Improve throughput / allocations of JsonNode.GetPath

src/libraries/Common/src/System/Text/ValueStringBuilder.cs

@@ -251,7 +251,7 @@ public unsafe void Append(char* value, int length)
             _pos += length;
         }
 
-        public void Append(ReadOnlySpan<char> value)
+        public void Append(scoped ReadOnlySpan<char> value)
         {
             int pos = _pos;
             if (pos > _chars.Length - value.Length)

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -23,6 +23,7 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
 
   <ItemGroup>
     <Compile Include="$(CommonPath)System\HexConverter.cs" Link="Common\System\HexConverter.cs" />
+    <Compile Include="$(CommonPath)System\Text\ValueStringBuilder.cs" Link="Common\ValueStringBuilder.cs" />
     <Compile Include="$(CommonPath)System\Text\Json\PooledByteBufferWriter.cs" Link="Common\System\Text\Json\PooledByteBufferWriter.cs" />
     <Compile Include="..\Common\JsonCamelCaseNamingPolicy.cs" Link="Common\System\Text\Json\JsonCamelCaseNamingPolicy.cs" />
     <Compile Include="..\Common\JsonNamingPolicy.cs" Link="Common\System\Text\Json\JsonNamingPolicy.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/JsonConstants.cs

@@ -74,6 +74,7 @@ internal static partial class JsonConstants
 
         public const int MaximumFormatBooleanLength = 5;
         public const int MaximumFormatInt64Length = 20;   // 19 + sign (i.e. -9223372036854775808)
+        public const int MaximumFormatUInt32Length = 10;  // i.e. 4294967295
         public const int MaximumFormatUInt64Length = 20;  // i.e. 18446744073709551615
         public const int MaximumFormatDoubleLength = 128;  // default (i.e. 'G'), using 128 (rather than say 32) to be future-proof.
         public const int MaximumFormatSingleLength = 128;  // default (i.e. 'G'), using 128 (rather than say 32) to be future-proof.

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonArray.cs

@@ -202,15 +202,26 @@ internal void SetItem(int index, JsonNode? value)
             List[index] = value;
         }
 
-        internal override void GetPath(List<string> path, JsonNode? child)
+        internal override void GetPath(ref ValueStringBuilder path, JsonNode? child)
         {
+            Parent?.GetPath(ref path, this);
+
             if (child != null)
             {
                 int index = List.IndexOf(child);
-                path.Add($"[{index}]");
+                Debug.Assert(index >= 0);
+
+                path.Append('[');
+#if NETCOREAPP
+                Span<char> chars = stackalloc char[JsonConstants.MaximumFormatUInt32Length];
+                bool formatted = ((uint)index).TryFormat(chars, out int charsWritten);
+                Debug.Assert(formatted);
+                path.Append(chars.Slice(0, charsWritten));
+#else
+                path.Append(index.ToString());
+#endif
+                path.Append(']');
             }
-
-            Parent?.GetPath(path, this);
         }
 
         /// <inheritdoc/>

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonNode.cs

@@ -131,19 +131,13 @@ public string GetPath()
                 return "$";
             }
 
-            var path = new List<string>();
-            GetPath(path, null);
-
-            var sb = new StringBuilder("$");
-            for (int i = path.Count - 1; i >= 0; i--)
-            {
-                sb.Append(path[i]);
-            }
-
-            return sb.ToString();
+            var path = new ValueStringBuilder(stackalloc char[JsonConstants.StackallocCharThreshold]);
+            path.Append('$');
+            GetPath(ref path, null);
+            return path.ToString();
         }
 
-        internal abstract void GetPath(List<string> path, JsonNode? child);
+        internal abstract void GetPath(ref ValueStringBuilder path, JsonNode? child);
 
         /// <summary>
         ///   Gets the root <see cref="JsonNode"/>.

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs

@@ -196,22 +196,25 @@ internal override bool DeepEqualsCore(JsonNode? node)
             return null;
         }
 
-        internal override void GetPath(List<string> path, JsonNode? child)
+        internal override void GetPath(ref ValueStringBuilder path, JsonNode? child)
         {
+            Parent?.GetPath(ref path, this);
+
             if (child != null)
             {
                 string propertyName = Dictionary.FindValue(child)!.Value.Key;
                 if (propertyName.AsSpan().ContainsSpecialCharacters())
                 {
-                    path.Add($"['{propertyName}']");
+                    path.Append("['");
+                    path.Append(propertyName);
+                    path.Append("']");
                 }
                 else
                 {
-                    path.Add($".{propertyName}");
+                    path.Append('.');
+                    path.Append(propertyName);
                 }
             }
-
-            Parent?.GetPath(path, this);
         }
 
         internal void SetItem(string propertyName, JsonNode? value)

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonValue.cs

@@ -90,11 +90,11 @@ private protected JsonValue(JsonNodeOptions? options = null) : base(options) { }
             return new JsonValueCustomized<T>(value, jsonTypeInfo, options);
         }
 
-        internal override void GetPath(List<string> path, JsonNode? child)
+        internal override void GetPath(ref ValueStringBuilder path, JsonNode? child)
         {
             Debug.Assert(child == null);
 
-            Parent?.GetPath(path, this);
+            Parent?.GetPath(ref path, this);
         }
 
         /// <summary>