[Wei Hu (live1206)]

Summary

Part of #12442

Opt-in compiler-backed C# structural reduction through C9, SDK-shaped cast/interface qualification, both Optional.IsDefined refinements, bounded whole-cref import activation/exception provenance, and branch-local static invocations. Enable with TYPESPEC_PROVIDER_REDUCER=structural; Roslyn remains the default.

Original writer provenance and shared original-compilation facts feed one rewrite traversal and final serialization. Unsupported cases retain their spelling, without a Roslyn fallback, per-edit production compilation or occurrence-acceptance cache.

Applied rules

• Supported type/declaration qualifications, namespace prefixes/aliases and constructed/nested/array/nullable forms; certified static type receivers, field/property this., attribute names/suffixes, optional defaults and grammar-safe parentheses/statement cleanup.
• Retained ((IPersistableModel<T>)this) receivers: recorded interface heads and enclosing SELF-model arguments only, with import witnesses and exact composed-type/parser/source proofs. No cast removal or foreign-model expansion.
• Optional.IsDefined: exact/actual ancestor namespace with no intervening namespace-local import activation and invariant affected statement/binding scope.
• Whole-type crefs: real exception provenance without changing qualified writer text; bare top-level nongeneric type import activation only when exact binding is preserved. The intentional diagnostic delta is limited to disappearance of unsuppressed default-Hidden CS8019 for the normal import genuinely made used. Other diagnostics/reporting/source checks remain.
• Branch-local static invocations: invariant complete method/occurrence in active parses; whole call disabled in inactive parses, with no overlapping active expression/argument/local-function source span. Original closure, outer-argument, source-observation, visibility, token and directive safeguards remain.

The paused serialization-context experiment and deferred imported-cref code-witness prototype are not included. Historical C3, C6 nameof, C7 method-call this and C10 SDK-shaped generic inference remain excluded.

Pre-merge saved AppService evaluation

Same saved input/reference, 2,301 emitted files. Counts are canonical syntax events, not diff lines or promises for other SDKs.

Checkpoint	Residual events
Initial structural slice	165,725
Historical C9	29,336
C9 + SDK-shaped casts	25,726
Both Optional refinements	23,747
Cref policy/provenance	20,718
Bounded branch-static rule	19,463

Latest delta: 1,255 static-receiver prefix removals /596 files—627 JsonDocument,627 JsonSerializer and1 BinaryData.FromString. All other categories unchanged; every changed output independently reconstructed through UTF16 deletions with exact UTF8/BOM parity. Remaining largest categories: type names3,940; method-call this.3,834; static receivers2,811; nameof2,399; crefs1,945.

Change-specific validation

Merged main at 6ba281975981910906590563363d16e74b5fbf47; resolved the binary-expression writer conflict while retaining both main's AddParentheses option and grammar-safe unparenthesized statement emission.

• Pre-merge latest epoch: 308 focused Roslyn5.9 tests, including21 new branch cases/direct checks and both unsafe CAE capture regressions. Six-configuration operation trees/calls/conversions/constants and complete compiler/emit diagnostic multisets/all endpoints; not explicit CFG comparison.
• The regressions retain qualification when another configuration selects a source-capturing overload or introduces a capturing closure only in the inactive parse. Forced edits still change real synthesized strings. The old directive-bearing receiver veto was traced and preserved, not relaxed to force a positive.
• Authentic original reconstruction:2,699 documents/20 references. Actual SDK ProcessDocument with all116 writer records/138 type nodes removes exactly2JSON prefixes, preserving exact constructed calls/receiver types and complete ordinary/unfiltered document diagnostic bags4→4. Original missing source-generator .Default errors are preserved, not repaired or ignored.
• One candidate-only saved-input replay:115.5s, zero baseline generations/builds, no fresh TypeSpec compilation or comparative speed claim. Actual runtime hashes, all2,301 output hashes and exact2,677-file SDK/unrelated-WIP restoration independently verified. The method smoke was20.08s total/7.78s first ProcessDocument including initial shared facts.
• Post-main-merge full suites: Core 3,966/3,966; client 2,671/2,671; input 307/307. Emitter 396 passed / 2 skipped. The merge's AddParentheses/statement-emission combination is covered by four new true/false option tests, all passing.
• Post-merge official unfiltered opt-in structural local regeneration/integration passed: Local 818/818; Spector 962 passed / 10 skipped. Temporary generated outputs and merged launcher settings were restored exactly.
• Post-merge wide Roslyn5.9 structural/provider/statement focus: 2,584/2,584 passed. An isolated-output fixture-path failure was corrected by supplying the existing shared TestProjects resource directory; the same built assembly and identical test-name multiset were rerun. Temporary package props were restored.

Limits

The saved SDK measurements above belong to the pre-merge source epoch. They are not post-merge whole-SDK semantic/compiler/emit/native/API clearance. Historical CAST validation included full implementation/local integration checks and net10 GA AppService1.5.0 API/byte-identical XML; that evidence is not automatically inherited by later permissions or the main merge. Broader SDK semantic/native/API/whole-XML validation and fresh netstandard2.0/net8.0/comparative profiling remain pending. The oversized context graph pipeline remains stopped.

This remains a draft prototype, not default enablement or a complete Roslyn-simplifier replacement. Large local captures/benchmarks and separately paused work are intentionally not committed. Status: packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/perf/ProviderReducerStatus.md.

- by copilot

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