From 0c1f89598c64a80eda69ba6eb22be1721aa3abd4 Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Thu, 28 May 2026 21:05:44 +0100
Subject: [PATCH] perf: drop LINQ join and bool double-alloc in
 TestNameFormatter

FormatArguments built a temporary string[] via Select and materialized it
through string.Join's IEnumerator; replace with a direct loop into a pooled
StringBuilder, avoiding the iterator, closure and intermediate array.

FormatArgumentValue formatted bool via b.ToString().ToLowerInvariant(), which
allocates twice; use the interned 'true'/'false' literals instead. Output is
byte-identical (lowercase) in both cases.

Closes #6030
Closes #6031
---
 TUnit.Core/Services/TestNameFormatter.cs | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/TUnit.Core/Services/TestNameFormatter.cs b/TUnit.Core/Services/TestNameFormatter.cs
index 3c6f108090..83dc031630 100644
--- a/TUnit.Core/Services/TestNameFormatter.cs
+++ b/TUnit.Core/Services/TestNameFormatter.cs
@@ -51,7 +51,7 @@ public string FormatArgumentValue(object? value)
             null => "null",
             string str => $"\"{str}\"",
             char ch => $"'{ch}'",
-            bool b => b.ToString().ToLowerInvariant(),
+            bool b => b ? "true" : "false",
             // Use InvariantCulture for numeric types to avoid culture-specific formatting issues
             double d => d.ToString(System.Globalization.CultureInfo.InvariantCulture),
             float f => f.ToString(System.Globalization.CultureInfo.InvariantCulture),
@@ -95,7 +95,26 @@ private string FormatArguments(object?[] args)
             return string.Empty;
         }
 
-        return string.Join(", ", args.Select(FormatArgumentValue));
+        // Build directly into a pooled StringBuilder to avoid the LINQ iterator/closure
+        // and the temporary string[] that Select + string.Join would materialize.
+        var builder = StringBuilderPool.Get();
+        try
+        {
+            for (var i = 0; i < args.Length; i++)
+            {
+                if (i > 0)
+                {
+                    builder.Append(", ");
+                }
+                builder.Append(FormatArgumentValue(args[i]));
+            }
+
+            return builder.ToString();
+        }
+        finally
+        {
+            StringBuilderPool.Return(builder);
+        }
     }
 
     private string FormatEnumerable(IEnumerable enumerable)