perf: change RemoveAt to set Contents to Doc.Null

Author: TimothyMakkison

Src/CSharpier/DocTypes/Doc.cs

@@ -56,6 +56,16 @@ public static Doc Concat(List<Doc> contents) =>
 
     public static Doc Concat(params Doc[] contents) => new Concat(contents);
 
+    public static Doc Concat(ref ValueListBuilder<Doc> contents)
+    {
+        return contents.Length switch
+        {
+            0 => Null,
+            1 => contents[0],
+            _ => Concat(contents.AsSpan().ToArray()),
+        };
+    }
+
     public static Doc Join(Doc separator, IEnumerable<Doc> enumerable)
     {
         var docs = new List<Doc>();

Src/CSharpier/SyntaxPrinter/SyntaxNodePrinters/CompilationUnit.cs

@@ -25,9 +25,14 @@ public static Doc Print(CompilationUnitSyntax node, PrintingContext context)
                 && previousList.Contents[^2] is HardLine
             )
             {
-                while (list.Contents[0] is HardLine { SkipBreakIfFirstInGroup: true })
+                for (
+                    var i = 0;
+                    i < list.Contents.Count
+                        && list.Contents[0] is HardLine { SkipBreakIfFirstInGroup: true };
+                    i++
+                )
                 {
-                    list.Contents.RemoveAt(0);
+                    list.Contents[i] = Doc.Null;
                 }
 
                 docs.Add(finalTrivia);

Src/CSharpier/SyntaxPrinter/Token.cs

@@ -253,7 +253,7 @@ private static Doc PrivatePrintLeadingTrivia(
 
                 if (
                     !(
-                        docs.Count > 1
+                        docs.Length > 1
                         && docs[^1] == Doc.HardLineSkipBreakIfFirstInGroup
                         && docs[^2] is LeadingComment { Type: CommentType.SingleLine }
                     )
@@ -374,7 +374,7 @@ void AddLeadingComment(CommentType commentType, ref ValueListBuilder<Doc> docs)
             context.State.NextTriviaNeedsLine = false;
         }
 
-        return docs.Length > 0 ? Doc.Concat(ref docs) : Doc.Null;
+        return Doc.Concat(ref docs);
     }
 
     private static bool IsSingleLineComment(SyntaxKind kind) =>

Src/CSharpier/Utilities/ValueListBuilder.cs

@@ -154,21 +154,6 @@ public ReadOnlySpan<T> AsSpan()
         return _span.Slice(0, _pos);
     }
 
-    public List<T> ToList()
-    {
-        var list = new List<T>(_pos);
-#if NETSTANDARD2_0
-        foreach (var item in _span[.._pos])
-        {
-            list.Add(item);
-        }
-#else
-        list.AddRange(_span[.._pos]);
-#endif
-
-        return list;
-    }
-
     public bool TryCopyTo(Span<T> destination, out int itemsWritten)
     {
         if (_span.Slice(0, _pos).TryCopyTo(destination))