chore: Add common shims for new language features

Handful of commonly used shim types to unlock new(-ish) language
features in older TFMs. Limited to features that are only gated by
symbol existence checks, but covers some fairly common features.

Signed-off-by: Austin Drenski <austin@austindrenski.io>

build/Common.prod.props

@@ -40,4 +40,8 @@
   <PropertyGroup Condition="'$(Deterministic)'=='true'">
     <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
   </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="$(MSBuildThisFileDirectory)../src/Shared/**" LinkBase="Shared" />
+  </ItemGroup>
 </Project>

build/Common.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <LangVersion>7.3</LangVersion>
+    <LangVersion>latest</LangVersion>
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
     <EnableNETAnalyzers>true</EnableNETAnalyzers>
   </PropertyGroup>

src/Shared/CallerArgumentExpressionAttribute.cs

@@ -0,0 +1,25 @@
+#nullable enable
+// @formatter:off
+// ReSharper disable All
+#if NETCOREAPP3_0_OR_GREATER
+// https://github.com/dotnet/runtime/issues/96197
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.CallerArgumentExpressionAttribute))]
+#else
+#pragma warning disable
+// 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
+{
+    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
+    internal sealed class CallerArgumentExpressionAttribute : Attribute
+    {
+        public CallerArgumentExpressionAttribute(string parameterName)
+        {
+            ParameterName = parameterName;
+        }
+
+        public string ParameterName { get; }
+    }
+}
+#endif

src/Shared/Check.cs

@@ -0,0 +1,22 @@
+#nullable enable
+using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace OpenFeature;
+
+[DebuggerStepThrough]
+static class Check
+{
+    public static T NotNull<T>(T? value, [CallerArgumentExpression("value")] string name = null!)
+    {
+#if NET8_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(value, name);
+#else
+        if (value is null)
+            throw new ArgumentNullException(name);
+#endif
+
+        return value;
+    }
+}

