Improve some string Rune helpers

[MihaZupan]

Method	Toolchain	Mean	Error	Ratio
ContainsBmp	\main\corerun.exe	11.018 ns	0.0416 ns	1.00
ContainsBmp	\pr\corerun.exe	5.335 ns	0.0090 ns	0.48
ContainsSupplementary	\main\corerun.exe	14.186 ns	0.0586 ns	1.00
ContainsSupplementary	\pr\corerun.exe	15.075 ns	0.0278 ns	1.06
StartsWithBmp	\main\corerun.exe	5.637 ns	0.0063 ns	1.00
StartsWithBmp	\pr\corerun.exe	2.193 ns	0.0021 ns	0.39
StartsWithSupplementary	\main\corerun.exe	9.371 ns	0.0070 ns	1.00
StartsWithSupplementary	\pr\corerun.exe	2.822 ns	0.0041 ns	0.30
EndsWithBmp	\main\corerun.exe	5.641 ns	0.0076 ns	1.00
EndsWithBmp	\pr\corerun.exe	2.505 ns	0.0022 ns	0.44
EndsWithSupplementary	\main\corerun.exe	9.400 ns	0.0273 ns	1.00
EndsWithSupplementary	\pr\corerun.exe	3.135 ns	0.0031 ns	0.33
ReplaceBothBmp	\main\corerun.exe	59.146 ns	0.6353 ns	1.00
ReplaceBothBmp	\pr\corerun.exe	29.462 ns	0.3781 ns	0.50
ReplaceSupplementary	\main\corerun.exe	62.393 ns	0.5636 ns	1.00
ReplaceSupplementary	\pr\corerun.exe	61.546 ns	0.5907 ns	0.99
SplitBmp	\main\corerun.exe	51.507 ns	0.8753 ns	1.00
SplitBmp	\pr\corerun.exe	51.466 ns	1.0191 ns	1.00
SplitSupplementary	\main\corerun.exe	49.510 ns	0.9627 ns	1.00
SplitSupplementary	\pr\corerun.exe	48.681 ns	0.4651 ns	0.98
TrimBmp	\main\corerun.exe	35.152 ns	0.1661 ns	1.00
TrimBmp	\pr\corerun.exe	14.858 ns	0.1258 ns	0.42
TrimSupplementary	\main\corerun.exe	23.888 ns	0.1723 ns	1.00
TrimSupplementary	\pr\corerun.exe	13.587 ns	0.1036 ns	0.57
TrimStartBmp	\main\corerun.exe	17.435 ns	0.1468 ns	1.00
TrimStartBmp	\pr\corerun.exe	12.658 ns	0.0918 ns	0.73
TrimStartSupplementary	\main\corerun.exe	17.189 ns	0.1117 ns	1.00
TrimStartSupplementary	\pr\corerun.exe	12.552 ns	0.0952 ns	0.73
TrimEndBmp	\main\corerun.exe	24.816 ns	0.1445 ns	1.00
TrimEndBmp	\pr\corerun.exe	11.842 ns	0.0948 ns	0.48
TrimEndSupplementary	\main\corerun.exe	19.257 ns	0.1592 ns	1.00
TrimEndSupplementary	\pr\corerun.exe	11.465 ns	0.1043 ns	0.60

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[MihaZupan]

@EgorBot

using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
 
BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser]
public class Bench
{
   private string _testString = default!;
   private string _testStringSupplementary = default!;
   private Rune _bmpRune;
   private Rune _supplementaryRune;
   private Rune _bmpReplacementRune;
   private Rune _supplementaryReplacementRune;

   [GlobalSetup]
   public void Setup()
   {
       _bmpRune = new Rune('a');
       _supplementaryRune = new Rune(0x1F600); // 😀
       _bmpReplacementRune = new Rune('b');
       _supplementaryReplacementRune = new Rune(0x1F601); // 😁

       _testString = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxa";
       _testStringSupplementary = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\U0001F600";
   }

   [Benchmark]
   public bool ContainsBmp() => _testString.Contains(_bmpRune);

   [Benchmark]
   public bool ContainsSupplementary() => _testStringSupplementary.Contains(_supplementaryRune);

   [Benchmark]
   public bool StartsWithBmp() => _testString.StartsWith(_bmpRune);

   [Benchmark]
   public bool StartsWithSupplementary() => _testStringSupplementary.StartsWith(_supplementaryRune);

   [Benchmark]
   public bool EndsWithBmp() => _testString.EndsWith(_bmpRune);

   [Benchmark]
   public bool EndsWithSupplementary() => _testStringSupplementary.EndsWith(_supplementaryRune);

   [Benchmark]
   public string ReplaceBothBmp() => _testString.Replace(_bmpRune, _bmpReplacementRune);

   [Benchmark]
   public string ReplaceSupplementary() => _testStringSupplementary.Replace(_supplementaryRune, _supplementaryReplacementRune);

   [Benchmark]
   public string[] SplitBmp() => _testString.Split(_bmpRune);

   [Benchmark]
   public string[] SplitSupplementary() => _testStringSupplementary.Split(_supplementaryRune);

   [Benchmark]
   public string TrimBmp() => "aaahello worldaaa".Trim(_bmpRune);

   [Benchmark]
   public string TrimSupplementary() => "\U0001F600hello world\U0001F600".Trim(_supplementaryRune);

   [Benchmark]
   public string TrimStartBmp() => "aaahello world".TrimStart(_bmpRune);

   [Benchmark]
   public string TrimStartSupplementary() => "\U0001F600hello world".TrimStart(_supplementaryRune);

   [Benchmark]
   public string TrimEndBmp() => "hello worldaaa".TrimEnd(_bmpRune);

   [Benchmark]
   public string TrimEndSupplementary() => "hello world\U0001F600".TrimEnd(_supplementaryRune);
}

[Copilot]

Pull request overview

This PR updates several string overloads that accept Rune to take faster paths for BMP runes and to simplify ordinal StartsWith / EndsWith checks.

Changes:

• Special-cases BMP Rune inputs to delegate to existing char overloads for Contains, Replace, and Split.
• Implements ordinal StartsWith(Rune) / EndsWith(Rune) via direct UTF-16 surrogate comparisons for non-BMP runes.
• Refactors Trim(Rune) / TrimStart(Rune) / TrimEnd(Rune) to use TrimHelper with a pinned UTF-16 span.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/String.Searching.cs	Adds BMP fast-path for Contains(Rune) by forwarding to Contains(char).
src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs	Adds BMP fast-paths for Replace / Split and refactors Trim* (Rune) implementations to call TrimHelper.
src/libraries/System.Private.CoreLib/src/System/String.Comparison.cs	Adds BMP fast-paths and direct surrogate-pair comparisons for ordinal StartsWith(Rune) / EndsWith(Rune).

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.