Implement parsing for records (#40467)

As agreed in LDM, the specification requires that a data contextual
keyword be present in front of the class and a parameter list to be
present after the identifier, for the type to be a record.

The absence of either of these two elements will be a semantic error,
not a parsing error.

eng/targets/Settings.props

@@ -17,7 +17,8 @@
     <DisableImplicitNuGetFallbackFolder>true</DisableImplicitNuGetFallbackFolder>
     <ToolsetPackagesDir>$(RepoRoot)build\ToolsetPackages\</ToolsetPackagesDir>
 
-    <RoslynPortableTargetFrameworks>net472;netcoreapp2.1</RoslynPortableTargetFrameworks>
+    <!-- PROTOTYPE: Swap the order of the frameworks back before merging -->
+    <RoslynPortableTargetFrameworks>netcoreapp2.1;net472</RoslynPortableTargetFrameworks>
     <RoslynCheckCodeStyle Condition="'$(ContinuousIntegrationBuild)' != 'true' or '$(RoslynEnforceCodeStyle)' == 'true'">true</RoslynCheckCodeStyle>
     <UseSharedCompilation>true</UseSharedCompilation>
 

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5927,4 +5927,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ExternEventInitializer" xml:space="preserve">
     <value>'{0}': extern event cannot have initializer</value>
   </data>
+  <data name="IDS_FeatureRecords" xml:space="preserve">
+    <value>records</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Declarations/DeclarationModifiers.cs

@@ -1,11 +1,12 @@
 // Copyright (c) Microsoft.  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 Microsoft.CodeAnalysis.CSharp
 {
     [Flags]
-    internal enum DeclarationModifiers
+    internal enum DeclarationModifiers : uint
     {
         None = 0,
         Abstract = 1 << 0,
@@ -34,9 +35,18 @@ internal enum DeclarationModifiers
         Async = 1 << 20,
         Ref = 1 << 21, // used only for structs
 
-        All = (1 << 22) - 1, // all modifiers
-        Unset = 1 << 22, // used when a modifiers value hasn't yet been computed
+        Data = 1 << 22,
+
+        All = (1 << 23) - 1, // all modifiers
+        Unset = 1 << 23, // used when a modifiers value hasn't yet been computed
 
         AccessibilityMask = PrivateProtected | Private | Protected | Internal | ProtectedInternal | Public,
     }
+
+    internal static class DeclarationModifierExtensions
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static bool HasFlag(this DeclarationModifiers modifiers, DeclarationModifiers flag)
+            => (modifiers & flag) != 0;
+    }
 }

src/Compilers/CSharp/Portable/Errors/MessageID.cs

@@ -185,6 +185,7 @@ internal enum MessageID
         IDS_FeatureSwitchExpression = MessageBase + 12763,
         IDS_FeatureAsyncUsing = MessageBase + 12764,
         IDS_FeatureLambdaDiscardParameters = MessageBase + 12765,
+        IDS_FeatureRecords = MessageBase + 12766,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -293,6 +294,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
             {
                 // Preview features.
                 case MessageID.IDS_FeatureLambdaDiscardParameters: // semantic check
+                case MessageID.IDS_FeatureRecords:
                     return LanguageVersion.Preview;
 
                 // C# 8.0 features.