Log Registry LongPathsEnabled in v:diagnostic (#10223)

* Add a message for LongPathsEnabled

* refactor LongPathsEnabled logic to a ternary enum

* Elaborate and comment the resource string

* translations

* add state for registry not set

* deduplicate

src/Framework/NativeMethods.cs

@@ -594,25 +594,70 @@ private static void SetMaxPath()
         }
     }
 
-    internal static bool IsMaxPathLegacyWindows()
+    internal enum LongPathsStatus
+    {
+        /// <summary>
+        ///  The registry key is set to 0 or does not exist.
+        /// </summary>
+        Disabled,
+
+        /// <summary>
+        /// The registry key does not exist.
+        /// </summary>
+        Missing,
+
+        /// <summary>
+        /// The registry key is set to 1.
+        /// </summary>
+        Enabled,
+
+        /// <summary>
+        /// Not on Windows.
+        /// </summary>
+        NotApplicable,
+    }
+
+    internal static LongPathsStatus IsLongPathsEnabled()
     {
+        if (!IsWindows)
+        {
+            return LongPathsStatus.NotApplicable;
+        }
+
         try
         {
-            return IsWindows && !IsLongPathsEnabledRegistry();
+            return IsLongPathsEnabledRegistry();
         }
         catch
         {
-            return true;
+            return LongPathsStatus.Disabled;
         }
     }
 
+    internal static bool IsMaxPathLegacyWindows()
+    {
+        var longPathsStatus = IsLongPathsEnabled();
+        return longPathsStatus == LongPathsStatus.Disabled || longPathsStatus == LongPathsStatus.Missing;
+    }
+
     [SupportedOSPlatform("windows")]
-    private static bool IsLongPathsEnabledRegistry()
+    private static LongPathsStatus IsLongPathsEnabledRegistry()
     {
         using (RegistryKey fileSystemKey = Registry.LocalMachine.OpenSubKey(WINDOWS_FILE_SYSTEM_REGISTRY_KEY))
         {
-            object longPathsEnabledValue = fileSystemKey?.GetValue(WINDOWS_LONG_PATHS_ENABLED_VALUE_NAME, 0);
-            return fileSystemKey != null && Convert.ToInt32(longPathsEnabledValue) == 1;
+            object longPathsEnabledValue = fileSystemKey?.GetValue(WINDOWS_LONG_PATHS_ENABLED_VALUE_NAME, -1);
+            if (fileSystemKey != null && Convert.ToInt32(longPathsEnabledValue) == -1)
+            {
+                return LongPathsStatus.Missing;
+            }
+            else if (fileSystemKey != null && Convert.ToInt32(longPathsEnabledValue) == 1)
+            {
+                return LongPathsStatus.Enabled;
+            }
+            else
+            { 
+                return LongPathsStatus.Disabled;
+            }
         }
     }
 

src/MSBuild/Resources/Strings.resx

@@ -1661,6 +1661,19 @@
     <value>succeeded: {0}</value>
     <comment>{0} whole number</comment>
   </data>
+  <data name="LongPaths" xml:space="preserve">
+    <value>Based on the Windows registry key LongPathsEnabled, the LongPaths feature is {0}.</value>
+    <comment>"Windows" is the OS, "LongPathsEnabled" should not be localized, and {0} will be "enabled"/"disabled"/"not set"</comment>
+  </data>
+  <data name="LongPaths_Enabled" xml:space="preserve">
+    <value>enabled</value>
+  </data>
+  <data name="LongPaths_Disabled" xml:space="preserve">
+    <value>disabled</value>
+  </data>
+  <data name="LongPaths_Missing" xml:space="preserve">
+    <value>not set</value>
+  </data>
   <!-- **** TerminalLogger strings end **** -->
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