Fix InvariantGlobalization=false in a trimmed app

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

@@ -6,8 +6,12 @@
       <method signature="System.Boolean IsEnabled(System.Diagnostics.Tracing.EventLevel,System.Diagnostics.Tracing.EventKeywords)" body="stub" value="false" />
       <method signature="System.Boolean IsEnabled(System.Diagnostics.Tracing.EventLevel,System.Diagnostics.Tracing.EventKeywords,System.Diagnostics.Tracing.EventChannel)" body="stub" value="false" />
     </type>
+    <!-- Note: Invariant=true and Invariant=false are substituted at different levels. This allows for the whole Settings nested class
+         to be trimmed when Invariant=true, and allows for the Settings static cctor (on Unix) to be preserved when Invariant=false. -->
     <type fullname="System.Globalization.GlobalizationMode">
       <method signature="System.Boolean get_Invariant()" body="stub" value="true" feature="System.Globalization.Invariant" featurevalue="true" />
+    </type>
+    <type fullname="System.Globalization.GlobalizationMode/Settings">
       <method signature="System.Boolean get_Invariant()" body="stub" value="false" feature="System.Globalization.Invariant" featurevalue="false" />
     </type>
     <type fullname="System.LocalAppContextSwitches">

src/libraries/System.Private.CoreLib/src/System/Globalization/GlobalizationMode.Unix.cs

@@ -7,42 +7,48 @@ internal static partial class GlobalizationMode
     {
         private static partial class Settings
         {
-            internal static readonly bool Invariant = GetGlobalizationInvariantMode();
-        }
-
-        internal static bool Invariant => Settings.Invariant;
-
-        internal static bool UseNls => false;
+            /// <summary>
+            /// Load ICU (when not in Invariant mode) in a static cctor to ensure it is loaded early in the process.
+            /// Other places, e.g. CompareInfo.GetSortKey, rely on ICU already being loaded before they are called.
+            /// </summary>
+            static Settings()
+            {
+                if (!Invariant)
+                {
+                    if (TryGetAppLocalIcuSwitchValue(out string? icuSuffixAndVersion))
+                    {
+                        LoadAppLocalIcu(icuSuffixAndVersion);
+                    }
+                    else
+                    {
+                        int loaded = LoadICU();
+                        if (loaded == 0)
+                        {
+                            Environment.FailFast(GetIcuLoadFailureMessage());
+                        }
+                    }
+                }
+            }
 
-        private static bool GetGlobalizationInvariantMode()
-        {
-            bool invariantEnabled = GetInvariantSwitchValue();
-            if (!invariantEnabled)
+            private static string GetIcuLoadFailureMessage()
             {
-                if (TryGetAppLocalIcuSwitchValue(out string? icuSuffixAndVersion))
+                // These strings can't go into resources, because a resource lookup requires globalization, which requires ICU
+                if (OperatingSystem.IsBrowser() || OperatingSystem.IsIOS() || OperatingSystem.IsAndroid())
                 {
-                    LoadAppLocalIcu(icuSuffixAndVersion);
+                    return "Unable to load required ICU Globalization data. Please see https://aka.ms/dotnet-missing-libicu for more information";
                 }
                 else
                 {
-                    int loaded = LoadICU();
-                    if (loaded == 0 && !OperatingSystem.IsBrowser())
-                    {
-                        // This can't go into resources, because a resource lookup requires globalization, which requires ICU
-                        string message = "Couldn't find a valid ICU package installed on the system. " +
-                                         "Please install libicu using your package manager and try again. " +
-                                         "Alternatively you can set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support. " +
-                                         "Please see https://aka.ms/dotnet-missing-libicu for more information.";
-                        Environment.FailFast(message);
-                    }
-
-                    // fallback to Invariant mode if LoadICU failed (Browser).
-                    return loaded == 0;
+                    return "Couldn't find a valid ICU package installed on the system. " +
+                        "Please install libicu using your package manager and try again. " +
+                        "Alternatively you can set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support. " +
+                        "Please see https://aka.ms/dotnet-missing-libicu for more information.";
                 }
             }
-            return invariantEnabled;
         }
 
+        internal static bool UseNls => false;
+
         private static void LoadAppLocalIcuCore(ReadOnlySpan<char> version, ReadOnlySpan<char> suffix)
         {
 

src/libraries/System.Private.CoreLib/src/System/Globalization/GlobalizationMode.Windows.cs

@@ -5,10 +5,6 @@ namespace System.Globalization
 {
     internal static partial class GlobalizationMode
     {
-        // Order of these properties in Windows matter because GetUseIcuMode is dependent on Invariant.
-        // So we need Invariant to be initialized first.
-        internal static bool Invariant { get; } = GetInvariantSwitchValue();
-
         internal static bool UseNls { get; } = !Invariant &&
             (AppContextConfigHelper.GetBooleanConfig("System.Globalization.UseNls", "DOTNET_SYSTEM_GLOBALIZATION_USENLS") ||
                 !LoadIcu());

src/libraries/System.Private.CoreLib/src/System/Globalization/GlobalizationMode.cs

