[Draft] Use hybrid C# reference map for post-processing

[live1206]

Summary

Adds a hybrid reference-map replacement for C# generated-code post-processing.

The hybrid path replaces broad Roslyn reference-map construction with:

• provider metadata for generated-code references
• explicit provider dependencies for known generated body-only references
• a small Roslyn scan only for custom/shared code roots

No benchmark measurement/profiling code is included in the production generator path in this PR.

Why

The earlier experimental PR measured full generation and identified Roslyn reference-map construction inside GeneratedCodeWorkspace.PostProcessAsync() as the largest hotspot.

This PR keeps generated output parity with the Roslyn cleanup path while moving generated-code reachability to provider metadata.

Latest Benchmark Data

Focused profile data from the benchmark PR, measuring reference-map construction/replacement rather than total full-generation wall time:

Path	Avg Time	Avg Allocated
Roslyn reference maps	405.8 ms	22.64 MB
Provider reference map replacement	226.4 ms	12.85 MB

Approximate focused improvement:

Time:       ~44.2% faster
Allocation: ~43.2% less

Profile breakdown:

Step	Avg Time	Avg Allocated
Roslyn public reference map	172.6 ms	11.43 MB
Roslyn all reference map	233.3 ms	11.21 MB
Provider map analysis	225.9 ms	12.81 MB
Provider candidate consumption	~0.55 ms	~38 KB

Profile notes:

• Data comes from POSTPROCESSING_BENCHMARK_PROFILE_STEPS=true on the full-generation benchmark so both Roslyn and provider-map paths run against real TypeProvider output.
• Roslyn rows are PostProcessor.Internalize.BuildPublicReferenceMapAsync and PostProcessor.Remove.BuildAllReferenceMapAsync.
• Provider rows are Generation.ProviderReferenceMapShadowAnalysis, PostProcessor.Internalize.UseShadowCandidates, PostProcessor.Remove.UseShadowCandidates, and PostProcessor.Remove.BuildShadowReferencedSet.
• Runtime: .NET 10.0.9, Ubuntu 26.04, AMD EPYC 7763.

Correctness Notes

The hybrid implementation preserves Roslyn cleanup behavior for generated output parity:

• model factory signatures and bodies do not keep otherwise-unused models alive
• MRW context/buildable attributes do not keep buildable-only models alive
• serialization providers are removable together with their owning model
• retained serialization providers report explicit helper dependencies such as ChangeTrackingDictionary and Optional
• collection-result providers report explicit body dependencies instead of relying on Roslyn body scanning
• client providers report explicit body dependencies for collection results, service method types, operation parameters, and operation response body/header types
• rest-client providers report explicit helper dependencies for generated collection parameter null checks, including ChangeTrackingList and ChangeTrackingDictionary
• generated body-only references are still handled for static helpers
• public discriminator subtypes stay public by matching Roslyn public-reference-map derived-class behavior
• internal/non-public discriminator constructor/property references do not make discriminator enum types public

Validation

Local validation performed while stabilizing the PR included:

• dotnet build packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Microsoft.TypeSpec.Generator.csproj -c Release
• focused generator tests covering post-processing/workspace/customization scenarios
• full generator test assembly: 1530/1530 passed
• regenerated and built previously failing generated projects/scenarios, including:
  • Sample-TypeSpec
  • Spector/http/authentication/api-key
  • Spector/http/parameters/collection-format
  • Spector/http/documentation
  • Spector/http/special-headers/repeatability
  • discriminator inheritance scenarios used by Spector tests

Implemented Generated Dependency Handling

This PR now avoids Roslyn body scanning for several generated cases:

• collection-result body dependencies are reported by CollectionResultDefinition
• client body dependencies are reported by ClientProvider
• rest-client helper dependencies are reported by RestClientProvider
• serialization provider helper dependencies are reported by serialization providers
• model factory is treated specially so unreachable model factory methods do not root models
• non-root MRW context/buildable attributes are excluded from model reachability

Custom/shared code references still use Roslyn because arbitrary user C# can reference generated types in ways providers cannot reliably describe.

Follow-Up Performance Opportunities

Potential next improvements, ordered by reward/risk:

Rank	Improvement	Reward	Risk	Notes
1	Precompute name lookup maps for AddMatchingName	Medium	Low	Avoid repeated full-node scans for helper/root matching.
2	Cache flattened provider lists and provider names	Medium	Low	Avoid repeated lazy provider/name materialization during analysis.
3	Conservative custom-code syntax prefilter	Medium	Medium/High	Can reduce custom Roslyn semantic work but must not miss arbitrary custom references.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10976

commit: dc6f0f2

[github-actions]

No changes needing a change description found.