Follow up on LDM decisions made 2022-01-26 (#59229)

LDM notes: https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-01-26.md
Feature spec: https://github.com/dotnet/csharplang/blob/main/proposals/utf8-string-literals.md

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

@@ -5913,7 +5913,6 @@ private BoundUTF8String BindUTF8StringLiteral(LiteralExpressionSyntax node, Bind
             var value = (string)node.Token.Value;
             var type = ArrayTypeSymbol.CreateSZArray(Compilation.Assembly, TypeWithAnnotations.Create(GetSpecialType(SpecialType.System_Byte, diagnostics, node)));
 
-            // PROTOTYPE(UTF8StringLiterals) : The result type is a deviation from the proposal, but I think it simplifies language rules. Will bring this for discussion to LDM.
             return new BoundUTF8String(node, value, type);
         }
 

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/ConversionsBase.cs

@@ -649,7 +649,8 @@ private Conversion ClassifyStandardImplicitConversion(BoundExpression sourceExpr
             // with the exception of the switch expression conversion.
 
             Conversion conversion = ClassifyImplicitBuiltInConversionFromExpression(sourceExpression, compilation, source, destination, ref useSiteInfo);
-            if (conversion.Exists)
+            if (conversion.Exists &&
+                !conversion.IsUtf8StringLiteral) // UTF-8 string conversion is not a standard conversion.
             {
                 Debug.Assert(IsStandardImplicitConversionFromExpression(conversion.Kind));
                 return conversion;
@@ -1107,7 +1108,7 @@ private Conversion ClassifyImplicitUtf8StringLiteralConversion(BoundExpression s
             ConstantValue constantValue = sourceExpression.ConstantValue;
 
             TypeSymbol destinationOriginalDefinition = destination.OriginalDefinition;
-            if (constantValue?.IsString == true && // PROTOTYPE(UTF8StringLiterals) : confirm if we actually want it to work with 'null' constant value.
+            if (constantValue is ({ IsString: true } or { IsNull: true }) &&
                 sourceExpression.Type?.SpecialType == SpecialType.System_String &&
                 (destination is ArrayTypeSymbol { IsSZArray: true, ElementType.SpecialType: SpecialType.System_Byte } || // byte[]
                  (destinationOriginalDefinition.TypeKind == TypeKind.Struct && destinationOriginalDefinition.IsRefLikeType &&

src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/OverloadResolution.cs

@@ -1669,19 +1669,6 @@ private BetterResult BetterFunctionMember<TMember>(
             Debug.Assert(m2.Result.IsValid);
             Debug.Assert(arguments != null);
 
-            // Prefer overloads that did not use the UTF8 string literal conversion to convert arguments.
-            // PROTOTYPE(UTF8StringLiterals) : Confirm this is the behavior we want. Similar to function types, this
-            //                                doesn't detect user defined conversions that became applicable due to
-            //                                UTF8 string literal conversions. But at the moment we do not allow
-            //                                implicit conversions like that and they are never applicable.
-            switch (RequiredUTF8StringLiteralConversion(m1), RequiredUTF8StringLiteralConversion(m2))
-            {
-                case (false, true):
-                    return BetterResult.Left;
-                case (true, false):
-                    return BetterResult.Right;
-            }
-
             // Prefer overloads that did not use the inferred type of lambdas or method groups
             // to infer generic method type arguments or to convert arguments.
             switch (RequiredFunctionType(m1), RequiredFunctionType(m2))
@@ -2139,23 +2126,6 @@ private static bool RequiredFunctionType<TMember>(MemberResolutionResult<TMember
             return conversionsOpt.Any(c => c.Kind == ConversionKind.FunctionType);
         }
 
-        /// <summary>
-        /// Returns true if the overload required a UTF8 string literal conversion to parameter types.
-        /// </summary>
-        private static bool RequiredUTF8StringLiteralConversion<TMember>(MemberResolutionResult<TMember> m)
-            where TMember : Symbol
-        {
-            Debug.Assert(m.Result.IsValid);
-
-            var conversionsOpt = m.Result.ConversionsOpt;
-            if (conversionsOpt.IsDefault)
-            {
-                return false;
-            }
-
-            return conversionsOpt.Any(c => c.Kind == ConversionKind.ImplicitUtf8StringLiteral);
-        }
-
         private static BetterResult PreferValOverInOrRefInterpolatedHandlerParameters<TMember>(
             ArrayBuilder<BoundExpression> arguments,
             MemberResolutionResult<TMember> m1,

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6947,4 +6947,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureUTF8StringLiterals" xml:space="preserve">
     <value>Utf8 String Literals</value>
   </data>
+  <data name="ERR_ExpressionTreeContainsUTF8StringLiterals" xml:space="preserve">
+    <value>An expression tree may not contain UTF8 string conversion or literal.</value>
+  </data>
 </root>

