Rewrite StringExtensions.ToString(IEnumerable<Rune>)

Appends rune chars to StringBuilder avoiding intermediate string allocation for each rune append.

Benchmarks/Text/StringExtensions/ToStringEnumerable.cs

@@ -0,0 +1,70 @@
+using System.Text;
+using BenchmarkDotNet.Attributes;
+using Tui = Terminal.Gui;
+
+namespace Terminal.Gui.Benchmarks.Text.StringExtensions;
+
+/// <summary>
+/// Benchmarks for <see cref="Tui.StringExtensions.ToString(IEnumerable{Rune})"/> performance fine-tuning.
+/// </summary>
+[MemoryDiagnoser]
+public class ToStringEnumerable
+{
+
+    /// <summary>
+    /// Benchmark for previous implementation.
+    /// </summary>
+    [Benchmark]
+    [ArgumentsSource (nameof (DataSource))]
+    public string Previous (IEnumerable<Rune> runes, int size)
+    {
+        return StringAppendInLoop (runes);
+    }
+
+    /// <summary>
+    /// Benchmark for current implementation with rune chars appending to StringBuilder.
+    /// </summary>
+    /// <param name="runes"></param>
+    /// <returns></returns>
+    [Benchmark (Baseline = true)]
+    [ArgumentsSource (nameof (DataSource))]
+    public string Current (IEnumerable<Rune> runes, int size)
+    {
+        return Tui.StringExtensions.ToString (runes);
+    }
+
+    /// <summary>
+    /// Previous implementation with string append in a loop.
+    /// </summary>
+    private static string StringAppendInLoop (IEnumerable<Rune> runes)
+    {
+        var str = string.Empty;
+
+        foreach (Rune rune in runes)
+        {
+            str += rune.ToString ();
+        }
+
+        return str;
+    }
+
+    public IEnumerable<object []> DataSource ()
+    {
+        string textSource =
+            """
+			Ĺόŕéḿ íṕśúḿ d́όĺόŕ śít́ áḿét́, ćόńśéćt́ét́úŕ ád́íṕíśćíńǵ éĺít́. Ṕŕáéśéńt́ q́úíś ĺúćt́úś éĺít́. Íńt́éǵéŕ út́ áŕćú éǵét́ d́όĺόŕ śćéĺéŕíśq́úé ḿát́t́íś áć ét́ d́íáḿ.
+			Ṕéĺĺéńt́éśq́úé śéd́ d́áṕíb́úś ḿáśśá, v́éĺ t́ŕíśt́íq́úé d́úí. Śéd́ v́ít́áé ńéq́úé éú v́éĺít́ όŕńáŕé áĺíq́úét́. Út́ q́úíś όŕćí t́éḿṕόŕ, t́éḿṕόŕ t́úŕṕíś íd́, t́éḿṕúś ńéq́úé.
+			Ṕŕáéśéńt́ śáṕíéń t́úŕṕíś, όŕńáŕé v́éĺ ḿáúŕíś át́, v́áŕíúś śúśćíṕít́ áńt́é. Út́ ṕúĺv́íńáŕ t́úŕṕíś ḿáśśá, q́úíś ćúŕśúś áŕćú f́áúćíb́úś íń.
+			Óŕćí v́áŕíúś ńát́όq́úé ṕéńát́íb́úś ét́ ḿáǵńíś d́íś ṕáŕt́úŕíéńt́ ḿόńt́éś, ńáśćét́úŕ ŕíd́íćúĺúś ḿúś. F́úśćé át́ éx́ b́ĺáńd́ít́, ćόńv́áĺĺíś q́úáḿ ét́, v́úĺṕút́át́é ĺáćúś.
+			Śúśṕéńd́íśśé śít́ áḿét́ áŕćú út́ áŕćú f́áúćíb́úś v́áŕíúś. V́ív́áḿúś śít́ áḿét́ ḿáx́íḿúś d́íáḿ. Ńáḿ éx́ ĺéό, ṕh́áŕét́ŕá éú ĺόb́όŕt́íś át́, t́ŕíśt́íq́úé út́ f́éĺíś.
+			""";
+
+        // Extra argument as workaround for the summary grouping different length collections to same baseline making comparison difficult.
+        int[] sizes = [1, 10, 100, textSource.Length / 2, textSource.Length];
+
+        foreach (int size in sizes)
+        {
+            yield return [textSource.EnumerateRunes ().Take (size).ToArray (), size];
+        }
+    }
+}

Benchmarks/Text/TextFormatter/StripCRLF.cs

@@ -77,7 +77,7 @@ public class StripCRLF
             }
         }
 
-        return StringExtensions.ToString (runes);
+        return Tui.StringExtensions.ToString (runes);
     }
 
     public IEnumerable<object []> DataSource ()

Terminal.Gui/Text/StringExtensions.cs

@@ -124,14 +124,16 @@ public static class StringExtensions
     /// <returns></returns>
     public static string ToString (IEnumerable<Rune> runes)
     {
-        var str = string.Empty;
-
+        StringBuilder stringBuilder = new();
+        const int maxCharsPerRune = 2;
+        Span<char> charBuffer = stackalloc char[maxCharsPerRune];
         foreach (Rune rune in runes)
         {
-            str += rune.ToString ();
+            int charsWritten = rune.EncodeToUtf16 (charBuffer);
+            ReadOnlySpan<char> runeChars = charBuffer [..charsWritten];
+            stringBuilder.Append (runeChars);
         }
-
-        return str;
+        return stringBuilder.ToString ();
     }
 
     /// <summary>Converts a byte generic collection into a string in the provided encoding (default is UTF8)</summary>