Answer Peek Definition from C# and Visual Basic with the F# source

docs/release-notes/.VisualStudio/18.vNext.md

@@ -5,6 +5,7 @@
 
 ### Fixed
 
+* Peek Definition from C# or Visual Basic on an F# symbol shows the F# declaration in place; it used to show the symbol decompiled into C#, while Go To Definition on the same symbol opened the F# file. ([PR #20520](https://github.com/dotnet/fsharp/pull/20520))
 * Improve Find All References performance by throttling parallel typechecks. ([PR #20128](https://github.com/dotnet/fsharp/pull/20128))
 * Fixed Rename incorrectly renaming `get` and `set` keywords for properties with explicit accessors. ([Issue #18270](https://github.com/dotnet/fsharp/issues/18270), [PR #19252](https://github.com/dotnet/fsharp/pull/19252))
 * Fixed Find All References crash when F# project contains non-F# files like `.cshtml`. ([Issue #16394](https://github.com/dotnet/fsharp/issues/16394), [PR #19252](https://github.com/dotnet/fsharp/pull/19252))
@@ -15,6 +16,7 @@
 * Fix doubled F# diagnostics in tooltips. ([Issue #16360](https://github.com/dotnet/fsharp/issues/16360))
 * Fix `NotSupportedException` in the memory-mapped-file optimization when copying `ReadOnlyMemory` into `MemoryMappedFileViewStream`. ([Issue #20263](https://github.com/dotnet/fsharp/issues/20263))
 * Reduce allocations in the VS project options reactor: the command-line options and project options caches and the mailbox reply payloads now hold struct tuples, and `IProjectSite.CompilationBinOutputPath` returns `string voption` picked with a new `Array.tryPickV`. ([PR #20413](https://github.com/dotnet/fsharp/pull/20413))
+* Go To Definition from C# or Visual Basic into an F# project no longer type checks the whole project: the file declaring the symbol is found from parsed declarations and checked alone, with the whole-project check as a fallback. ([PR #20465](https://github.com/dotnet/fsharp/pull/20465))
 
 ### Changed
 

vsintegration/src/FSharp.Editor/Common/CancellableTasks.fs

@@ -1147,6 +1147,17 @@ module CancellableTasks =
                 return results
             }
 
+        /// Runs the chooser over the items one at a time and stops at the first ValueSome.
+        let rec tryPick (chooser: 'T -> CancellableTask<'U voption>) (items: 'T list) : CancellableTask<'U voption> =
+            match items with
+            | [] -> singleton ValueNone
+            | item :: rest ->
+                cancellableTask {
+                    match! chooser item with
+                    | ValueSome picked -> return ValueSome picked
+                    | ValueNone -> return! tryPick chooser rest
+                }
+
         let inline ignore ([<InlineIfLambda>] ctask: CancellableTask<_>) = toUnit ctask
 
         /// If this CancellableTask gets canceled for another reason than the token being canceled, return the specified value.

vsintegration/src/FSharp.Editor/FSharp.Editor.fsproj

@@ -92,6 +92,7 @@
     <Compile Include="Navigation\GoToDefinitionService.fs" />
     <Compile Include="Navigation\NavigationBarItemService.fs" />
     <Compile Include="Navigation\NavigateToSearchService.fs" />
+    <Compile Include="Navigation\CrossLanguageSymbolNavigation.fs" />
     <Compile Include="Navigation\FindUsagesService.fs" />
     <Compile Include="Navigation\FindDefinitionService.fs" />
     <Compile Include="QuickInfo\WpfFactories.fs" />