NormalizeWhitespace: don't add space after colon in interpolation format clause (#24494)

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

@@ -385,7 +385,7 @@ private static bool NeedsSeparator(SyntaxToken token, SyntaxToken next)
 
             if (token.IsKind(SyntaxKind.ColonToken))
             {
-                return true;
+                return !token.Parent.IsKind(SyntaxKind.InterpolationFormatClause);
             }
 
             if (next.IsKind(SyntaxKind.ColonToken))

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

@@ -274,6 +274,14 @@ public void TestSpacingOnInvocationLikeKeywords()
             TestNormalizeExpression("__arglist (a)", "__arglist(a)");
         }
 
+        [Fact]
+        [WorkItem(24454, "https://github.com/dotnet/roslyn/issues/24454")]
+        public void TestSpacingOnInterpolatedString()
+        {
+            TestNormalizeExpression("$\"{3:C}\"", "$\"{3:C}\"");
+            TestNormalizeExpression("$\"{3: C}\"", "$\"{3: C}\"");
+        }
+
         [Fact]
         [WorkItem(23618, "https://github.com/dotnet/roslyn/issues/23618")]
         public void TestSpacingOnMethodConstraint()