Reduce allocations for cached command lookups

[Przemysław Kłys (PrzemyslawKlys)]

PR Summary

Repeated command lookups allocate a closure, a Lazy<CommandInfo>, and an uppercased key even when the result is already cached. Return the existing lazy value first, move miss-only allocations into a helper, and use StringComparer.OrdinalIgnoreCase for hashing to match key equality.

Command discovery, command-type filters, bypass behavior, and the winning lazy value on concurrent misses are unchanged. Tests cover cached identity across casing and equivalent command types, separate type filters, and culture-independent lookup.

With command names extracted from Locksmith and Locksmith2, cached-lookup medians fall by 55–61% across two CPU cache domains (two warmups and seven rotated samples per lane). The measured C# lookup loop allocates zero bytes after the change, versus 203 MB / 333 MB before it for 890,000 / 1,400,000 lookups. These are cached-lookup results, not whole-formatting or build speedups. Baseline: 411c3d0b197b488285bddd2375b292ab7b8db5e8.

Validation: net8 and net462 builds; 41 focused tests pass with one skipped on each PowerShell Core and Windows PowerShell. Full formatting of both modules produces identical bytes and diagnostics before/after on each runtime. Locksmith2 retains an existing attribute-construction diagnostic in this isolated test.

Related performance discussion: #1528.

PR Checklist

• PR has a meaningful title
• Summarized changes
• Change is not breaking
• C#, PowerShell script and module files have the correct copyright header
• Added tests for the changed contracts
• This PR is ready for review and is not Work in Progress

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused optimization preserves existing semantics and is adequately covered by targeted tests.

Review effort: Balanced
Findings: None

What changed in this PR

Optimizes cached command lookups while preserving lookup behavior and thread safety.

Changes:

• Avoids miss-only allocations on cache hits.
• Uses ordinal case-insensitive hashing.
• Adds cache identity, filtering, and culture tests.

File	Description
Engine/​CommandInfoCache.cs	Adds allocation-free cache-hit path and matching hash semantics.
Tests/​Engine/​CommandInfoCache.tests.ps1	Tests casing, command types, and culture independence.

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[Jesse Houwing (jessehouwing)]

Fixed in #2206 with more significant improvements.

Verdict: PR #2221’s implementation is already covered in #2206. The two substantive optimizations are present:

• CommandInfoCache.cs:121-149 checks the cache before creating the Lazy factory. Cached hits therefore avoid the closure and Lazy allocations; concurrent misses still share the winning entry.
• CommandLookupKey.cs:24-37 keeps the original name and hashes with StringComparer.OrdinalIgnoreCase, avoiding the uppercase-string allocation while matching the key’s equality comparison.

[Przemysław Kłys (PrzemyslawKlys)]

Thanks for pointing this out. I checked #2206 and confirmed that it already includes both the cache-hit allocation change and ordinal case-insensitive hashing. Closing this PR as superseded by #2206.

[Jesse Houwing (jessehouwing)]

I did steal your additional tests! Thanks! Would be helpful if you can run the code from that PR and report your findings.