Unsafe evolution: allow await in unsafe context

src/Compilers/CSharp/Portable/Binder/BinderFlags.cs

@@ -19,7 +19,7 @@ internal enum BinderFlags : uint
         SuppressObsoleteChecks = 1 << 1,
         ConstructorInitializer = 1 << 2,
         FieldInitializer = 1 << 3,
-        // Previously used, can be reused in the future
+        InFixedStatement = 1 << 4, // body or initializer of a 'fixed' statement (used to disallow 'await')
         CollectionInitializerAddMethod = 1 << 5,   // used for collection initializer add method overload resolution diagnostics
         AttributeArgument = 1 << 6,
         GenericConstraintsClause = 1 << 7, // "where" clause (used for cycle checking)
@@ -120,6 +120,6 @@ internal enum BinderFlags : uint
 
         // Groups
 
-        AllClearedAtExecutableCodeBoundary = InLockBody | InCatchBlock | InCatchFilter | InFinallyBlock | InTryBlockOfTryCatch | InNestedFinallyBlock,
+        AllClearedAtExecutableCodeBoundary = InLockBody | InCatchBlock | InCatchFilter | InFinallyBlock | InTryBlockOfTryCatch | InNestedFinallyBlock | InFixedStatement,
     }
 }

src/Compilers/CSharp/Portable/Binder/Binder_Await.cs

@@ -218,9 +218,12 @@ private bool ReportBadAwaitWithoutAsync(SyntaxNodeOrToken nodeOrToken, BindingDi
         /// <returns>True if errors were found.</returns>
         private bool ReportBadAwaitContext(SyntaxNodeOrToken nodeOrToken, BindingDiagnosticBag diagnostics)
         {
-            if (this.InUnsafeRegion && !this.Flags.Includes(BinderFlags.AllowAwaitInUnsafeContext))
+            // Better error location than `nodeOrToken` but using it only for new diagnostics to avoid test churn.
+            var awaitNodeOrToken = nodeOrToken.AsNode() is AwaitExpressionSyntax awaitExpression ? awaitExpression.AwaitKeyword : nodeOrToken;
+
+            if (this.Flags.Includes(BinderFlags.InFixedStatement) && !this.Flags.Includes(BinderFlags.AllowAwaitInUnsafeContext))
             {
-                Error(diagnostics, ErrorCode.ERR_AwaitInUnsafeContext, nodeOrToken.GetLocation()!);
+                Error(diagnostics, ErrorCode.ERR_BadAwaitInFixed, awaitNodeOrToken);
                 return true;
             }
             else if (this.Flags.Includes(BinderFlags.InLockBody))
@@ -233,6 +236,11 @@ private bool ReportBadAwaitContext(SyntaxNodeOrToken nodeOrToken, BindingDiagnos
                 Error(diagnostics, ErrorCode.ERR_BadAwaitInCatchFilter, nodeOrToken.GetLocation()!);
                 return true;
             }
+            else if (this.InUnsafeRegion && !this.Flags.Includes(BinderFlags.AllowAwaitInUnsafeContext) &&
+                !CheckFeatureAvailability(awaitNodeOrToken, MessageID.IDS_FeatureUnsafeEvolution, diagnostics))
+            {
+                return true;
+            }
             else if (this.Flags.Includes(BinderFlags.InFinallyBlock) &&
                 (nodeOrToken.SyntaxTree as CSharpSyntaxTree)?.Options?.IsFeatureEnabled(MessageID.IDS_AwaitInCatchAndFinally) == false)
             {

src/Compilers/CSharp/Portable/Binder/Binder_Symbols.cs

@@ -2819,6 +2819,8 @@ protected AssemblySymbol GetForwardedToAssembly(string name, int arity, ref Name
         }
 
 #nullable enable
+        internal static bool CheckFeatureAvailability(SyntaxNodeOrToken syntax, MessageID feature, BindingDiagnosticBag diagnostics, Location? location = null)
+            => CheckFeatureAvailability(syntax.SyntaxTree!, feature, diagnostics.DiagnosticBag, (location, syntax), static tuple => tuple.location ?? tuple.syntax.GetLocation()!);
 
         internal static bool CheckFeatureAvailability(SyntaxNode syntax, MessageID feature, BindingDiagnosticBag diagnostics, Location? location = null)
             => CheckFeatureAvailability(syntax, feature, diagnostics.DiagnosticBag, location);

src/Compilers/CSharp/Portable/Binder/FixedStatementBinder.cs

@@ -20,7 +20,7 @@ internal sealed class FixedStatementBinder : LocalScopeBinder
         private readonly FixedStatementSyntax _syntax;
 
         public FixedStatementBinder(Binder enclosing, FixedStatementSyntax syntax)
-            : base(enclosing)
+            : base(enclosing, enclosing.Flags | BinderFlags.InFixedStatement)
         {
             Debug.Assert(syntax != null);
             _syntax = syntax;

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -3831,8 +3831,8 @@ Give the compiler some way to differentiate the methods. For example, you can gi
   <data name="ERR_BadAwaitInStaticVariableInitializer" xml:space="preserve">
     <value>The 'await' operator cannot be used in a static script variable initializer.</value>
   </data>
-  <data name="ERR_AwaitInUnsafeContext" xml:space="preserve">
-    <value>Cannot await in an unsafe context</value>
+  <data name="ERR_BadAwaitInFixed" xml:space="preserve">
+    <value>Cannot await in the body or the initializer of a 'fixed' statement</value>
   </data>
   <data name="ERR_BadAsyncLacksBody" xml:space="preserve">
     <value>The 'async' modifier can only be used in methods that have a body.</value>

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

@@ -1103,7 +1103,7 @@ internal enum ErrorCode
         ERR_BadAwaitArgIntrinsic = 4001,
         // 4002 unused
         ERR_BadAwaitAsIdentifier = 4003,
-        ERR_AwaitInUnsafeContext = 4004,
+        // ERR_AwaitInUnsafeContext = 4004, // replaced with a langversion error
         ERR_UnsafeAsyncArgType = 4005,
         ERR_VarargsAsync = 4006,
         ERR_ByRefTypeAndAwait = 4007,
@@ -2505,6 +2505,7 @@ internal enum ErrorCode
         ERR_NoBreakId = 9393,
         ERR_NoContinueId = 9394,
         ERR_ClosedBadDerivedTypesProperty = 9395,
+        ERR_BadAwaitInFixed = 9396,
 
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)

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

@@ -1550,7 +1550,7 @@ or ErrorCode.WRN_CLS_BadInterface
                 or ErrorCode.FTL_BadChecksumAlgorithm
                 or ErrorCode.ERR_BadAwaitArgIntrinsic
                 or ErrorCode.ERR_BadAwaitAsIdentifier
-                or ErrorCode.ERR_AwaitInUnsafeContext
+                or ErrorCode.ERR_BadAwaitInFixed
                 or ErrorCode.ERR_UnsafeAsyncArgType
                 or ErrorCode.ERR_VarargsAsync
                 or ErrorCode.ERR_BadAwaitArgVoidCall