Search each file of a multi-targeted F# project once in Find All References and Rename

[xperiandri]

Description

Find All References and Rename on a symbol of a multi-targeted F# project did every piece of work once per target framework. Measured in the debugger on a solution with 135 project instances: a scope of two projects (the two instances of one .fsproj) grew to 86 instances of 26 project files after adding dependents, 1246 documents in all; the snapshots of all 86 were built one after another before the first search started; every file was type checked once per instance, and FCS never shares those checks between instances; Roslyn's window hides the duplicate results, so the user only sees the time. On top of that the "skip the files before the declaration" optimisation only applied in the instance owning the first DocumentId of the declaration, because Project.FindFSharpReferencesAsync compared document.Project = this by reference.

Change (SymbolHelpers.fs, WorkspaceExtensions.fs):

• getSymbolUsesInProjects keeps F# projects only (which also stops getFSharpOptionsForProject from being asked about a C# project in the external-symbol branch) and, with EnableFastFindReferencesAndRename on (the default), groups the instances of one project file. The instance searched in full is the one of the current document, else one in its dependency closure, else the first; the other instances only search files compiled solely there and files whose parse tree has conditional compilation directives (ParsedInput.Trivia.ConditionalDirectives), since those are the files whose sources can differ between instances.
• Searches start as soon as a project's snapshot is ready instead of after all snapshots; snapshots are built only for the transparent compiler (Document.FindFSharpReferencesAsync takes an FSharpProjectSnapshot voption and picks the FCS overload from it); one SemaphoreSlim(ProcessorCount) bounds the concurrent file checks across the whole search instead of per project.
• Project.FindFSharpReferencesAsync resolves the declaration document inside its own instance, so the compile-order skip works in every instance.
• The two dead arms of findSymbolUses (CurrentDocument/SignatureAndImplementation inside the | scope -> fallback) are gone; the outer match is exhaustive.

Behaviour change with the option off: every instance is still searched in full, but the concurrency bound now applies across the solution where the code used to run one project's documents sequentially and projects without a limit. Residual risk with the option on: an instance can resolve an overload differently without any #if (an extension member shadowed by a newer BCL intrinsic); the ranking covers the polyfill case within the current project's world, and turning the option off restores the full search.

Rename needs no change: getSymbolUsesInSolution now yields one DocumentId per file plus the instance-specific ones, and Roslyn merges the edits of linked documents.

Tests: MultiTargetFindReferencesTests loads one project as two instances (one without FOO and without the fourth file, one with both) through the new RoslynTestHelpers.CreateMultiTargetSolution and, starting from either instance, checks that Find All References reports the signature, the plain use, the use under #if FOO and the use in the instance-only file exactly once each, and that Rename gets one document per file, owned by an instance that compiles it. The first commit (test helpers) is shared with #20462; the second moves the IFSharpFindUsagesContext stub into RoslynTestHelpers.

No timings are claimed: on the solution above the search scope goes from 86 instances to 26, plus the conditional and instance-only files of the other 60.

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md