Fix NormalizeWhitespace - String Interpolation with e.g. Initializer-expressions

src/Compilers/CSharp/Portable/Syntax/SyntaxNormalizer.cs

@@ -27,6 +27,7 @@ internal class SyntaxNormalizer : CSharpSyntaxRewriter
 
         private bool _afterLineBreak;
         private bool _afterIndentation;
+        private bool _inSingleLineInterpolation;
 
         // CONSIDER: if we become concerned about space, we shouldn't actually need any 
         // of the values between indentations[0] and indentations[initialDepth] (exclusive).
@@ -177,6 +178,11 @@ private static bool NeedsIndentAfterLineBreak(SyntaxToken token)
 
         private int LineBreaksAfter(SyntaxToken currentToken, SyntaxToken nextToken)
         {
+            if (_inSingleLineInterpolation)
+            {
+                return 0;
+            }
+
             if (currentToken.IsKind(SyntaxKind.EndOfDirectiveToken))
             {
                 return 1;
@@ -915,5 +921,24 @@ node is QueryExpressionSyntax ||
 
             return 0;
         }
+
+        public override SyntaxNode? VisitInterpolatedStringExpression(InterpolatedStringExpressionSyntax node)
+        {
+            if (node.StringStartToken.Kind() == SyntaxKind.InterpolatedStringStartToken)
+            {
+                var old = _inSingleLineInterpolation;
+                _inSingleLineInterpolation = true;
+                try
+                {
+                    return base.VisitInterpolatedStringExpression(node);
+                }
+                finally
+                {
+                    _inSingleLineInterpolation = old;
+                }
+            }
+
+            return base.VisitInterpolatedStringExpression(node);
+        }
     }
 }

src/Compilers/CSharp/Test/Syntax/Syntax/SyntaxFactoryTests.cs

@@ -534,6 +534,18 @@ public void TestSpacingOnTernary()
             Assert.Equal("x is DateTime ? y : z", syntaxNode2.ToFullString());
         }
 
+        [Fact]
+        [WorkItem(50742, "https://github.com/dotnet/roslyn/issues/50742")]
+        public void TestLineBreakInterpolations()
+        {
+            var code = @"$""Printed: { new Printer() { TextToPrint = ""Hello world!"" }.PrintedText }""";
+
+            var syntaxNode = SyntaxFactory.ParseExpression(code).NormalizeWhitespace();
+            var aText = syntaxNode.ToFullString();
+
+            Assert.True(!aText.Contains('\n'));
+        }
+
         [Fact]
         [WorkItem(21231, "https://github.com/dotnet/roslyn/issues/21231")]
         public void TestSpacingOnCoalescing()