Change "OSX" to "macOS" on user messages.

src/Microsoft.DotNet.Analyzers.Compatibility.Tests/ExceptionAnalyzerTests.cs

@@ -71,7 +71,7 @@ static void Main(string[] args)
             ";
 
             var expected = @"
-                PC001: RegistryKey.OpenSubKey(string) isn't supported on MacOSX
+                PC001: RegistryKey.OpenSubKey(string) isn't supported on macOS
             ";
 
             var settings = @"
@@ -101,7 +101,7 @@ static void Main(string[] args)
 
             var settings = @"
                 PlatformCompatIgnore=Linux
-                MacOSX
+                macOS
             ";
 
             AssertNoMatch(source, settings);
@@ -151,7 +151,7 @@ static void Main(string[] args)
             ";
 
             var expected = @"
-                PC001: RegistryKey.OpenSubKey(string) isn't supported on Linux, MacOSX
+                PC001: RegistryKey.OpenSubKey(string) isn't supported on Linux and macOS
             ";
 
             AssertMatch(source, expected);
@@ -176,7 +176,7 @@ static void Main(string[] args)
             ";
 
             var expected = @"
-                PC001: NamedPipeClientStream.NamedPipeClientStream(string) isn't supported on Linux, MacOSX
+                PC001: NamedPipeClientStream.NamedPipeClientStream(string) isn't supported on Linux and macOS
             ";
 
             AssertMatch(source, expected);
@@ -201,7 +201,7 @@ static void Main(string[] args)
             ";
 
             var expected = @"
-                PC001: Console.WindowWidth isn't supported on Linux, MacOSX
+                PC001: Console.WindowWidth isn't supported on Linux and macOS
             ";
 
             AssertMatch(source, expected);
@@ -231,7 +231,7 @@ private void MyException_SerializeObjectState(object sender, SafeSerializationEv
             ";
 
             var expected = @"
-                PC001: Exception.SerializeObjectState isn't supported on Linux, MacOSX, Windows
+                PC001: Exception.SerializeObjectState isn't supported on Linux, macOS, and Windows
             ";
 
             AssertMatch(source, expected);
@@ -258,7 +258,7 @@ static void Main(string[] args)
             ";
 
             var expected = @"
-                PC001: CngAlgorithm.operator ==(CngAlgorithm, CngAlgorithm) isn't supported on Linux, MacOSX
+                PC001: CngAlgorithm.operator ==(CngAlgorithm, CngAlgorithm) isn't supported on Linux and macOS
             ";
 
             AssertMatch(source, expected);

src/Microsoft.DotNet.Analyzers.Compatibility/Exceptions/ExceptionAnalyzer.cs

@@ -66,7 +66,7 @@ private void AnalyzeSymbol(SymbolUsageAnalysisContext context, PlatformCompatOpt
 
             var api = symbol.ToDisplayString(SymbolDisplayFormat.CSharpShortErrorMessageFormat);
             var location = context.GetLocation();
-            var list = maskedPlatforms.ToString();
+            var list = maskedPlatforms.ToFriendlyString();
             var diagnostic = Diagnostic.Create(Rule, location, api, list);
             context.ReportDiagnostic(diagnostic);
         }

src/Microsoft.DotNet.Analyzers.Compatibility/Exceptions/ExceptionDocument.Parser.cs

@@ -51,7 +51,7 @@ private static bool TryParsePlatformName(string text, out Platform platform)
                         platform = Platform.Linux;
                         return true;
                     case "osx":
-                        platform = Platform.MacOSX;
+                        platform = Platform.MacOS;
                         return true;
                     case "win":
                         platform = Platform.Windows;

src/Microsoft.DotNet.Analyzers.Compatibility/Platform.cs

@@ -7,7 +7,55 @@ public enum Platform
     {
         None = 0,
         Linux = 1,
-        MacOSX = 2,
+        MacOS = 2,
         Windows = 4
     }
+
+    public static class PlatformEnumHelpers
+    {
+        private const string Linux = "Linux";
+        private const string MacOS = "macOS";
+        private const string Windows = "Windows";
+
+        private static readonly string[] s_mapPlatformsToFriendlyString = new[]
+            {
+                String.Empty,
+                Linux,
+                MacOS,
+                $"{Linux} and {MacOS}",
+                $"{Windows}",
+                $"{Linux} and {Windows}",
+                $"{MacOS} and {Windows}",
+                $"{Linux}, {MacOS}, and {Windows}"
+            };
+
+        public static string ToFriendlyString(this Platform platform)
+        {
+            var friendlyStringIndex = (int)platform;
+            if (friendlyStringIndex < 0 && friendlyStringIndex >= s_mapPlatformsToFriendlyString.Length)
+                throw new ArgumentOutOfRangeException(nameof(platform));
+
+            return s_mapPlatformsToFriendlyString[friendlyStringIndex];
+        }
+
+        public static bool TryParse(string trimmedPlatformName, out Platform platform)
+        {
+            platform = Platform.None;
+
+            switch (trimmedPlatformName)
+            {
+                case Linux:
+                    platform = Platform.Linux;
+                    break;
+                case MacOS:
+                    platform = Platform.MacOS;
+                    break;
+                case Windows:
+                    platform = Platform.Windows;
+                    break;
+            }
+
+            return platform != Platform.None;
+        }
+    }
 }

src/Microsoft.DotNet.Analyzers.Compatibility/PlatformCompatOptions.cs

@@ -32,7 +32,7 @@ public static Platform ParseIgnoredPlatforms(ImmutableDictionary<string, string>
                 foreach (var name in names)
                 {
                     var trimmedNamed = name.Trim();
-                    if (Enum.TryParse<Platform>(trimmedNamed, out var platform))
+                    if (PlatformEnumHelpers.TryParse(trimmedNamed, out var platform))
                         result |= platform;
                 }
             }