Add a refactoring between option and struct partial active patterns

[xperiandri]

let (|Even|_|) x = if x % 2 = 0 then Some () else None
// ⟷
[<return: Struct>]
let (|Even|_|) x = if x % 2 = 0 then ValueSome () else ValueNone

Match sites don't change, so only the definition is edited. A local active pattern can't carry the attribute, so it converts to struct only on F# 9 or later. Not offered when a result isn't built from Some/None (or ValueSome/ValueNone), or when the file has a paired .fsi.

🤖 Generated with Claude Code

[github-actions]

❗ Release notes required

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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

[kerams]

I appreciate all these PRs, they're bringing F# (VS) tooling closer to what C#'s had for ages, but I feel like you're just creating an ever-increasing mountain of work to review. I get anxious with every new PR notification, and it's not even my job to review them. Maybe take a break for a couple of weeks, making sure the existing PRs get over the line, and see where things stand afterwards?

[github-actions]

🔍 Tooling Safety Check — Affects-Design-Time
Affects-Design-Time: Modifies Visual Studio code executed while projects are open.

Generated by PR Tooling Safety Check · gpt56 1.8M · ◷

[T-Gro]

🤖🕵️ Please shorten the description using this guidance. Focus on the problem and why the change is needed, in simplified technical English. Leave the implementation inventory to the Files tab and retain necessary caveats.

[xperiandri]

Covered cases

Converts

• Module-level pattern gets [<return: Struct>] on its own line above let: let (|Even|_|) x = if x % 2 = 0 then Some () else None → [<return: Struct>] + let (|Even|_|) x = if x % 2 = 0 then ValueSome () else ValueNone
• match branches: | v when v > 0 -> Some v | _ -> None → | v when v > 0 -> ValueSome v | _ -> ValueNone
• A raising branch plus try … with: if s.Length = 0 then failwith "empty" else try Some(int s) with _ -> None → … try ValueSome(int s) with _ -> ValueNone
• Return type annotation: (x: int) : unit option = … Some () else None → (x: int) : unit voption = … ValueSome () else ValueNone
• Local active pattern inside let f v = … changes only Some/None → ValueSome/ValueNone, with no attribute added
• An existing attribute stays and the new one goes on the next line: [<CompiledName("EvenPattern")>] → [<CompiledName("EvenPattern")>] + [<return: Struct>]
• In a let rec … and group, only the pattern under the caret converts, with the attribute placed after and: and (|B|_|) x = … → and [<return: Struct>] (|B|_|) x = … ValueSome () else ValueNone
• Back, from an annotation with no attribute: : unit voption = … ValueSome () else ValueNone → : unit option = … Some () else None
• Back, removing [<return: Struct>] both from the same line ([<return: Struct>] let (|Even|_|) x = … → let (|Even|_|) x = …) and from a shared list ([<CompiledName("EvenPattern"); return: Struct>] → [<CompiledName("EvenPattern")>])
• Round trip gives back the original text for the match, CompiledName, rec … and, module-level and local cases
• Action titles: Use struct return for active pattern / Use option return for active pattern, exactly one action each
• Each converted document type-checks without errors

Not offered

• Patterns that are not single-case partial: let (|Even|Odd|) x = if x % 2 = 0 then Even else Odd, let (|A|B|_|) x = …
• Results not all built from cases of one option kind: Some x |> Option.filter (fun v -> v > 0), : unit option = failwith "never", then Some () else ValueNone
• Boolean-returning pattern: let (|Even|_|) x = x % 2 = 0
• Caret on Some in the body instead of the pattern header
• Ordinary function: let even x = if x % 2 = 0 then Some () else None
• Return annotation that is not an option: let (|Even|_|) x : int list = …
• Local active pattern converted to struct before F# 9 (--langversion:8.0)
• File that has a signature file: val (|Even|_|): int -> unit option