Add `Async.AwaitTask` overloads which helps with CancellationToken passing, and a new warning

[jwosty]

I propose we add two Async.AwaitTask overloads:

• Async.AwaitTask(makeTask: CancellationToken -> Task<'a>) -> Async<'a>
• Async.AwaitTask(makeTask: CancellationToken -> Task) -> Async<unit>

I also propose that an accompanying compiler warning be added. In situations where the task method could have been provided a CancellationToken (either via an optional parameter or a separate overload), the compiler should emit a warning.

The warning could say something like: "This task-returning method can accept a CancellationToken (through an overload or optional parameter), but is not provided one. Please either use Async.AwaitTask(makeTask: CancellationToken -> Task<'a>), or capture and provide a CancellationToken manually."

Surely there are better wordings for such a warning; improvements are welcome.

For example, the following, which forgets to pass a CancellationToken, would now emit a warning:

let doWorkBad () = async {
    do! Async.AwaitTask (File.WriteAllTextAsync ("file.txt", "Hello, world!"))
}

To make the warning go away, you could use the new overload:

let doWorkGood () = async {
    do! Async.AwaitTask (fun ct -> File.WriteAllTextAsync ("file.txt", "Hello, world!", ct))
}

Or capture and pass it yourself:

let doWork () = async {
    let! ct = Async.CancellationToken
    do! Async.AwaitTask (File.WriteAllTextAsync ("file.txt", "Hello, world!", ct))
}

The existing way of approaching this problem in F# is ...

For the new method, you can write your own extensions today:

type Async =
    static member AwaitTask (makeTask: CancellationToken -> Task<'a>) = async {
        let! ct = Async.CancellationToken
        let! result = Async.AwaitTask (makeTask ct)
        return result
    }
    static member AwaitTask (makeTask: CancellationToken -> Task) = async {
        let! ct = Async.CancellationToken
        let! result = Async.AwaitTask (makeTask ct)
        return result
    }

Pros and Cons

The advantages of making this adjustment to F# are:

• Makes it harder to forget to pass a CancellationToken when bridging from Async to Task code, which is a common source of bugs
• Makes it easier to do the actual passing of the CancellationToken (reduces common boilerplate)

The disadvantages of making this adjustment to F# are:

• More library and compiler complexity

Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions: (put links to related suggestions here)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• [x] This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• [x] I have searched both open and closed suggestions on this site and believe this is not a duplicate
• [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• [x] This is not a breaking change to the F# language design
• [x] I or my company would be willing to help implement and/or test this

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