@@ -8,11 +8,18 @@ namespace System.Globalization
 {
     internal static partial class GlobalizationMode
     {
+        // split from GlobalizationMode so the whole class can be trimmed when Invariant=true.
         private static partial class Settings
         {
             internal static readonly bool PredefinedCulturesOnly = AppContextConfigHelper.GetBooleanConfig("System.Globalization.PredefinedCulturesOnly", "DOTNET_SYSTEM_GLOBALIZATION_PREDEFINED_CULTURES_ONLY");
+            internal static bool Invariant { get; } = GetInvariantSwitchValue();
         }
 
+        // Note: Invariant=true and Invariant=false are substituted at different levels in the ILLink.Substitutions file.
+        // This allows for the whole Settings nested class to be trimmed when Invariant=true, and allows for the Settings
+        // static cctor (on Unix) to be preserved when Invariant=false.
+        internal static bool Invariant => Settings.Invariant;
+
         internal static bool PredefinedCulturesOnly => !Invariant && Settings.PredefinedCulturesOnly;
 
         private static bool GetInvariantSwitchValue() =>

src/libraries/System.Runtime/tests/TrimmingTests/InvariantGlobalizationFalse.cs

@@ -0,0 +1,26 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Globalization;
+using System.Threading;
+
+/// <summary>
+/// Ensures setting InvariantGlobalization = false still works in a trimmed app.
+/// </summary>
+class Program
+{
+    static int Main(string[] args)
+    {
+        // since we are using Invariant GlobalizationMode = false, setting the culture matters.
+        // The app will always use the current culture, so in the Turkish culture, 'i' ToUpper will NOT be "I"
+        Thread.CurrentThread.CurrentCulture = new CultureInfo("tr-TR");
+        if ("i".ToUpper() == "I")
+        {
+            // 'i' ToUpper was "I", but shouldn't be in the Turkish culture, so fail
+            return -1;
+        }
+
+        return 100;
+    }
+}

src/libraries/System.Runtime/tests/TrimmingTests/InvariantGlobalizationTrue.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.
+
+using System;
+using System.Globalization;
+using System.Reflection;
+using System.Threading;
+
+/// <summary>
+/// Ensures setting InvariantGlobalization = true still works in a trimmed app.
+/// </summary>
+class Program
+{
+    static int Main(string[] args)
+    {
+        // since we are using Invariant GlobalizationMode = true, setting the culture doesn't matter.
+        // The app will always use Invariant mode, so even in the Turkish culture, 'i' ToUpper will be "I"
+        Thread.CurrentThread.CurrentCulture = new CultureInfo("tr-TR");
+        if ("i".ToUpper() != "I")
+        {
+            // 'i' ToUpper was not "I", so fail
+            return -1;
+        }
+
+        // Ensure the internal GlobalizationMode class is trimmed correctly
+        Type globalizationMode = GetCoreLibType("System.Globalization.GlobalizationMode");
+        const BindingFlags allStatics = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static;
+        foreach (MemberInfo member in globalizationMode.GetMembers(allStatics))
+        {
+            // properties and their backing getter methods are OK
+            if (member is PropertyInfo || member.Name.StartsWith("get_"))
+            {
+                continue;
+            }
+
+            if (OperatingSystem.IsWindows())
+            {
+                // Windows still contains a static cctor and a backing field for UseNls
+                if (member is ConstructorInfo || (member is FieldInfo field && field.Name.Contains("UseNls")))
+                {
+                    continue;
+                }
+            }
+
+            // Some unexpected member was left on GlobalizationMode, fail
+            Console.WriteLine($"Member '{member.Name}' was not trimmed from GlobalizationMode, but should have been.");
+            return -2;
+        }
+
+        return 100;
+    }
+
+    private static Type GetCoreLibType(string name) =>
+        typeof(object).Assembly.GetType(name, throwOnError: true);
+}

src/libraries/System.Runtime/tests/TrimmingTests/System.Runtime.TrimmingTests.proj

@@ -11,6 +11,12 @@
     <TestConsoleAppSourceFiles Include="GenericArraySortHelperTest.cs" />
     <TestConsoleAppSourceFiles Include="InheritedAttributeTests.cs" />
     <TestConsoleAppSourceFiles Include="InterfacesOnArrays.cs" />
+    <TestConsoleAppSourceFiles Include="InvariantGlobalizationFalse.cs">
+      <ExtraTrimmerArgs>--feature System.Globalization.Invariant false</ExtraTrimmerArgs>
+    </TestConsoleAppSourceFiles>
+    <TestConsoleAppSourceFiles Include="InvariantGlobalizationTrue.cs">
+      <ExtraTrimmerArgs>--feature System.Globalization.Invariant true</ExtraTrimmerArgs>
+    </TestConsoleAppSourceFiles>
     <TestConsoleAppSourceFiles Include="StackFrameHelperTest.cs">
       <!-- There is a bug with the linker where it is corrupting the pdbs while trimming
       causing the framework to not be able to get source line info any longer. This