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 +921,29 @@ node is QueryExpressionSyntax ||
 
             return 0;
         }
+
+        public override SyntaxNode? VisitInterpolatedStringExpression(InterpolatedStringExpressionSyntax node)
+        {
+            if (node.StringStartToken.Kind() == SyntaxKind.InterpolatedStringStartToken)
+            {
+                //Just for non verbatim strings we want to make sure that the formatting of interpolations does not emit line breaks.
+                //See: https://github.com/dotnet/roslyn/issues/50742
+                //
+                //The flag _inSingleLineInterpolation is set to true while visiting InterpolatedStringExpressionSyntax and checked in LineBreaksAfter
+                //to suppress adding newlines.
+                var old = _inSingleLineInterpolation;
+                _inSingleLineInterpolation = true;
+                try
+                {
+                    return base.VisitInterpolatedStringExpression(node);
+                }
+                finally
+                {
+                    _inSingleLineInterpolation = old;
+                }
+            }
+
+            return base.VisitInterpolatedStringExpression(node);
+        }
     }
 }

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

@@ -16,6 +16,33 @@ namespace Microsoft.CodeAnalysis.CSharp.UnitTests
 {
     public class SyntaxNormalizerTests
     {
+        [Fact, WorkItem(50742, "https://github.com/dotnet/roslyn/issues/50742")]
+        public void TestLineBreakInterpolations()
+        {
+            TestNormalizeExpression(
+                @"$""Printed: {                    new Printer() { TextToPrint = ""Hello world!"" }.PrintedText }""",
+                @"$""Printed: {new Printer(){TextToPrint = ""Hello world!""}.PrintedText}"""
+            );
+        }
+
+        [Fact, WorkItem(50742, "https://github.com/dotnet/roslyn/issues/50742")]
+        public void TestVerbatimStringInterpolationWithLineBreaks()
+        {
+            TestNormalizeStatement(@"Console.WriteLine($@""Test with line
+breaks
+{
+                new[]{
+     1, 2, 3
+  }[2]
+}
+            "");",
+            @"Console.WriteLine($@""Test with line
+breaks
+{new[]{1, 2, 3}[2]}
+            "");"
+            );
+        }
+
         [Fact]
         public void TestNormalizeExpression1()
         {