When there is an error in an interpolated string, take the next quote as a close quote (#44899)

Fixes #44789

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

@@ -372,6 +372,12 @@ private void ScanInterpolatedStringLiteralContents(ArrayBuilder<Interpolation> i
 
                     switch (lexer.TextWindow.PeekChar())
                     {
+                        case '"' when RecoveringFromRunawayLexing():
+                            // When recovering from mismatched delimiters, we consume the next
+                            // quote character as the close quote for the interpolated string. In
+                            // practice this gets us out of trouble in scenarios we've encountered.
+                            // See, for example, https://github.com/dotnet/roslyn/issues/44789
+                            return;
                         case '"':
                             if (isVerbatim && lexer.TextWindow.PeekChar(1) == '"')
                             {
@@ -588,13 +594,19 @@ private void ScanInterpolatedStringLiteralHoleBalancedText(char endingChar, bool
                             }
 
                             goto default;
+                        case '"' when RecoveringFromRunawayLexing():
+                            // When recovering from mismatched delimiters, we consume the next
+                            // quote character as the close quote for the interpolated string. In
+                            // practice this gets us out of trouble in scenarios we've encountered.
+                            // See, for example, https://github.com/dotnet/roslyn/issues/44789
+                            return;
                         case '"':
                         case '\'':
                             // handle string or character literal inside an expression hole.
                             ScanInterpolatedStringLiteralNestedString();
                             continue;
                         case '@':
-                            if (lexer.TextWindow.PeekChar(1) == '"')
+                            if (lexer.TextWindow.PeekChar(1) == '"' && !RecoveringFromRunawayLexing())
                             {
                                 // check for verbatim string inside an expression hole.
                                 ScanInterpolatedStringLiteralNestedVerbatimString();
@@ -655,6 +667,12 @@ private void ScanInterpolatedStringLiteralHoleBalancedText(char endingChar, bool
                 }
             }
 
+            /// <summary>
+            /// The lexer can run away consuming the rest of the input when delimiters are mismatched.
+            /// This is a test for when we are attempting to recover from that situation.
+            /// </summary>
+            private bool RecoveringFromRunawayLexing() => this.error != null;
+
             private void ScanInterpolatedStringLiteralNestedComment()
             {
                 Debug.Assert(lexer.TextWindow.PeekChar() == '/');

src/Compilers/CSharp/Test/Semantic/Semantics/InterpolationTests.cs

@@ -151,14 +151,8 @@ public static void Main(string[] args)
             // too many diagnostics perhaps, but it starts the right way.
             CreateCompilationWithMscorlib45(source).VerifyDiagnostics(
                 // (5,71): error CS8077: A single-line comment may not be used in an interpolated string.
-                //         Console.WriteLine("Jenny don\'t change your number \{ 8675309 // ");
-                Diagnostic(ErrorCode.ERR_SingleLineCommentInExpressionHole, "//").WithLocation(5, 71),
-                // (5,77): error CS1026: ) expected
-                //         Console.WriteLine("Jenny don\'t change your number \{ 8675309 // ");
-                Diagnostic(ErrorCode.ERR_CloseParenExpected, "").WithLocation(5, 77),
-                // (5,77): error CS1002: ; expected
-                //         Console.WriteLine("Jenny don\'t change your number \{ 8675309 // ");
-                Diagnostic(ErrorCode.ERR_SemicolonExpected, "").WithLocation(5, 77)
+                //         Console.WriteLine($"Jenny don\'t change your number { 8675309 // ");
+                Diagnostic(ErrorCode.ERR_SingleLineCommentInExpressionHole, "//").WithLocation(5, 71)
                 );
         }
 

