiler feature.
+    /// </summary>
+    public string FeatureName { get; }
+
+    /// <summary>
+    /// If true, the compiler can choose to allow access to the location where this attribute is applied if it does not understand <see cref="FeatureName"/>.
+    /// </summary>
+    public bool IsOptional { get; init; }
+
+    /// <summary>
+    /// The <see cref="FeatureName"/> used for the ref structs C# feature.
+    /// </summary>
+    public const string RefStructs = nameof(RefStructs);
+
+    /// <summary>
+    /// The <see cref="FeatureName"/> used for the required members C# feature.
+    /// </summary>
+    public const string RequiredMembers = nameof(RequiredMembers);
+}