Hot reload

[NatElkins]

F# Hot Reload (Edit-and-Continue) Implementation

This PR introduces hot reload infrastructure for F# on top of .NET's MetadataUpdater.ApplyUpdate pipeline, with Roslyn-parity-oriented metadata behavior and multi-generation delta chaining.

Status Snapshot (2026-02-07)

• PR: NatElkins/fsharp#1
• Branch head: hot-reload@c454aeb
• Scope has evolved significantly since the original PR text; this description reflects the current state.

Verified in this branch

• ./.dotnet/dotnet build FSharp.sln -c Debug -v minimal ✅
• ./.dotnet/dotnet test tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj -c Debug --no-build --filter FullyQualifiedName~HotReload -v minimal ✅ (258 passed)
• ./.dotnet/dotnet test tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj -c Debug --no-build --filter FullyQualifiedName~HotReload -v minimal ✅ (101 passed)

What This PR Adds

• Hot reload session lifecycle in FSharpChecker:
  • StartHotReloadSession
  • EmitHotReloadDelta
  • EndHotReloadSession
  • HotReloadSessionActive
  • HotReloadCapabilities
• Typed-tree diff + rude-edit classification pipeline for deciding delta vs rebuild.
• Baseline capture (FSharpEmitBaseline) and generation chaining (EncId/EncBaseId) across multiple edits.
• Delta emitters for metadata/IL/PDB with EncLog/EncMap tracking.
• Runtime integration path exercised by component tests and demo tooling.
• In-repo demo app (tests/projects/HotReloadDemo/HotReloadDemoApp) and smoke script (tests/scripts/hot-reload-demo-smoke.sh).

Recent Architecture/Parity Direction (late 2025)

• Significant SRM boundary reduction/refactoring in hot reload paths:
  • byte-based baseline metadata readers
  • unified handle/index abstractions (BinaryConstants/delta token helpers)
  • PDB parity tests and shared reader/writer utilities
• Roslyn parity fixes for StandAloneSig row handling and metadata sizing/encoding behaviors.
• Method additions enabled with Roslyn-parity restrictions (c454aeb).

Supported vs Unsupported (Current)

Supported

• Method-body edits with metadata/IL/PDB delta emission.
• Multi-generation sessions.
• Runtime apply scenarios validated by component tests (when runtime prerequisites are set).
• Limited method additions under emitter restrictions (no incompatible type-layout changes).

Still rejected / rebuild required

• Structural edits that change type layout/signature semantics beyond supported boundaries.
• Edits that introduce unsupported metadata/token mapping cases (reported as UnsupportedEdit/rude edits).

Runtime Apply Capability Model

• Capability flag is intentionally conservative:
  • RuntimeApply is only advertised when the runtime supports it and FSHARP_HOTRELOAD_ENABLE_RUNTIME_APPLY=1 is set.
• Demo smoke supports both modes:
  • default: emit + validate deltas without runtime apply
  • opt-in: runtime apply path via HOTRELOAD_SMOKE_RUNTIME_APPLY=1

Remaining Follow-ups (Not Claimed Done Here)

• Broaden structural edit coverage (especially synthesized/complex scenarios) while preserving Roslyn parity.
• Improve telemetry/diagnostics surfacing for hosts.
• Wire official dotnet watch integration path.
• Continue tightening mdv + runtime parity checks as the final acceptance bar.

Key Files

• Service/session API: src/Compiler/Service/service.fs, src/Compiler/HotReload/EditAndContinueLanguageService.fs
• Delta emission: src/Compiler/CodeGen/IlxDeltaEmitter.fs, src/Compiler/CodeGen/FSharpDeltaMetadataWriter.fs, src/Compiler/CodeGen/DeltaMetadataSerializer.fs
• Baseline/capabilities: src/Compiler/CodeGen/HotReloadBaseline.fs, src/Compiler/HotReload/HotReloadCapabilities.fs
• Tests:
  • tests/FSharp.Compiler.Service.Tests/HotReload/*
  • tests/FSharp.Compiler.ComponentTests/HotReload/*