Color byref and outref types as mutable

[cartermp]

Consider this horribly imperative program:

type C() =
    let mutable nums = [| 1; 3; 7; 15; 31; 63; 127; 255; 511; 1023 |]

    override __.ToString() = String.Join(' ', nums)

    member __.FindLargestSmaller(target: int) =
        let mutable ctr = nums.Length - 1

        while ctr > 0 && nums.[ctr] >= target do ctr <- ctr - 1

        if ctr > 0 then &nums.[ctr] else &nums.[0]

[<EntryPoint>]
let main argv =
    let c = C()
    printfn "Original sequence: %s" (c.ToString())

    let num = 16
    let v = &c.FindLargestSmaller(num)
    v <- v*2

    printfn "New sequence:      %s" (c.ToString())

    0 // return an integer exit code

// Gives:
// Original sequence: 1 3 7 15 31 63 127 255 511 1023
// New sequence:      1 3 7 30 31 63 127 255 511 1023

Note that v is not mutable - it doesn't need to be, since it's a pointer.

Except it's not colored as if it could be mutated like that.

[cartermp]

Note that you cannot assign to an inref:

let f1 (x: inref<int>) = x <- 1

So even though it's not the same thing as mutability, since you can only read the value, it's fine if it's not colored.