Avoid allocation when parsing whitespace-free UTF-8 X-format Guids

[EgorBo]

Fix an inverted whitespace check in UTF-8 X-format Guid parsing. The initial scan stops at the first non-whitespace rune, causing whitespace-free input to allocate and copy a temporary byte array.

Stop at the first whitespace rune instead, enabling the existing return-original-span path. For the canonical X-format input, allocations drop from 96 bytes to zero per parse.

using System;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser]
public class Bench {
    [Benchmark]
    public Guid Parse() =>
        Guid.Parse("{0x00112233,0x4455,0x6677,{0x88,0x99,0xaa,0xbb,0xcc,0xdd,0xee,0xff}}"u8);
}

Method	Toolchain	Mean	Error	Ratio	Gen0	Allocated	Alloc Ratio
Parse	main	157.9 ns	0.20 ns	1.36	0.0114	96 B	NA
Parse	PR #135272	116.2 ns	0.45 ns	1.00	-	-	NA

[EgorBo]

@EgorBot -arm

using System;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser]
public class Bench {
    [Benchmark]
    public Guid Parse() =>
        Guid.Parse("{0x00112233,0x4455,0x6677,{0x88,0x99,0xaa,0xbb,0xcc,0xdd,0xee,0xff}}"u8);
}

Note

Benchmark comment generated with GitHub Copilot.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused correction matches the helper’s intended behavior, with existing tests covering UTF-8 X-format inputs both with and without whitespace.

Review effort: Balanced
Findings: None

What changed in this PR

Corrects UTF-8 X-format GUID whitespace detection, avoiding an unnecessary allocation for canonical inputs.

Changes:

• Stops scanning at the first whitespace rune.
• Preserves the original span when no whitespace exists.

File	Description
src/​libraries/​System.Private.CoreLib/​src/​System/​Guid.cs	Fixes the inverted whitespace condition.

[EgorBo]

PTAL @MihaZupan

[MihaZupan]

Nice catch

[EgorBo]

/ba-g #135283