Add swithces for enabling NSJ deserialization

Author: Nigusu-Allehu

src/NuGet.Core/NuGet.Common/AotCompatibilityAttributes.cs

@@ -0,0 +1,14 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+#if !NET9_0_OR_GREATER
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Property, Inherited = false)]
+    internal sealed class FeatureSwitchDefinitionAttribute : Attribute
+    {
+        public FeatureSwitchDefinitionAttribute(string switchName) => SwitchName = switchName;
+        public string SwitchName { get; }
+    }
+}
+#endif

src/NuGet.Core/NuGet.Common/NuGetFeatureFlags.cs

@@ -0,0 +1,32 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Runtime.CompilerServices;
+
+namespace NuGet.Common
+{
+    public static class NuGetFeatureFlags
+    {
+        internal const string UsesNSJDeserializationSwitchName = "NuGet.UsesNSJDeserialization";
+        internal const string UsesNSJDeserializationEnvVar = "NUGET_USES_NSJ_DESERIALIZATION";
+
+        /// <summary>
+        /// <see langword="true"/> to use Newtonsoft.Json (default); <see langword="false"/> to use System.Text.Json.
+        /// Override via AppContext switch <c>NuGet.UsesNSJDeserialization</c> or env var <c>NUGET_USES_NSJ_DESERIALIZATION</c>.
+        /// </summary>
+        [FeatureSwitchDefinition(UsesNSJDeserializationSwitchName)]
+        public static bool UsesNSJDeserialization { get; } = GetSwitch(EnvironmentVariableWrapper.Instance);
+
+        internal static bool GetSwitch(IEnvironmentVariableReader env)
+        {
+            if (AppContext.TryGetSwitch(UsesNSJDeserializationSwitchName, out bool switchValue))
+            {
+                return switchValue;
+            }
+
+            string? envValue = env.GetEnvironmentVariable(UsesNSJDeserializationEnvVar);
+            return !string.Equals(envValue, "false", StringComparison.OrdinalIgnoreCase);
+        }
+    }
+}

src/NuGet.Core/NuGet.Common/PublicAPI.Unshipped.txt

@@ -1 +1,3 @@
 #nullable enable
+NuGet.Common.NuGetFeatureFlags
+static NuGet.Common.NuGetFeatureFlags.UsesNSJDeserialization.get -> bool

test/NuGet.Core.Tests/NuGet.Common.Test/NuGetFeatureFlagsTests.cs

@@ -0,0 +1,50 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using Test.Utility;
+using Xunit;
+
+namespace NuGet.Common.Test
+{
+    public class NuGetFeatureFlagsTests
+    {
+        [Fact]
+        public void GetSwitch_WhenEnvVarNotSet_ReturnsTrue()
+        {
+            bool result = NuGetFeatureFlags.GetSwitch(TestEnvironmentVariableReader.EmptyInstance);
+
+            Assert.True(result);
+        }
+
+        [Theory]
+        [InlineData("false")]
+        [InlineData("False")]
+        [InlineData("FALSE")]
+        public void GetSwitch_WhenEnvVarSetToFalse_ReturnsFalse(string value)
+        {
+            var env = new TestEnvironmentVariableReader(
+                new Dictionary<string, string> { [NuGetFeatureFlags.UsesNSJDeserializationEnvVar] = value });
+
+            bool result = NuGetFeatureFlags.GetSwitch(env);
+
+            Assert.False(result);
+        }
+
+        [Theory]
+        [InlineData("true")]
+        [InlineData("True")]
+        [InlineData("0")]
+        [InlineData("1")]
+        [InlineData("anything")]
+        public void GetSwitch_WhenEnvVarSetToTrueOrUnrecognized_ReturnsTrue(string value)
+        {
+            var env = new TestEnvironmentVariableReader(
+                new Dictionary<string, string> { [NuGetFeatureFlags.UsesNSJDeserializationEnvVar] = value });
+
+            bool result = NuGetFeatureFlags.GetSwitch(env);
+
+            Assert.True(result);
+        }
+    }
+}