[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

Latest data from benchmark PR #10885 after porting the current #10976 hybrid analyzer changes (including generated body invocation edges and base-preserved reachability).

Benchmark output root: /tmp/typespec-final-hybrid-bench-20260624-0943.

Full-generation BenchmarkDotNet, averaged across 3 local runs:

Mode	Avg Mean	Avg Allocated
Roslyn reference maps	895.8 ms	67.78 MB
Provider reference map	859.9 ms	57.75 MB

Approximate full-generation improvement:

Time:       ~4.0% faster
Allocation: ~14.8% less

Focused profile data, using 72 Roslyn-mode and 72 provider-mode invocations from the same runs (median):

Path	Median Time	Median Allocated
Roslyn reference-map construction	357.2 ms	23.70 MB
Provider map analysis + candidate consumption	263.2 ms	17.83 MB
Provider candidate consumption only	0.79 ms	0.06 MB

Approximate focused reference-map improvement:

Time:       ~26.3% faster
Allocation: ~24.8% less

Profile notes:

• Data comes from POSTPROCESSING_BENCHMARK_PROFILE_STEPS=true on the full-generation benchmark so both 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.Internalize.UseShadowPublicizeCandidates, 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
• current union/variant roots participate in internalization, matching Roslyn _typesToKeep behavior
• remove reachability includes current discriminator derived-model edges after root discovery
• previous generated files under src/Generated are not scanned; reachability is based on current providers/generated workspace plus custom/API roots, matching Roslyn's workspace inputs
• internal/non-public discriminator constructor/property references do not make discriminator enum types public

Azure SDK DPG Regen Note

Azure/azure-sdk-for-net#60128 completed a full DPG regen with no sdk/**/api/** changes.

The regen has a few extra generated internal model/serialization files. This is expected: the hybrid provider map keeps conservative client body dependencies from service metadata to avoid broad Roslyn body-reference scans and preserve correctness for body-only generated references. These files are internal implementation details and do not affect public API.

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

Latest Safe Provider-Map Optimization (2026-06-24)

Pushed safe optimization commits:

• Production PR: 88bea5758
• Benchmark PR: 7b3c0f1e5

Changes:

• Removed the global Roslyn SymbolFinder.FindReferencesAsync helper lookup from generated-body dependency discovery.
• Kept correctness by scanning current generated body syntax for both type references and invocation containing types, even when provider body dependencies are available.
• Reused the flattened generated-provider list during graph construction/base-preserved dependency traversal.
• Skipped XML doc parsing unless the doc contains a type cref marker.

Safe two-pass focused profile aggregate (/tmp/typespec-provider-map-safe-final-20260624-1320, /tmp/typespec-provider-map-safe-final2-20260624-1320) compared with the previous hotspot baseline (/tmp/typespec-provider-map-hotspots-20260624-1320):

Path	Before Median	After Median	Delta
Provider map analysis + candidate consumption	231.5 ms, 18.03 MB	188.8 ms, 7.89 MB	~18.4% faster, ~56.2% less allocation
Provider analysis only	231.3 ms, 17.98 MB	188.0 ms, 7.83 MB	~18.7% faster, ~56.4% less allocation

Validation after the safe correction:

• npm run build:generator
• pwsh ./eng/scripts/Generate.ps1 -filter Sample-TypeSpec with no checked-in generated diffs

Latest 3-Run Benchmark Data (2026-06-24 14:03 UTC)

Benchmark branch: mtg-manual-name-reduction-experiment at 7b3c0f1e5. Artifacts: /tmp/typespec-latest-safe-bench-20260624-1403/run-{1,2,3}.

Full-generation BenchmarkDotNet

Mode	Run means	Avg mean	Avg allocated
Roslyn reference maps	1295.0, 1029.8, 1633.4 ms	1319.4 ms	67.52 MB
Provider reference map	1008.0, 785.1, 795.0 ms	862.7 ms	47.95 MB

Approximate full-generation improvement:

Time:       ~34.6% faster
Allocation: ~29.0% less

Focused reference-map profile

Profile data uses 71 Roslyn-mode and 71 provider-mode invocations from the same three runs (median):

Path	Median Time	Median Allocated
Roslyn reference-map construction	429.7 ms	23.65 MB
Provider map analysis + candidate consumption	198.7 ms	7.87 MB
Provider analysis only	197.0 ms	7.81 MB
Provider candidate consumption only	0.98 ms	0.06 MB

Approximate focused reference-map replacement improvement:

Time:       ~53.8% faster
Allocation: ~66.7% less

Latest Helper-Fix Benchmark Data (2026-06-24 14:36 UTC)

Benchmark branch: mtg-manual-name-reduction-experiment at b14b1f282. Artifacts: /tmp/typespec-helper-fix-bench-20260624-1436/run-{1,2,3}. This includes the CI helper-root fix that explicitly preserves generated ClientUriBuilder, ClientPipelineExtensions, and CancellationTokenExtensions without restoring the broad Roslyn SymbolFinder helper lookup.

Full-generation BenchmarkDotNet

Mode	Run means	Avg mean	Avg allocated
Roslyn reference maps	1025.7, 881.6, 1007.9 ms	971.7 ms	67.64 MB
Provider reference map	809.8, 693.8, 842.7 ms	782.1 ms	48.12 MB

Approximate full-generation improvement:

Time:       ~19.5% faster
Allocation: ~28.9% less

Focused reference-map profile

Profile data uses 72 Roslyn-mode and 72 provider-mode invocations from the same three runs (median):

Path	Median Time	Median Allocated
Roslyn reference-map construction	349.0 ms	23.57 MB
Provider map analysis + candidate consumption	172.6 ms	7.88 MB
Provider analysis only	171.3 ms	7.82 MB
Provider candidate consumption only	0.80 ms	0.06 MB

Approximate focused reference-map replacement improvement:

Time:       ~50.6% faster
Allocation: ~66.6% less

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