Fix parsing of nested verbatim interpolated strings started with @$ (#44924)

* Fix parsing of nested verbatim interpolated strings started with @$

* Issue error for alt nested interpolated string in 7.3 or below. Better unit tests and test the error.

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

@@ -612,6 +612,27 @@ private void ScanInterpolatedStringLiteralHoleBalancedText(char endingChar, bool
                                 ScanInterpolatedStringLiteralNestedVerbatimString();
                                 continue;
                             }
+                            else if (lexer.TextWindow.PeekChar(1) == '$' && lexer.TextWindow.PeekChar(2) == '"')
+                            {
+                                lexer.CheckFeatureAvailability(MessageID.IDS_FeatureAltInterpolatedVerbatimStrings);
+                                var interpolations = (ArrayBuilder<Interpolation>)null;
+                                var info = default(TokenInfo);
+                                bool wasVerbatim = this.isVerbatim;
+                                bool wasAllowNewlines = this.allowNewlines;
+                                try
+                                {
+                                    this.isVerbatim = true;
+                                    this.allowNewlines = true;
+                                    bool closeQuoteMissing;
+                                    ScanInterpolatedStringLiteralTop(interpolations, ref info, out closeQuoteMissing);
+                                }
+                                finally
+                                {
+                                    this.isVerbatim = wasVerbatim;
+                                    this.allowNewlines = wasAllowNewlines;
+                                }
+                                continue;
+                            }
 
                             goto default;
                         case '/':

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

-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
@@ -92,6 +92,79 @@ public void TestAltInterpolatedVerbatimString_CSharp8()
             EOF();
         }
 
+        [Fact]
+        public void TestNestedAltInterpolatedVerbatimString_CSharp73()
+        {
+            UsingExpression("$@\"aaa{@$\"bbb\nccc\"}ddd\"", TestOptions.Regular7_3,
+                // (1,8): error CS8401: To use '@$' instead of '$@' for an interpolated verbatim string, please use language version '8.0' or greater.
+                // $@"aaa{@$"bbb
+                Diagnostic(ErrorCode.ERR_AltInterpolatedVerbatimStringsNotAvailable, @"@$""").WithArguments("8.0").WithLocation(1, 8)
+                );
+
+            N(SyntaxKind.InterpolatedStringExpression);
+            {
+                N(SyntaxKind.InterpolatedVerbatimStringStartToken);
+                N(SyntaxKind.InterpolatedStringText);
+                {
+                    N(SyntaxKind.InterpolatedStringTextToken, "aaa");
+                }
+                N(SyntaxKind.Interpolation);
+                {
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.InterpolatedStringExpression);
+                    {
+                        N(SyntaxKind.InterpolatedVerbatimStringStartToken);
+                        N(SyntaxKind.InterpolatedStringText);
+                        {
+                            N(SyntaxKind.InterpolatedStringTextToken, "bbb\nccc");
+                        }
+                        N(SyntaxKind.InterpolatedStringEndToken);
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.InterpolatedStringText);
+                {
+                    N(SyntaxKind.InterpolatedStringTextToken, "ddd");
+                }
+                N(SyntaxKind.InterpolatedStringEndToken);
+            }
+            EOF();
+        }
+
+        [Fact]
+        public void TestNestedAltInterpolatedVerbatimString_CSharp8()
+        {
+            UsingExpression("$@\"aaa{@$\"bbb\nccc\"}ddd\"", TestOptions.Regular8);
+            N(SyntaxKind.InterpolatedStringExpression);
+            {
+                N(SyntaxKind.InterpolatedVerbatimStringStartToken);
+                N(SyntaxKind.InterpolatedStringText);
+                {
+                    N(SyntaxKind.InterpolatedStringTextToken, "aaa");
+                }
+                N(SyntaxKind.Interpolation);
+                {
+                    N(SyntaxKind.OpenBraceToken);
+                    N(SyntaxKind.InterpolatedStringExpression);
+                    {
+                        N(SyntaxKind.InterpolatedVerbatimStringStartToken);
+                        N(SyntaxKind.InterpolatedStringText);
+                        {
+                            N(SyntaxKind.InterpolatedStringTextToken, "bbb\nccc");
+                        }
+                        N(SyntaxKind.InterpolatedStringEndToken);
+                    }
+                    N(SyntaxKind.CloseBraceToken);
+                }
+                N(SyntaxKind.InterpolatedStringText);
+                {
+                    N(SyntaxKind.InterpolatedStringTextToken, "ddd");
+                }
+                N(SyntaxKind.InterpolatedStringEndToken);
+            }
+            EOF();
+        }
+
         [Fact]
         public void TestName()
         {