Allow _ in async use! _ pattern (lift FS1228 restriction)

[edgarfgp]

Description

Fixes #16343

Before

open System
let doSomething () =
    async {
        use! _ = Async.OnCancel (fun () -> printfn "disposed") // FS1228: 'use!' bindings must be of the form 'use! <var> = <expr>'
        use! res = Async.OnCancel (fun () -> printfn "disposed")
        return ()
    }

After

open System
let doSomething () =
    async {
        use! _ = Async.OnCancel (fun () -> printfn "disposed") //  Works
        use! res = Async.OnCancel (fun () -> printfn "disposed")
        return ()
    }

Checklist

• Test cases added
• Release notes entry updated

[github-actions]

❗ Release notes required

@edgarfgp,

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Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/9.0.300.md	No release notes found or release notes format is not correct

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
LanguageFeatures.fsi	docs/release-notes/.Language/preview.md

[auduchinok]

This is a change in what does the language allow. I think it should be guarded with a language version, so the tooling could suggest using _ safely only when the supported language version is used.

[T-Gro]

I was expecting a call to "Using" somewhere, but cannot find it.
Is it implicit by recursive calling into a different handler?

[edgarfgp]

The way I understood the logic was that _ (wild) will need to be Bind in the same way that ident(any ident) __ (double underscore). But Im not very familiar with the CE Translation logic.

Tested this to "Using" and it does not work.

[T-Gro]

If it only does "Bind" and not an "Using", it will lack the CE's implementation of disposal (which can be anything the CE wants to do to cleanup).

[T-Gro]

Please do add positive tests which include a run operation and demonstrate that a resource which is being use! _ ='d get's cleaned up with a custom .Using method of the builder.