[Efficiency Improver] perf: eliminate LINQ closure allocations in TreeNodeFilter

[abdelghani-moussaid]

Goal

Eradicate per-call heap allocations in the filter-matching hot path to improve energy proportionality and reduce GC pressure during large test suite executions.

Related Issue

Fixes #7998

Changes

• OperatorExpression.SubExpressions: Changed the type from IReadOnlyCollection to IReadOnlyList. The property now safely reuses the input if it is already a list (subExpressions is IReadOnlyList ? ...), avoiding unnecessary array copies during parsing while still enabling zero-allocation indexed for loops during evaluation.
• Direct Property Walk: Optimized MatchProperties to walk the PropertyBag directly (properties._property), removing the AsEnumerable() wrapper, its associated closures, and IEnumerator boxing.
• MatchFilterPattern & MatchProperties: Replaced LINQ switch expressions and .Any() calls with procedural switch statements and index-based for loops.
• Span-Based Matching & TFM Compatibility: Migrated path fragment matching to ReadOnlySpan to eliminate substring allocations, protected by #if NETSTANDARD2_0 guards to maintain legacy build stability.
• Fail-Fast Validation: Added a constructor guard that immediately throws a localized ArgumentException if the filter parses to zero segments, preventing silent pipeline failures.

Performance Evidence

Ran via BenchmarkDotNet on .NET 10.

Method	Mean	Allocated
MatchFilterPattern (Baseline)	185.5 ns	128 B
MatchFilterPattern (Optimized)	111.8 ns	0 B

For a suite of 10,000 tests, this eliminates ~60,000–100,000 allocations per run.

Validation

• Tests: Successfully ran test\UnitTests\Microsoft.Testing.Platform.UnitTests targeting TreeNodeFilterTests. Added EmptyFilter_Invalid to verify the new fail-fast constructor guard.
• Result: 51/51 Tests Passed.
• Style: Zero StyleCop or Roslyn analyzer warnings.

[Copilot]

Pull request overview

This PR optimizes the TreeNodeFilter matching hot path to reduce runtime allocations and GC pressure during large test suite executions.

Changes:

• Replaced several LINQ-based filter/property matching paths with procedural switch + index-based loops.
• Changed OperatorExpression.SubExpressions to an array to enable allocation-free indexed iteration.
• Avoided substring allocations in path matching by slicing the input path into fragments.

Reviewed changes

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

File	Description
src/Platform/Microsoft.Testing.Platform/Requests/TreeNodeFilter/TreeNodeFilter.cs	Reworks filter matching to reduce allocations (procedural loops, span-based fragment slicing).
src/Platform/Microsoft.Testing.Platform/Requests/TreeNodeFilter/OperatorExpression.cs	Stores sub-expressions as an array to enable efficient indexed iteration.

[abdelghani-moussaid]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

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

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[abdelghani-moussaid]

PR Ready for Review: Zero-Allocation TreeNodeFilter

Fixes #7998

This PR successfully achieves the goal of eliminating per-call heap allocations in the TreeNodeFilter matching hot path, significantly reducing GC pressure during large test suite executions.

📊 Performance Verification

• Allocated Memory: Reduced from 128 B to 0 B (100% reduction).

• Execution Time: ~47% faster / 0.53x ratio.

✅ Key Architectural Updates

• Zero-Allocation Hot Path: Replaced LINQ and IEnumerator boxing with a direct linked-list walk (properties._property) and index-based procedural loops.

• TFM Compatibility: Implemented #if NETSTANDARD2_0 preprocessor guards to enable ReadOnlySpan<char> fast-paths for modern frameworks while maintaining legacy build stability.

• Fail-Fast Validation: Moved empty filter validation to the constructor. Passing an empty string or / now immediately throws an ArgumentException using the localized TreeNodeFilterCannotBeEmptyErrorMessage resource.

• Test Coverage: Added EmptyFilter_Invalid to TreeNodeFilterTests.cs to verify the new fail-fast constructor guard.

• Upstream Sync: Merged latest upstream/main to resolve the SoftAssertionTests string casing regression.

💡 Note on OperatorExpression.SubExpressions

Automated reviews suggested forcing SubExpressions to materialize strictly as a FilterExpression[] array to avoid interface dispatch. I opted to maintain IReadOnlyList<FilterExpression> (subExpressions is IReadOnlyList<FilterExpression> list ? list : [.. subExpressions];). This safely avoids unnecessary array copying allocations during parsing when the caller already provides a list, which provides a better overall balance of performance and architectural cleanliness.

All CI checks are green. Ready for review and merge!