Support ListCollector

[nojaf]

Description

It would be great if Fable could support ListCollector<'T>

Repro code

    let cutOffLast list =
        let mutable headList = ListCollector<'a>()

        let rec visit list =
            match list with
            | []
            | [ _ ] -> ()
            | head :: tail ->
                headList.Add(head)
                visit tail

        visit list
        headList.Close()

Expected and actual results

Expected:

// Something along the lines of
// let mutable headList = ListCollector<'a>()
const headList = [];

// headList.Add(head)
headList.push(head);

// headList.Close()
import { ofArray } from "fable-library/List.js";
ofArray(headList);

Actual: error FABLE: Microsoft.FSharp.Core.CompilerServices.ListCollector1.Close is not supported by Fable`.

Related information

• Fable version: dotnet fable --version: 4.9.0
• Operating system: Windows

I might wanna dig into this if approved.

[MangelMaxime]

It seems to be a small enough type to write Source so fo me it is ok to add it.

Because this is an F# type, what we do in general is we take heavy inspiration from the FSharp.Core implementation and include it inside of the fable-library folder. For example, this is what we did for StringBuilder.

The only things I have question about is that ListCollector is using RuntimeHelpers.FreshConsNoTail which is defined as:

let inline FreshConsNoTail head = head :: (# "ldnull" : 'T list #)

so it is emitting IL code and we will not be able to do that same goes for SetFreshConsTail. But I don't think this is blocking.

[ncave]

Closing as implemented by #3745, feel free to reopen if needed.