add FeatureGuardAttribute (#249)

Author: SimonCropp

apiCount.include.md

@@ -1 +1 @@
-**API count: 402**
+**API count: 404**

readme.md

@@ -12,7 +12,7 @@ The package targets `netstandard2.0` and is designed to support the following ru
  * `net5.0`, `net6.0`, `net7.0`, `net8.0`, `net9.0`
 
 
-**API count: 402**<!-- singleLineInclude: apiCount. path: /apiCount.include.md -->
+**API count: 404**<!-- singleLineInclude: apiCount. path: /apiCount.include.md -->
 
 
 **See [Milestones](../../milestones?state=closed) for release notes.**

src/Consume/Consume.cs

@@ -48,6 +48,7 @@ class Consume
         type = typeof(NotNullWhenAttribute);
         type = typeof(CallerArgumentExpressionAttribute);
         type = typeof(IsExternalInit);
+        type = typeof(FeatureGuardAttribute);
         type = typeof(ModuleInitializerAttribute);
         type = typeof(RequiredMemberAttribute);
         type = typeof(SetsRequiredMembersAttribute);

src/Polyfill/FeatureGuardAttribute.cs

@@ -0,0 +1,47 @@
+// <auto-generated />
+#pragma warning disable
+
+#if !NET9_0_OR_GREATER
+
+namespace System.Diagnostics.CodeAnalysis;
+
+using Diagnostics;
+using Diagnostics.CodeAnalysis;
+using Link = ComponentModel.DescriptionAttribute;
+
+
+/// <summary>
+/// Indicates that the specified public static boolean get-only property
+/// guards access to the specified feature.
+/// </summary>
+/// <remarks>
+/// Analyzers can use this to prevent warnings on calls to code that is
+/// annotated as requiring that feature, when the callsite is guarded by a
+/// call to the property.
+/// </remarks>
+[ExcludeFromCodeCoverage]
+[DebuggerNonUserCode]
+[AttributeUsage(AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+//Link:https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.featureguardattribute
+#if PolyPublic
+public
+#endif
+sealed class FeatureGuardAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="FeatureGuardAttribute"/> class
+    /// with the specified feature type.
+    /// </summary>
+    /// <param name="featureType">
+    /// The type that represents the feature guarded by the property.
+    /// </param>
+    public FeatureGuardAttribute(Type featureType) =>
+        FeatureType = featureType;
+
+    /// <summary>
+    /// The type that represents the feature guarded by the property.
+    /// </summary>
+    public Type FeatureType { get; }
+}
+
+#endif