src/Compilers/CSharp/Test/Syntax/Parsing/ExpressionParsingTests.cs

@@ -5236,5 +5236,118 @@ public void ArrayCreation_ElementAccess()
             }
             EOF();
         }
+
+        [Fact, WorkItem(44789, "https://github.com/dotnet/roslyn/issues/44789")]
+        public void MismatchedInterpolatedStringContents_01()
+        {
+            var text =
+@"class A
+{
+    void M()
+    {
+        if (b)
+        {
+            A B = new C($@""{D(.E}"");
+            N.O("""", P.Q);
+            R.S(T);
+            U.V(W.X, Y.Z);
+        }
+    }
+
+    string M() => """";
+}";
+            var tree = ParseTree(text, TestOptions.Regular);
+            // Note that the parser eventually syncs back up and stops producing diagnostics.
+            tree.GetDiagnostics().Verify(
+                // (7,31): error CS1001: Identifier expected
+                //             A B = new C($@"{D(.E}");
+                Diagnostic(ErrorCode.ERR_IdentifierExpected, ".").WithLocation(7, 31),
+                // (7,33): error CS1003: Syntax error, ')' expected
+                //             A B = new C($@"{D(.E}");
+                Diagnostic(ErrorCode.ERR_SyntaxError, "}").WithArguments(")").WithLocation(7, 33),
+                // (7,33): error CS1003: Syntax error, ',' expected
+                //             A B = new C($@"{D(.E}");
+                Diagnostic(ErrorCode.ERR_SyntaxError, "}").WithArguments(",", "}").WithLocation(7, 33),
+                // (7,34): error CS1026: ) expected
+                //             A B = new C($@"{D(.E}");
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "").WithLocation(7, 34)
+                );
+        }
+
+        [Fact, WorkItem(44789, "https://github.com/dotnet/roslyn/issues/44789")]
+        public void MismatchedInterpolatedStringContents_02()
+        {
+            var text =
+@"class A
+{
+    void M()
+    {
+        if (b)
+        {
+            A B = new C($@""{D(.E}\F\G{H}_{I.J.K(""L"")}.M"");
+            N.O("""", P.Q);
+            R.S(T);
+            U.V(W.X, Y.Z);
+        }
+    }
+
+    string M() => """";
+}";
+            var tree = ParseTree(text, TestOptions.Regular);
+            // Note that the parser eventually syncs back up and stops producing diagnostics.
+            tree.GetDiagnostics().Verify(
+                    // (7,31): error CS1001: Identifier expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_IdentifierExpected, ".").WithLocation(7, 31),
+                    // (7,33): error CS1003: Syntax error, ')' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "}").WithArguments(")").WithLocation(7, 33),
+                    // (7,33): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "}").WithArguments(",", "}").WithLocation(7, 33),
+                    // (7,34): error CS1056: Unexpected character '\'
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_UnexpectedCharacter, "").WithArguments("\\").WithLocation(7, 34),
+                    // (7,35): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "F").WithArguments(",", "").WithLocation(7, 35),
+                    // (7,36): error CS1056: Unexpected character '\'
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_UnexpectedCharacter, "").WithArguments("\\").WithLocation(7, 36),
+                    // (7,37): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "G").WithArguments(",", "").WithLocation(7, 37),
+                    // (7,38): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "{").WithArguments(",", "{").WithLocation(7, 38),
+                    // (7,39): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "H").WithArguments(",", "").WithLocation(7, 39),
+                    // (7,40): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "}").WithArguments(",", "}").WithLocation(7, 40),
+                    // (7,41): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "_").WithArguments(",", "").WithLocation(7, 41),
+                    // (7,42): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "{").WithArguments(",", "{").WithLocation(7, 42),
+                    // (7,43): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "I").WithArguments(",", "").WithLocation(7, 43),
+                    // (7,49): error CS1026: ) expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_CloseParenExpected, "").WithLocation(7, 49),
+                    // (7,49): error CS1026: ) expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_CloseParenExpected, "").WithLocation(7, 49),
+                    // (7,50): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, "L").WithArguments(",", "").WithLocation(7, 50),
+                    // (7,51): error CS1003: Syntax error, ',' expected
+                    //             A B = new C($@"{D(.E}\F\G{H}_{I.J.K("L")}.M");
+                    Diagnostic(ErrorCode.ERR_SyntaxError, @""")}.M""").WithArguments(",", "").WithLocation(7, 51)
+                );
+        }
     }
 }