src/Shared/Index.cs

@@ -0,0 +1,166 @@
+#nullable enable
+// @formatter:off
+// ReSharper disable All
+#if NET5_0_OR_GREATER
+// https://github.com/dotnet/runtime/issues/96197
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Index))]
+#else
+#pragma warning disable
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+
+namespace System
+{
+    /// <summary>Represent a type can be used to index a collection either from the start or the end.</summary>
+    /// <remarks>
+    /// Index is used by the C# compiler to support the new index syntax
+    /// <code>
+    /// int[] someArray = new int[5] { 1, 2, 3, 4, 5 } ;
+    /// int lastElement = someArray[^1]; // lastElement = 5
+    /// </code>
+    /// </remarks>
+    readonly struct Index : IEquatable<Index>
+    {
+        private readonly int _value;
+
+        /// <summary>Construct an Index using a value and indicating if the index is from the start or from the end.</summary>
+        /// <param name="value">The index value. it has to be zero or positive number.</param>
+        /// <param name="fromEnd">Indicating if the index is from the start or from the end.</param>
+        /// <remarks>
+        /// If the Index constructed from the end, index value 1 means pointing at the last element and index value 0 means pointing at beyond last element.
+        /// </remarks>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Index(int value, bool fromEnd = false)
+        {
+            if (value < 0)
+            {
+                // ThrowHelper.ThrowValueArgumentOutOfRange_NeedNonNegNumException();
+                throw new ArgumentOutOfRangeException(nameof(value));
+            }
+
+            if (fromEnd)
+                _value = ~value;
+            else
+                _value = value;
+        }
+
+        // The following private constructors mainly created for perf reason to avoid the checks
+        private Index(int value)
+        {
+            _value = value;
+        }
+
+        /// <summary>Create an Index pointing at first element.</summary>
+        public static Index Start => new Index(0);
+
+        /// <summary>Create an Index pointing at beyond last element.</summary>
+        public static Index End => new Index(~0);
+
+        /// <summary>Create an Index from the start at the position indicated by the value.</summary>
+        /// <param name="value">The index value from the start.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Index FromStart(int value)
+        {
+            if (value < 0)
+            {
+                // ThrowHelper.ThrowValueArgumentOutOfRange_NeedNonNegNumException();
+                throw new ArgumentOutOfRangeException(nameof(value));
+            }
+
+            return new Index(value);
+        }
+
+        /// <summary>Create an Index from the end at the position indicated by the value.</summary>
+        /// <param name="value">The index value from the end.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Index FromEnd(int value)
+        {
+            if (value < 0)
+            {
+                // ThrowHelper.ThrowValueArgumentOutOfRange_NeedNonNegNumException();
+                throw new ArgumentOutOfRangeException(nameof(value));
+            }
+
+            return new Index(~value);
+        }
+
+        /// <summary>Returns the index value.</summary>
+        public int Value
+        {
+            get
+            {
+                if (_value < 0)
+                    return ~_value;
+                else
+                    return _value;
+            }
+        }
+
+        /// <summary>Indicates whether the index is from the start or the end.</summary>
+        public bool IsFromEnd => _value < 0;
+
+        /// <summary>Calculate the offset from the start using the giving collection length.</summary>
+        /// <param name="length">The length of the collection that the Index will be used with. length has to be a positive value</param>
+        /// <remarks>
+        /// For performance reason, we don't validate the input length parameter and the returned offset value against negative values.
+        /// we don't validate either the returned offset is greater than the input length.
+        /// It is expected Index will be used with collections which always have non negative length/count. If the returned offset is negative and
+        /// then used to index a collection will get out of range exception which will be same affect as the validation.
+        /// </remarks>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public int GetOffset(int length)
+        {
+            int offset = _value;
+            if (IsFromEnd)
+            {
+                // offset = length - (~value)
+                // offset = length + (~(~value) + 1)
+                // offset = length + value + 1
+
+                offset += length + 1;
+            }
+            return offset;
+        }
+
+        /// <summary>Indicates whether the current Index object is equal to another object of the same type.</summary>
+        /// <param name="value">An object to compare with this object</param>
+        public override bool Equals([NotNullWhen(true)] object? value) => value is Index && _value == ((Index)value)._value;
+
+        /// <summary>Indicates whether the current Index object is equal to another Index object.</summary>
+        /// <param name="other">An object to compare with this object</param>
+        public bool Equals(Index other) => _value == other._value;
+
+        /// <summary>Returns the hash code for this instance.</summary>
+        public override int GetHashCode() => _value;
+
+        /// <summary>Converts integer number to an Index.</summary>
+        public static implicit operator Index(int value) => FromStart(value);
+
+        /// <summary>Converts the value of the current Index object to its equivalent string representation.</summary>
+        public override string ToString()
+        {
+            if (IsFromEnd)
+                return ToStringFromEnd();
+
+            return ((uint)Value).ToString();
+        }
+
+        private string ToStringFromEnd()
+        {
+#if (!NETSTANDARD2_0 && !NETFRAMEWORK)
+            Span<char> span = stackalloc char[11]; // 1 for ^ and 10 for longest possible uint value
+            bool formatted = ((uint)Value).TryFormat(span.Slice(1), out int charsWritten);
+            Debug.Assert(formatted);
+            span[0] = '^';
+            return new string(span.Slice(0, charsWritten + 1));
+#else
+            return '^' + Value.ToString();
+#endif
+        }
+    }
+}
+#endif

src/Shared/IsExternalInit.cs

@@ -0,0 +1,25 @@
+#nullable enable
+// @formatter:off
+// ReSharper disable All
+#if NET5_0_OR_GREATER
+// https://github.com/dotnet/runtime/issues/96197
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]
+#else
+#pragma warning disable
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.ComponentModel;
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Reserved to be used by the compiler for tracking metadata.
+    /// This class should not be used by developers in source code.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    static class IsExternalInit
+    {
+    }
+}
+#endif