Implement ExperimentalAttribute

src/libraries/System.Private.CoreLib/src/ILLink/ILLink.LinkAttributes.Shared.xml

@@ -141,6 +141,9 @@
     <type fullname="System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverageAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
+    <type fullname="System.Diagnostics.CodeAnalysis.ExperimentalAttribute">
+      <attribute internal="RemoveAttributeInstances" />
+    </type>
 
     <!-- System.Reflection -->
     <type fullname="System.Reflection.AssemblyCompanyAttribute">

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -259,6 +259,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\DynamicallyAccessedMembersAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\DynamicDependencyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\ExcludeFromCodeCoverageAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\ExperimentalAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\NullableAttributes.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\UnscopedRefAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\CodeAnalysis\RequiresAssemblyFilesAttribute.cs" />

src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/ExperimentalAttribute.cs

@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    ///  Indicates that an API is experimental and it may change in the future.
+    /// </summary>
+    /// <remarks>
+    ///   This attribute allows call sites to be flagged with a diagnostic that indicates that an experimental
+    ///   feature is used. Authors can use this attribute to ship preview features in their assemblies.
+    /// </remarks>
+    [AttributeUsage(AttributeTargets.Assembly |
+                    AttributeTargets.Module |
+                    AttributeTargets.Class |
+                    AttributeTargets.Struct |
+                    AttributeTargets.Enum |
+                    AttributeTargets.Constructor |
+                    AttributeTargets.Method |
+                    AttributeTargets.Property |
+                    AttributeTargets.Field |
+                    AttributeTargets.Event |
+                    AttributeTargets.Interface |
+                    AttributeTargets.Delegate, Inherited = false)]
+    public sealed class ExperimentalAttribute : Attribute
+    {
+        /// <summary>
+        ///  Initializes a new instance of the <see cref="ExperimentalAttribute"/> class, specifying the ID that the compiler will use
+        ///  when reporting a use of the API the attribute applies to.
+        /// </summary>
+        /// <param name="diagnosticId">The ID that the compiler will use when reporting a use of the API the attribute applies to.</param>
+        public ExperimentalAttribute(string diagnosticId)
+        {
+            DiagnosticId = diagnosticId;
+        }
+
+        /// <summary>
+        ///  Gets or sets the ID that the compiler will use when reporting a use of the API the attribute applies to.
+        /// </summary>
+        /// <value>The unique diagnostic ID.</value>
+        /// <remarks>
+        ///  The diagnostic ID is shown in build output for warnings and errors.
+        ///  <para>This property represents the unique ID that can be used to suppress the warnings or errors, if needed.</para>
+        /// </remarks>
+        public string DiagnosticId { get; }
+
+        /// <summary>
+        ///  Gets or sets the URL for corresponding documentation.
+        ///  The API accepts a format string instead of an actual URL, creating a generic URL that includes the diagnostic ID.
+        /// </summary>
+        /// <value>The format string that represents a URL to corresponding documentation.</value>
+        /// <remarks>An example format string is <c>https://contoso.com/obsoletion-warnings/{0}</c>.</remarks>
+        public string? UrlFormat { get; set; }
+    }
+}

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -8281,6 +8281,13 @@ public sealed partial class ExcludeFromCodeCoverageAttribute : System.Attribute
         public ExcludeFromCodeCoverageAttribute() { }
         public string? Justification { get { throw null; } set { } }
     }
+    [System.AttributeUsage(System.AttributeTargets.Assembly |  System.AttributeTargets.Module | System.AttributeTargets.Class | System.AttributeTargets.Struct | System.AttributeTargets.Enum | System.AttributeTargets.Constructor | System.AttributeTargets.Method | System.AttributeTargets.Property | System.AttributeTargets.Field | System.AttributeTargets.Event | System.AttributeTargets.Interface | System.AttributeTargets.Delegate, Inherited = false)]
+    public sealed class ExperimentalAttribute : System.Attribute
+    {
+        public ExperimentalAttribute(string diagnosticId) { }
+        public string DiagnosticId { get { throw null; } }
+        public string? UrlFormat { get; set; }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.Property | System.AttributeTargets.ReturnValue, Inherited=false)]
     public sealed partial class MaybeNullAttribute : System.Attribute
     {

src/libraries/System.Runtime/tests/System.Runtime.Tests.csproj

@@ -68,6 +68,7 @@
     <Compile Include="System\Attributes.cs" />
     <Compile Include="System\AttributeUsageAttributeTests.cs" />
     <Compile Include="System\BadImageFormatExceptionTests.cs" />
+    <Compile Include="System\Diagnostics\CodeAnalysis\ExperimentalAttributeTests.cs" />
     <Compile Include="System\NumberFormatTestHelper.cs" />
     <Compile Include="System\StringTests.GenericMath.cs" />
     <Compile Include="System\BooleanTests.cs" />

src/libraries/System.Runtime/tests/System/Diagnostics/CodeAnalysis/ExperimentalAttributeTests.cs

@@ -0,0 +1,40 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace System.Diagnostics.CodeAnalysis.Tests
+{
+    public class ExperimentalAttributeTests
+    {
+        [Theory]
+        // Note: Once the compiler support is implemented these should fail
+        [InlineData("")]
+        [InlineData(" ")]
+        [InlineData("\t")]
+        // Note end
+        [InlineData("diagnosticId")]
+        public void TestConstructor(string expected)
+        {
+            var attr = new ExperimentalAttribute(expected);
+
+            Assert.Equal(expected, attr.DiagnosticId);
+            Assert.Null(attr.UrlFormat);
+        }
+
+        [Theory]
+        [InlineData(null)]
+        [InlineData("")]
+        [InlineData("https://contoso.com/obsoletion-warnings/{0}")]
+        public void TestSetUrlFormat(string urlFormat)
+        {
+            var attr = new ExperimentalAttribute("diagnosticId")
+            {
+                UrlFormat = urlFormat
+            };
+
+            Assert.Equal("diagnosticId", attr.DiagnosticId);
+            Assert.Equal(urlFormat, attr.UrlFormat);
+        }
+    }
+}