Let Peek Definition ask another language for a metadata symbol's source

[xperiandri]

Peek Definition on a symbol Roslyn sees as metadata generates metadata-as-source for it, even when
another language owns the source. For F# that means peeking an F# symbol from C# or Visual Basic shows
decompiled C#, while Go To Definition on the same symbol opens the F# file.

The two commands take different routes. Go To Definition reaches
VisualStudioSymbolNavigationService.GetNavigableLocationAsync, which asks every exported
ICrossLanguageSymbolNavigationService before falling back to metadata-as-source ("See if there's
another .Net language service that can handle navigating to this metadata symbol (for example, F#)").
Peek goes through PeekableItemFactory.GetPeekableItemsAsync, which only consults
ISymbolNavigationService.GetExternalNavigationSymbolLocationAsync — the RQName and
IVsSymbolicNavigationNotify path — and otherwise builds a DefinitionPeekableItem over
metadata-as-source. ICrossLanguageSymbolNavigationService is never asked; that one call in
VisualStudioSymbolNavigationService is its only consumer.

Peek cannot use the existing member: it shows a file in place, so it needs a file and a position —
ExternalFilePeekableItem takes a FileLinePositionSpan — while TryGetNavigableLocationAsync hands
back an INavigableLocation that can only navigate.

This adds that member and has Peek ask it.

• ICrossLanguageSymbolNavigationService.TryGetNavigableFileLocationAsync returns
  (string filePath, LinePosition linePosition)? for the same assembly name and documentation comment
  id — the shape GetExternalNavigationSymbolLocationAsync already returns, so its result drops into
  the existing ExternalFilePeekableItem branch.
• PeekableItemFactory asks the cross-language services when the external navigation location comes
  back empty, in the order VisualStudioSymbolNavigationService uses: a symbol with a source location
  and one metadata-as-source cannot navigate to are left alone, and only a navigable metadata symbol is
  offered to the other language before falling back.
• IFSharpCrossLanguageSymbolNavigationService2 derives from IFSharpCrossLanguageSymbolNavigationService
  and carries the new member; the F# adapter implements it by casting the imported service, and returns
  null when the F# language service does not implement it. As with IFSharpAdvancedNavigateToSearchService
  in Let F# take part in the Navigate To search that runs while the solution loads #85213, it is a separate optional interface rather than a member on the existing one: the
  implementation lives outside this repository and Roslyn reaches Visual Studio ahead of it, so a
  member added to the existing interface would fail type load during MEF composition for an F#
  language service built against the previous version. The 2 suffix follows FSharpNavigationOptions2
  in the same file.

ICrossLanguageSymbolNavigationService is internal and its only implementation is the F# adapter, so
adding the member there changes nothing else. The type forward and the InternalAPI.Unshipped.txt
entries for both external access assemblies are added alongside.

The consumer is dotnet/fsharp#20520, which cannot merge until this has flowed. It is
written and its tests pass against a local build of this branch.

PeekTests composes a test ICrossLanguageSymbolNavigationService that owns a referenced metadata
assembly. Peek shows the file it hands back for a type, a method, and a member of a constructed generic
type, which the service is asked about by its definition's id (M:Box`1.Set(`0)). The other tests pin
what must not change: a symbol in source and a metadata symbol metadata-as-source cannot show are not
offered to the other language, a location GetExternalNavigationSymbolLocationAsync found wins, and
with no answer Peek still shows metadata-as-source. Without the new branch in PeekableItemFactory, the
four tests that rely on it fail.

This branch builds with -p:RunAnalyzersDuringBuild=true without warnings. Both halves were deployed
to the experimental hive against a mixed C#/F# solution: Peek Definition from C# on an F# symbol now
shows the F# declaration in place, and Go To Definition is unchanged.

🤖 Generated with Claude Code

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[Copilot]

🟡 Changes recommended

Address the two moderate findings in PeekableItemFactory.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR enables Peek Definition to show cross-language source, such as F# declarations, instead of metadata-as-source.

Changes:

• Adds cross-language file-location navigation.
• Updates Peek Definition fallback behavior.
• Adds versioned F# adapters, type forwarding, and API baselines.

File summaries

File	Summary
src/VisualStudio/ExternalAccess/FSharp/TypeForwards.cs	Forwards the new F# interface.
src/VisualStudio/ExternalAccess/FSharp/InternalAPI.Unshipped.txt	Records F# API additions.
src/VisualStudio/ExternalAccess/Core/InternalAPI.Unshipped.txt	Records shared API additions.
src/VisualStudio/ExternalAccess/Core/FSharp/Navigation/IFSharpCrossLanguageSymbolNavigationService.cs	Adds the optional versioned F# contract.
src/VisualStudio/ExternalAccess/Core/FSharp/Internal/Navigation/FSharpCrossLanguageSymbolNavigationService.cs	Adapts the optional F# implementation.
src/Features/Core/Portable/Navigation/ICrossLanguageSymbolNavigationService.cs	Adds the file-location navigation contract.
src/EditorFeatures/Core/Peek/PeekableItemFactory.cs	Queries cross-language services before metadata fallback. Findings: moderate (3 votes), add regression coverage; moderate (1 vote), normalize to OriginalDefinition before generating the documentation ID.

Review details

Suppressed comments (1)

src/EditorFeatures/Core/Peek/PeekableItemFactory.cs:115

• This lookup uses the constructed symbol's documentation ID, while Go To Definition first normalizes to symbol.OriginalDefinition (VisualStudioSymbolNavigationService.cs:54). For a metadata member/type instantiated with concrete generic arguments, GetDocumentationCommentId() can include substituted types, so the F# service may not recognize the ID and Peek falls back to decompiled C# even though Go To Definition succeeds. Normalize to symbol.OriginalDefinition before generating the ID.

        var docCommentId = symbol.GetDocumentationCommentId();

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

No unresolved review issues were identified, and the changes include regression coverage.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

Add adapter-level coverage for the v2 implementation and legacy-interface fallback.

Review details

Suppressed comments (1)

src/VisualStudio/ExternalAccess/Core/FSharp/Internal/Navigation/FSharpCrossLanguageSymbolNavigationService.cs:58

• The added Peek tests export a test ICrossLanguageSymbolNavigationService directly, so they bypass this adapter's IFSharpCrossLanguageSymbolNavigationService2 cast and forwarding. A regression in the external-access version detection would leave F# Peek broken while all current tests still pass; add an adapter-level test covering a v2 implementation and the old-interface-only fallback.

        if (_underlyingService is not IFSharpCrossLanguageSymbolNavigationService2 fileLocationService)
            return null;

        return await fileLocationService.TryGetNavigableFileLocationAsync(
            assemblyName, documentationCommentId, cancellationToken).ConfigureAwait(false);

• Files reviewed: 8/8 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

The reviewed changes have no unresolved blocking issues.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0 new
• Review effort level: Lite