Add workspace/didChangeWorkspaceFolders handler with graph-based project unloading

[Copilot]

Roslyn LSP had no handler for workspace/didChangeWorkspaceFolders, so projects loaded from removed workspace roots were never unloaded. Critically, projects outside workspace folders that are still transitively referenced by in-scope projects must be retained.

Core algorithm (LanguageServerProjectLoader)

New UnloadProjectsNotReachableFromWorkspaceFoldersAsync performs a graph-based BFS from retained roots:

1. Seed reachable set: tracked projects whose paths are under an active workspace folder (PathUtilities.IsChildPath)
2. BFS expand: follow Project.ProjectReferences from each reachable project, retaining any transitively referenced tracked project
3. Unload everything not in the reachable set via existing TryUnloadProject_NoLockAsync

Non-absolute paths are skipped (containment undefined). Empty workspace folder set → unload all.

Workspace folder state (LanguageServerProjectSystem)

• _currentWorkspaceFolderPaths tracks the active set (guarded by _folderPathsLock)
• SetInitialWorkspaceFolderPaths seeds state from initializeParams.WorkspaceFolders at startup (called from AutoLoadProjectsInitializer)
• OnWorkspaceFoldersChangedAsync(added, removed, ct) applies deltas using PathUtilities.Comparer, then calls the unload algorithm

New LSP handler (DidChangeWorkspaceFoldersHandler)

Handles workspace/didChangeWorkspaceFolders, extracts file: URI folder paths, and delegates to LanguageServerProjectSystem.OnWorkspaceFoldersChangedAsync. Non-file URIs are skipped with a warning.

Capability advertisement (DefaultCapabilitiesProvider)

Sets workspace.workspaceFolders = { supported: true, changeNotifications: true } so clients know to send folder-change notifications.

Tests (WorkspaceFolderChangeTests)

Seven focused unit tests using a TestAccessor to inject fake Primordial project state without running design-time builds:

• Project inside workspace folder → retained
• Project outside workspace folder → unloaded
• Project outside but directly referenced by in-scope project → retained
• Transitively referenced chain (App → Core → Util, only App in folder) → all retained
• Unreferenced external project → unloaded
• Path prefix safety (/repo folder does not match /repo2/proj.csproj)
• All workspace folders removed → all projects unloaded

Original prompt

Implement support in dotnet/roslyn for unloading Roslyn LSP projects when workspace/didChangeWorkspaceFolders removes the workspace-folder roots that keep those projects in scope.

Repository: dotnet/roslyn
Base branch: main

Context and current state:

• The project loading/unloading logic lives in src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/HostWorkspace/LanguageServerProjectLoader.cs.
• LanguageServerProjectLoader already supports explicit unloads via TryUnloadProjectAsync(...) and UnloadAllProjectsAsync(), and it tracks loaded projects in _loadedProjects keyed by project path.
• LanguageServerProjectSystem in src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/HostWorkspace/LanguageServerProjectSystem.cs handles project loading through OpenProjectsAsync(...) and OpenSolutionAsync(...).
• Roslyn’s LSP protocol layer already contains protocol types/constants for workspace folders, including DidChangeWorkspaceFoldersParams, WorkspaceFoldersChangeEvent, and Methods.WorkspaceDidChangeWorkspaceFoldersName.
• There does not appear to be an existing Roslyn LSP handler wiring workspace/didChangeWorkspaceFolders to project unloading.

Important additional requirement:

• Do NOT unload projects merely because their own project path is outside the current workspace folders if they are still referenced (transitively) by a project that remains in-scope.
• In other words, when workspace folders change, retained projects should be:
  1. tracked projects whose project path is under one of the active workspace folders, plus
  2. any tracked projects transitively referenced by those retained root projects.
• Only unload tracked projects that are outside the active workspace folders AND unreachable from any retained root project.

Implementation goals:

1. Add support for handling workspace/didChangeWorkspaceFolders in the Roslyn LSP server.

   • Create an appropriate notification handler in the HostWorkspace area.
   • Route the notification to LanguageServerProjectSystem or another appropriate abstraction.
   • Follow existing Roslyn MEF/LSP handler conventions.

2. Track workspace folders in the project system.

   • Add APIs/state to LanguageServerProjectSystem for setting/updating the active workspace folder paths.
   • Normalize file paths appropriately.
   • On initialization/startup, ensure the initial workspace folder state is available if needed.

3. Extend LanguageServerProjectLoader with a graph-based unload operation.

   • Add an internal API with a name like UnloadProjectsNotReachableFromWorkspaceFoldersAsync(...).
   • Safely snapshot tracked projects under _gate.
   • Treat tracked project path as the node identity.
   • Determine retained root tracked projects by path containment under active workspace folders.
   • Build a reverse map from workspace ProjectId to tracked project path.
   • Use Roslyn workspace Project.ProjectReferences to compute the transitive closure from retained roots.
   • Be conservative around multi-targeting: if any backing ProjectId for a tracked path is reachable, retain that tracked path.
   • Be conservative around primordial projects / partially loaded states if needed to avoid unloading dependencies too aggressively while loads are in flight.
   • Use existing unload primitives (TryUnloadProject_NoLockAsync, LoadedProject.Dispose(), primordial project removal) rather than inventing new removal mechanics.

4. Keep race behavior safe.

   • ReloadProjectAsync(...) already checks _loadedProjects before proceeding; preserve or extend this model so that folder-change-driven unloads do not leave inconsistent state.
   • Avoid holding _gate during expensive work unless already part of the surrounding design.

5. Add logging where useful for diagnosis.

   • For example, log when workspace folders change and when projects are unloaded because they are no longer reachable from active workspace folders.

6. Add or update tests in src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer.UnitTests/.
   Please add focused tests covering at least:

   • A project inside the workspace folders remains loaded.
   • A project outside all workspace folders is unloaded after a workspace folder change.
   • A project outside all workspace folders but referenced by an in-scope project remains loaded.
   • Transitive references are retained.
   • A no-longer-referenced external project is unloaded.
   • Prefix/nesting path safety if relevant.

Helpful files:

• src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/HostWorkspace/LanguageServerProjectLoader.cs
• src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/HostWorkspace/LanguageServerProjectSystem.cs
• src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/HostWorkspace/OpenProjectsHandler.cs
• src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/HostWorkspace/OpenSolutionHandler.cs
• `src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/Host...

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