perf: use ValueListBuilder for PrivatePrintLeadingTrivia

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.ToList()),
+        };
+    }
+
     public static Doc Join(Doc separator, IEnumerable<Doc> array)
     {
         var docs = new List<Doc>();

Src/CSharpier/SyntaxPrinter/Token.cs

@@ -233,7 +233,7 @@ private static Doc PrivatePrintLeadingTrivia(
             return Doc.Null;
         }
 
-        var docs = new List<Doc>();
+        var docs = new ValueListBuilder<Doc>([null, null, null, null, null, null, null, null]);
 
         // we don't print any new lines until we run into a comment or directive
         // the PrintExtraNewLines method takes care of printing the initial new lines for a given node
@@ -246,18 +246,18 @@ private static Doc PrivatePrintLeadingTrivia(
 
             if (printNewLines && kind == SyntaxKind.EndOfLineTrivia)
             {
-                if (docs.Count > 0 && docs[^1] == Doc.HardLineSkipBreakIfFirstInGroup)
+                if (docs.Length > 0 && docs[^1] == Doc.HardLineSkipBreakIfFirstInGroup)
                 {
                     printNewLines = false;
                 }
-                docs.Add(Doc.HardLineSkipBreakIfFirstInGroup);
+                docs.Append(Doc.HardLineSkipBreakIfFirstInGroup);
             }
             if (kind is not (SyntaxKind.EndOfLineTrivia or SyntaxKind.WhitespaceTrivia))
             {
                 printNewLines = true;
             }
 
-            void AddLeadingComment(CommentType commentType)
+            void AddLeadingComment(CommentType commentType, ref ValueListBuilder<Doc> docs)
             {
                 var comment = trivia.ToFullString().TrimEnd('\n', '\r');
                 if (
@@ -269,43 +269,43 @@ void AddLeadingComment(CommentType commentType)
                     comment = leadingTrivia[x - 1] + comment;
                 }
 
-                docs.Add(Doc.LeadingComment(comment, commentType));
+                docs.Append(Doc.LeadingComment(comment, commentType));
             }
 
             if (IsSingleLineComment(kind))
             {
-                AddLeadingComment(CommentType.SingleLine);
-                docs.Add(
+                AddLeadingComment(CommentType.SingleLine, ref docs);
+                docs.Append(
                     kind == SyntaxKind.SingleLineDocumentationCommentTrivia
                         ? Doc.HardLineSkipBreakIfFirstInGroup
                         : Doc.Null
                 );
             }
             else if (IsMultiLineComment(kind))
             {
-                AddLeadingComment(CommentType.MultiLine);
+                AddLeadingComment(CommentType.MultiLine, ref docs);
             }
             else if (kind == SyntaxKind.DisabledTextTrivia)
             {
-                docs.Add(Doc.Trim, trivia.ToString());
+                docs.Append(Doc.Trim, trivia.ToString());
             }
             else if (IsRegion(kind))
             {
                 var triviaText = trivia.ToString();
-                docs.Add(Doc.HardLineIfNoPreviousLine);
-                docs.Add(Doc.Trim);
-                docs.Add(
+                docs.Append(Doc.HardLineIfNoPreviousLine);
+                docs.Append(Doc.Trim);
+                docs.Append(
                     kind == SyntaxKind.RegionDirectiveTrivia
                         ? Doc.BeginRegion(triviaText)
                         : Doc.EndRegion(triviaText)
                 );
-                docs.Add(Doc.HardLine);
+                docs.Append(Doc.HardLine);
             }
             else if (trivia.IsDirective)
             {
                 var triviaText = trivia.ToString();
 
-                docs.Add(
+                docs.Append(
                     // adding two of these to ensure we get a new line when a directive follows a trailing comment
                     Doc.HardLineIfNoPreviousLineSkipBreakIfFirstInGroup,
                     Doc.HardLineIfNoPreviousLineSkipBreakIfFirstInGroup,
@@ -323,16 +323,16 @@ void AddLeadingComment(CommentType commentType)
                     )
                     {
                         x++;
-                        docs.Add(Doc.HardLineSkipBreakIfFirstInGroup);
+                        docs.Append(Doc.HardLineSkipBreakIfFirstInGroup);
                     }
                     printNewLines = false;
                 }
             }
         }
 
-        while (skipLastHardline && docs.Count != 0 && docs.Last() is HardLine or NullDoc)
+        while (skipLastHardline && docs.Length != 0 && docs[^1] is HardLine or NullDoc)
         {
-            docs.RemoveAt(docs.Count - 1);
+            docs.Pop();
         }
 
         if (context.State.NextTriviaNeedsLine)
@@ -343,7 +343,7 @@ void AddLeadingComment(CommentType commentType)
             }
             else
             {
-                var index = docs.Count - 1;
+                var index = docs.Length - 1;
                 while (
                     index >= 0
                     && (docs[index] is HardLine or LeadingComment || docs[index] == Doc.Null)
@@ -354,7 +354,7 @@ void AddLeadingComment(CommentType commentType)
                 // this handles an edge case where we get here but already added the line
                 // it relates to the fact that single line comments include new line directives
                 if (
-                    index + 2 >= docs.Count
+                    index + 2 >= docs.Length
                     || !(docs[index + 1] is HardLine && docs[index + 2] is HardLine)
                 )
                 {
@@ -364,7 +364,7 @@ void AddLeadingComment(CommentType commentType)
             context.State.NextTriviaNeedsLine = false;
         }
 
-        return docs.Count > 0 ? Doc.Concat(docs) : Doc.Null;
+        return docs.Length > 0 ? Doc.Concat(ref docs) : Doc.Null;
     }
 
     private static bool IsSingleLineComment(SyntaxKind kind) =>

Src/CSharpier/Utilities/ValueListBuilder.cs

@@ -89,18 +89,19 @@ private void AppendMultiChar(scoped ReadOnlySpan<T> source)
         _pos += source.Length;
     }
 
-    public void Insert(int index, scoped ReadOnlySpan<T> source)
+    public void Insert(int index, T item)
     {
-        Debug.Assert(index == 0, "Implementation currently only supports index == 0");
-
-        if ((uint)(_pos + source.Length) > (uint)_span.Length)
+        int pos = _pos;
+        if ((uint)pos >= (uint)_span.Length)
         {
-            Grow(source.Length);
+            Grow(1);
         }
 
-        _span.Slice(0, _pos).CopyTo(_span.Slice(source.Length));
-        source.CopyTo(_span);
-        _pos += source.Length;
+        var sp = _span;
+
+        _span.Slice(index, _pos - index).CopyTo(_span.Slice(index + 1));
+        _span[index] = item;
+        _pos += 1;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -141,11 +142,33 @@ private void AddWithResize(T item)
         _pos = pos + 1;
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public T Pop()
+    {
+        _pos--;
+        return _span[_pos];
+    }
+
     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))