From 3819819d27f25864b5ccc97eb37e523efe627fe5 Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Fri, 11 Sep 2026 16:21:18 +0100
Subject: [PATCH] perf: avoid splitting raw generated source into line strings

---
 src/TUnit.Core.SourceGenerator/CodeWriter.cs  | 63 ++++++++++++-------
 .../CodeWriterTests.cs                        | 52 +++++++++++++++
 2 files changed, 94 insertions(+), 21 deletions(-)
 create mode 100644 tests/TUnit.Core.SourceGenerator.Tests/CodeWriterTests.cs

diff --git a/src/TUnit.Core.SourceGenerator/CodeWriter.cs b/src/TUnit.Core.SourceGenerator/CodeWriter.cs
index f8d77b5bcf2..1bbcb50b7c3 100644
--- a/src/TUnit.Core.SourceGenerator/CodeWriter.cs
+++ b/src/TUnit.Core.SourceGenerator/CodeWriter.cs
@@ -215,35 +215,56 @@ public ICodeWriter AppendRaw(string multilineText)
             return this;
         }
 
-        var lines = multilineText.Split(["\r\n", "\r", "\n"], StringSplitOptions.None);
-
-        // Skip leading empty lines
-        var startIndex = 0;
-        while (startIndex < lines.Length && string.IsNullOrWhiteSpace(lines[startIndex]))
+        var position = 0;
+        var hasContent = false;
+        var pendingBlankLines = 0;
+        while (position < multilineText.Length)
         {
-            startIndex++;
-        }
+            var lineStart = position;
+            while (position < multilineText.Length && multilineText[position] is not '\r' and not '\n')
+            {
+                position++;
+            }
 
-        // Skip trailing empty lines
-        var endIndex = lines.Length - 1;
-        while (endIndex >= startIndex && string.IsNullOrWhiteSpace(lines[endIndex]))
-        {
-            endIndex--;
-        }
+            var lineEnd = position;
+            if (position < multilineText.Length)
+            {
+                var newline = multilineText[position++];
+                if (newline == '\r' && position < multilineText.Length && multilineText[position] == '\n')
+                {
+                    position++;
+                }
+            }
 
-        // Process remaining lines, preserving blank lines within the content
-        for (var i = startIndex; i <= endIndex; i++)
-        {
-            var line = lines[i].TrimEnd();
-            if (line.Length == 0)
+            while (lineEnd > lineStart && char.IsWhiteSpace(multilineText[lineEnd - 1]))
+            {
+                lineEnd--;
+            }
+
+            if (lineEnd == lineStart)
+            {
+                if (hasContent)
+                {
+                    pendingBlankLines++;
+                }
+                continue;
+            }
+
+            // Delay blank lines so leading/trailing ones are omitted, while interior ones survive.
+            while (pendingBlankLines > 0)
             {
-                // Preserve blank lines by forcing a newline even when already at line start
                 _builder.AppendLine();
+                pendingBlankLines--;
             }
-            else
+
+            if (_isNewLine)
             {
-                AppendLine(line);
+                _builder.Append(GetIndentation(_indentLevel));
             }
+            _builder.Append(multilineText, lineStart, lineEnd - lineStart);
+            _builder.AppendLine();
+            _isNewLine = true;
+            hasContent = true;
         }
 
         return this;
diff --git a/tests/TUnit.Core.SourceGenerator.Tests/CodeWriterTests.cs b/tests/TUnit.Core.SourceGenerator.Tests/CodeWriterTests.cs
new file mode 100644
index 00000000000..79e24f5c628
--- /dev/null
+++ b/tests/TUnit.Core.SourceGenerator.Tests/CodeWriterTests.cs
@@ -0,0 +1,52 @@
+namespace TUnit.Core.SourceGenerator.Tests;
+
+public class CodeWriterTests
+{
+    [Test]
+    [Arguments("", "")]
+    [Arguments(" \t\r\n\u00a0\r\u2003\n", "")]
+    [Arguments("first", "    first\n")]
+    [Arguments("first\nsecond", "    first\n    second\n")]
+    [Arguments("first\rsecond", "    first\n    second\n")]
+    [Arguments("first\r\nsecond", "    first\n    second\n")]
+    [Arguments("\r\n \t\n  first \t\r\n\r\n\t\rsecond\n \t\n", "      first\n\n\n    second\n")]
+    [Arguments("first\u00a0\u2003\r\n\u00a0\nsecond\u0085", "    first\n\n    second\n")]
+    [Arguments("first\r\r\nsecond\n\n", "    first\n\n    second\n")]
+    [Arguments("first\u2028middle\nsecond", "    first\u2028middle\n    second\n")]
+    public async Task AppendRaw_PreservesFormatting(string input, string expected)
+    {
+        var writer = new CodeWriter(includeHeader: false);
+        writer.Indent();
+        writer.AppendRaw(input);
+        writer.Append("tail");
+
+        await Assert.That(writer.ToString()).IsEqualTo(expected.Replace("\n", Environment.NewLine) + "    tail");
+    }
+
+    [Test]
+    public async Task AppendRaw_PreservesPartialLineAndSubsequentWrites()
+    {
+        var writer = new CodeWriter(indentString: "--", includeHeader: false);
+        writer.Indent();
+        writer.Append("prefix:");
+        writer.AppendRaw("\n  \r\nfirst \t\r\n\t\n second\r\n\n");
+        writer.AppendRaw("third\n");
+        writer.Append("tail");
+
+        await Assert.That(writer.ToString()).IsEqualTo(
+            "--prefix:first\n\n-- second\n--third\n--tail".Replace("\n", Environment.NewLine));
+    }
+
+    [Test]
+    public async Task AppendRaw_EmptyInputDoesNotEndPartialLine()
+    {
+        var writer = new CodeWriter(includeHeader: false);
+        writer.Append("prefix:");
+        writer.AppendRaw(null!);
+        writer.AppendRaw("");
+        writer.AppendRaw(" \t\r\n\u00a0\n");
+        writer.Append("tail");
+
+        await Assert.That(writer.ToString()).IsEqualTo("prefix:tail");
+    }
+}