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

@@ -2028,6 +2028,7 @@ internal enum ErrorCode
         WRN_NullCheckingOnNullableType = 8995,
 
         ERR_CannotBeConvertedToUTF8 = 9100, // PROTOTYPE(UTF8StringLiterals) : pack numbers
+        ERR_ExpressionTreeContainsUTF8StringLiterals = 9101,
 
         #endregion
 

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -7723,7 +7723,14 @@ private TypeWithState VisitConversion(
                     break;
 
                 case ConversionKind.ImplicitUtf8StringLiteral:
-                    resultState = NullableFlowState.NotNull; // PROTOTYPE(UTF8StringLiterals) : Adjust if we actually want it to work with 'null' value.
+                    if (targetType.IsReferenceType)
+                    {
+                        resultState = getConversionResultState(operandType);
+                    }
+                    else
+                    {
+                        resultState = NullableFlowState.NotNull;
+                    }
                     break;
 
                 case ConversionKind.ObjectCreation:

src/Compilers/CSharp/Portable/Lowering/DiagnosticsPass_ExpressionTrees.cs

@@ -761,6 +761,13 @@ public override BoundNode VisitConversion(BoundConversion node)
                     }
                     break;
 
+                case ConversionKind.ImplicitUtf8StringLiteral:
+                    if (_inExpressionLambda)
+                    {
+                        Error(ErrorCode.ERR_ExpressionTreeContainsUTF8StringLiterals, node);
+                    }
+                    break;
+
                 default:
 
                     if (_inExpressionLambda && node.Conversion.Method is MethodSymbol method && method.IsAbstract && method.IsStatic)
@@ -776,6 +783,16 @@ public override BoundNode VisitConversion(BoundConversion node)
             return result;
         }
 
+        public override BoundNode VisitUTF8String(BoundUTF8String node)
+        {
+            if (_inExpressionLambda)
+            {
+                Error(ErrorCode.ERR_ExpressionTreeContainsUTF8StringLiterals, node);
+            }
+
+            return null;
+        }
+
         public override BoundNode VisitDelegateCreationExpression(BoundDelegateCreationExpression node)
         {
             if (node.Argument.Kind != BoundKind.MethodGroup)

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Conversion.cs

@@ -79,7 +79,11 @@ public override BoundNode VisitConversion(BoundConversion node)
         private BoundNode RewriteUtf8StringLiteralConversion(BoundConversion node)
         {
             string? value = node.Operand.ConstantValue?.StringValue;
-            Debug.Assert(value != null); // PROTOTYPE(UTF8StringLiterals) : Adjust if we actually want it to work with 'null' value.
+
+            if (value == null)
+            {
+                return new BoundDefaultExpression(node.Syntax, node.Type);
+            }
 
             ArrayTypeSymbol byteArray;
 

src/Compilers/CSharp/Portable/Parser/Lexer_StringLiteral.cs

@@ -77,11 +77,9 @@ private void ScanStringLiteral(ref TokenInfo info, bool inDirective)
             }
             else
             {
-                // PROTOTYPE(UTF8StringLiterals) : Should the suffix be case-insensitive?
-                if (!inDirective && TextWindow.PeekChar() == 'u' && TextWindow.PeekChar(1) == '8')
+                if (!inDirective && ScanUTF8Suffix())
                 {
                     info.Kind = SyntaxKind.UTF8StringLiteralToken;
-                    TextWindow.AdvanceChar(2);
                 }
                 else
                 {
@@ -101,6 +99,17 @@ private void ScanStringLiteral(ref TokenInfo info, bool inDirective)
             }
         }
 
+        private bool ScanUTF8Suffix()
+        {
+            if (TextWindow.PeekChar() is ('u' or 'U') && TextWindow.PeekChar(1) == '8')
+            {
+                TextWindow.AdvanceChar(2);
+                return true;
+            }
+
+            return false;
+        }
+
         private char ScanEscapeSequence(out char surrogateCharacter)
         {
             var start = TextWindow.Position;
@@ -194,11 +203,9 @@ private void ScanVerbatimStringLiteral(ref TokenInfo info)
                 _builder.Append(ch);
             }
 
-            // PROTOTYPE(UTF8StringLiterals) : Should the suffix be case-insensitive?
-            if (TextWindow.PeekChar() == 'u' && TextWindow.PeekChar(1) == '8')
+            if (ScanUTF8Suffix())
             {
                 info.Kind = SyntaxKind.UTF8StringLiteralToken;
-                TextWindow.AdvanceChar(2);
             }
             else
             {