Add go-to implementation that would navigate to signature file

[Krzysztof-Cieslak]

Details

Relevant Ionide issue: ionide/ionide-vscode-fsharp#1966

Checklist

• I have looked through existing issues to make sure that this feature has not been requested before
• I have provided a descriptive title for this issue
• I am aware that even valid feature requests may be rejected if they do not align with the project's goals
• I or my company would be willing to contribute this feature

[github-actions]

🤖 This is an automated response from Repo Assist.

Thanks for filing this, @Krzysztof-Cieslak! The "Go to Implementation" feature (textDocument/implementation) is a great addition for F# development — especially for navigating from a .fsi signature file declaration directly to the .fs implementation.

Current state

FSAC currently handles textDocument/definition which navigates to the declaration. A dedicated "Go to Implementation" flow would complement this by navigating the other direction: from a .fsi signature to its .fs counterpart, or from an abstract/virtual member to its concrete implementations.

Implementation sketch

The LSP standard for this is textDocument/implementation. The FSAC entry points are in AdaptiveFSharpLspServer.fs. A basic implementation could:

1. For .fsi → .fs navigation: Given a position in a .fsi file, look up the corresponding .fs file in the same project (they share a name) and navigate to the same symbol by name/position.
2. For abstract/virtual/interface members: Use FSharpCheckFileResults.GetSymbolUseAtLocation to find the symbol, then use FSharpCheckProjectResults.GetAllUsesOfAllSymbols or the FindAllReferences machinery to locate override/implementation sites.

The Ionide issue linked (ionide-vscode-fsharp#1966) seems to focus primarily on the .fsi ↔ .fs navigation case.

Related prior art

• textDocument/definition already jumps to the symbol definition — the .fsi case would be a natural extension of that path
• The FindAllReferences infrastructure in AdaptiveFSharpLspServer.fs is a good reference for how to traverse the project symbol graph

I'm happy to take a first pass at implementing the .fsi → .fs case if that's a good starting point — let me know!

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