Add InlineArrayX types

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

@@ -857,6 +857,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\InternalsVisibleToAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\IntrinsicAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\IsByRefLikeAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\InlineArray.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\InlineArrayAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\IsConst.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\IsExternalInit.cs" />
@@ -2790,4 +2791,4 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Wasi\WasiPollWorld.wit.imports.wasi.io.v0_2_0.IPoll.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Wasi\WasiPollWorld.wit.imports.wasi.io.v0_2_0.PollInterop.cs" />
   </ItemGroup>
-</Project>
+</Project>

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/InlineArray.cs

@@ -0,0 +1,145 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 2.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(2)]
+    public struct InlineArray2<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 3.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(3)]
+    public struct InlineArray3<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 4.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(4)]
+    public struct InlineArray4<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 5.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(5)]
+    public struct InlineArray5<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 6.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(6)]
+    public struct InlineArray6<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 7.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(7)]
+    public struct InlineArray7<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 8.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(8)]
+    public struct InlineArray8<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 9.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(9)]
+    public struct InlineArray9<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 10.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(10)]
+    public struct InlineArray10<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 11.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(11)]
+    public struct InlineArray11<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 12.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(12)]
+    public struct InlineArray12<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 13.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(13)]
+    public struct InlineArray13<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 14.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(14)]
+    public struct InlineArray14<T>
+    {
+        private T t;
+    }
+
+    /// <summary>
+    /// Represents an inline array of <typeparamref name="T"/> with a fixed length of 15.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    [InlineArray(15)]
+    public struct InlineArray15<T>
+    {
+        private T t;
+    }
+}

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

@@ -13490,6 +13490,76 @@ public sealed partial class IndexerNameAttribute : System.Attribute
     {
         public IndexerNameAttribute(string indexerName) { }
     }
+    [System.Runtime.CompilerServices.InlineArray(2)]
+    public struct InlineArray2<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(3)]
+    public struct InlineArray3<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(4)]
+    public struct InlineArray4<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(5)]
+    public struct InlineArray5<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(6)]
+    public struct InlineArray6<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(7)]
+    public struct InlineArray7<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(8)]
+    public struct InlineArray8<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(9)]
+    public struct InlineArray9<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(10)]
+    public struct InlineArray10<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(11)]
+    public struct InlineArray11<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(12)]
+    public struct InlineArray12<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(13)]
+    public struct InlineArray13<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(14)]
+    public struct InlineArray14<T>
+    {
+        private T t;
+    }
+    [System.Runtime.CompilerServices.InlineArray(15)]
+    public struct InlineArray15<T>
+    {
+        private T t;
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Struct, AllowMultiple=false)]
     public sealed partial class InlineArrayAttribute : System.Attribute
     {

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/InlineArrayTests.cs

@@ -0,0 +1,124 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Reflection;
+using Xunit;
+
+namespace System.Runtime.CompilerServices.Tests
+{
+    public static class InlineArrayTests
+    {
+        private static void WriteRead(Span<int> span, int expectedLength)
+        {
+            Assert.Equal(expectedLength, span.Length);
+
+            for (int i = 0; i < span.Length; i++)
+            {
+                span[i] = i + 1;
+            }
+
+            for (int i = 0; i < span.Length; i++)
+            {
+                Assert.Equal(i + 1, span[i]);
+            }
+        }
+
+        [Fact]
+        public void InlineArray2Test()
+        {
+            InlineArray2<int> inlineArray2 = new();
+            WriteRead(inlineArray2, 2);
+        }
+
+        [Fact]
+        public void InlineArray3Test()
+        {
+            InlineArray3<int> inlineArray3 = new();
+            WriteRead(inlineArray3, 3);
+        }
+
+        [Fact]
+        public void InlineArray4Test()
+        {
+            InlineArray4<int> inlineArray4 = new();
+            WriteRead(inlineArray4, 4);
+        }
+
+        [Fact]
+        public void InlineArray5Test()
+        {
+            InlineArray5<int> inlineArray5 = new();
+            WriteRead(inlineArray5, 5);
+        }
+
+        [Fact]
+        public void InlineArray6Test()
+        {
+            InlineArray6<int> inlineArray6 = new();
+            WriteRead(inlineArray6, 6);
+        }
+
+        [Fact]
+        public void InlineArray7Test()
+        {
+            InlineArray7<int> inlineArray7 = new();
+            WriteRead(inlineArray7, 7);
+        }
+
+        [Fact]
+        public void InlineArray8Test()
+        {
+            InlineArray8<int> inlineArray8 = new();
+            WriteRead(inlineArray8, 8);
+        }
+
+        [Fact]
+        public void InlineArray9Test()
+        {
+            InlineArray9<int> inlineArray9 = new();
+            WriteRead(inlineArray9, 9);
+        }
+
+        [Fact]
+        public void InlineArray10Test()
+        {
+            InlineArray10<int> inlineArray10 = new();
+            WriteRead(inlineArray10, 10);
+        }
+
+        [Fact]
+        public void InlineArray11Test()
+        {
+            InlineArray11<int> inlineArray11 = new();
+            WriteRead(inlineArray11, 11);
+        }
+
+        [Fact]
+        public void InlineArray12Test()
+        {
+            InlineArray12<int> inlineArray12 = new();
+            WriteRead(inlineArray12, 12);
+        }
+
+        [Fact]
+        public void InlineArray13Test()
+        {
+            InlineArray13<int> inlineArray13 = new();
+            WriteRead(inlineArray13, 13);
+        }
+
+        [Fact]
+        public void InlineArray14Test()
+        {
+            InlineArray14<int> inlineArray14 = new();
+            WriteRead(inlineArray14, 14);
+        }
+
+        [Fact]
+        public void InlineArray15Test()
+        {
+            InlineArray15<int> inlineArray15 = new();
+            WriteRead(inlineArray15, 15);
+        }
+    }
+}