From 7448c298277be21ea23aedd6e8874246f0c89671 Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Thu, 28 May 2026 19:31:04 +0100
Subject: [PATCH 1/2] perf: collapse Replace chain in
 TestNameFormatter.BuildTestId

Replace the four chained string.Replace calls (each allocating a new
string) with a single pooled StringBuilder pass. Only the final ToString
allocates; output is byte-identical since StringBuilder.Replace shares
ordinal, case-sensitive, all-occurrences semantics with string.Replace.

Closes #6032
---
 TUnit.Core/Services/TestNameFormatter.cs | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/TUnit.Core/Services/TestNameFormatter.cs b/TUnit.Core/Services/TestNameFormatter.cs
index e9f582ab69..5427cb85c6 100644
--- a/TUnit.Core/Services/TestNameFormatter.cs
+++ b/TUnit.Core/Services/TestNameFormatter.cs
@@ -1,5 +1,6 @@
 using System.Collections;
 using System.Text;
+using TUnit.Core.Helpers;
 using TUnit.Core.Interfaces;
 
 namespace TUnit.Core.Services;
@@ -68,11 +69,23 @@ public string BuildTestId(
         int classDataIndex = 0,
         int methodDataIndex = 0)
     {
-        return template
-            .Replace("{TestIndex}", testIndex.ToString())
-            .Replace("{RepeatIndex}", repeatIndex.ToString())
-            .Replace("{ClassDataIndex}", classDataIndex.ToString())
-            .Replace("{MethodDataIndex}", methodDataIndex.ToString());
+        // Mutate a pooled StringBuilder in place then materialize once, instead of
+        // allocating a new string per Replace call. Only the final string allocates.
+        var builder = StringBuilderPool.Get();
+        try
+        {
+            return builder
+                .Append(template)
+                .Replace("{TestIndex}", testIndex.ToString())
+                .Replace("{RepeatIndex}", repeatIndex.ToString())
+                .Replace("{ClassDataIndex}", classDataIndex.ToString())
+                .Replace("{MethodDataIndex}", methodDataIndex.ToString())
+                .ToString();
+        }
+        finally
+        {
+            StringBuilderPool.Return(builder);
+        }
     }
 
     private string FormatArguments(object?[] args)

From f04b14102fe46afec5e1046dfc53b432349d7e2a Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Thu, 28 May 2026 20:30:34 +0100
Subject: [PATCH 2/2] perf: pool StringBuilder in
 TestNameFormatter.FormatEnumerable
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Address review on PR #6083: FormatEnumerable allocated a fresh StringBuilder
on every call on the discovery hot path. Use StringBuilderPool like
BuildTestId already does. Reentrant-safe for nested enumerables.

(The remaining int.ToString() allocations the reviewer noted are left as-is
per their guidance — the stackalloc workaround adds complexity for a marginal
win.)
---
 TUnit.Core/Services/TestNameFormatter.cs | 32 ++++++++++++++++--------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/TUnit.Core/Services/TestNameFormatter.cs b/TUnit.Core/Services/TestNameFormatter.cs
index 5427cb85c6..3c6f108090 100644
--- a/TUnit.Core/Services/TestNameFormatter.cs
+++ b/TUnit.Core/Services/TestNameFormatter.cs
@@ -100,20 +100,30 @@ private string FormatArguments(object?[] args)
 
     private string FormatEnumerable(IEnumerable enumerable)
     {
-        var sb = new StringBuilder("[");
-        var first = true;
-
-        foreach (var item in enumerable)
+        // Pool the builder like BuildTestId. Reentrant-safe: a nested enumerable draws a
+        // distinct instance from the pool, and each Get is balanced by a Return.
+        var sb = StringBuilderPool.Get();
+        try
         {
-            if (!first)
+            sb.Append('[');
+            var first = true;
+
+            foreach (var item in enumerable)
             {
-                sb.Append(", ");
+                if (!first)
+                {
+                    sb.Append(", ");
+                }
+                first = false;
+                sb.Append(FormatArgumentValue(item));
             }
-            first = false;
-            sb.Append(FormatArgumentValue(item));
-        }
 
-        sb.Append(']');
-        return sb.ToString();
+            sb.Append(']');
+            return sb.ToString();
+        }
+        finally
+        {
+            StringBuilderPool.Return(sb);
+        }
     }
 }