Optimize: Reduce allocations in source generator hot paths

[DJGosnell]

Summary

• Closes Performance: Reduce allocations and redundant work in source generator hot paths #25

Reduces allocations and redundant work across 14 files in the source generator's hot paths. With ~3,000 interceptors in the test project, these optimizations target per-site and per-interceptor overhead: single-pass collection partitioning, a BuilderKind enum replacing scattered string.Contains() checks, pre-grouped field lookups, StringBuilder for in-loop string mutations, mutable translation context, ConditionalWeakTable-backed ToDisplayString caching, and deferred dictionary updates.

Reason for Change

The generator produces correct output but has accumulated allocation-heavy patterns that compound across ~3,000 interceptor sites — triple-filtering collections, duplicating partitions, allocating intermediate strings in loops, copying dictionaries on every join, and performing repeated string.Contains() type classification across 5 files.

Impact

• Finding 1 (HIGH): Single-pass loop replaces 3 sequential .Where().ToList() calls for site classification
• Finding 2 (HIGH): Single-pass scalar/collection parameter split with pre-allocated lists
• Finding 3 (HIGH): StringBuilder.Replace for SQL placeholder and raw SQL parameter substitution loops
• Finding 4 (HIGH): Combined GetConstantValue + GetTypeInfo into single flow, eliminating redundant semantic model query per non-constant captured expression
• Finding 5 (MEDIUM): Pre-grouped Dictionary<string, List<CachedExtractorField>> eliminates per-interceptor O(N) linear scans
• Finding 6 (MEDIUM): BuilderKind enum replaces ~15 scattered string.Contains() checks across 5 files with single classification at site creation
• Finding 7 (MEDIUM): WithJoinedEntity() mutates dictionary in place — callers reassign immediately so immutability was unnecessary
• Finding 8 (MEDIUM): SymbolDisplayCache using ConditionalWeakTable<ITypeSymbol, string> caches display strings across ~3,000 invocations for ~20 unique entity types
• Finding 9 (LOW): ToArray() instead of ToList() for join entity type collections that are only iterated
• Finding 10 (LOW): Deferred update pattern — iterate dictionary directly, collect pending updates, apply after loop
• Finding 11 (LOW): Single-pass StringBuilder escaping; shared EscapeForCSharpString helper

Migration Steps

None — internal generator changes only.

Performance Considerations

All changes reduce allocations and CPU work. The ConditionalWeakTable cache replaces far more string allocations than it introduces. The BuilderKind enum adds one byte per UsageSiteInfo but eliminates multiple string scans downstream.

Security Considerations

None.

Breaking Changes

• Consumer-facing: None
• Internal:
  • BuilderKind enum and property added to UsageSiteInfo
  • staticFields parameter changed from List to pre-filtered List? in GenerateWhereInterceptor, GenerateJoinedWhereInterceptor, GenerateModificationWhereInterceptor
  • ExpressionTranslationContext.WithJoinedEntity() now mutates in place instead of returning a copy
  • TryInlineConstant removed (inlined into TranslateCapturedValue with TryInlineConstantFromCompilation fallback)
  • SymbolDisplayCache utility added; Analyzers project now links Utilities/ folder

🤖 Generated with Claude Code