The code to answer the question on SO (https://stackoverflow.com/questions/76832545/passing-cancellationtoken-to-iasyncenumerable-in-f)

DamnedCode.fs

open System
open System.Collections.Generic
open System.IO
open System.Runtime.CompilerServices
open System.Threading
open System.Threading.Tasks
open FSharp.Control

module AsyncEnum =
    
    let inline retype<'T,'U> (x:'T) : 'U = (# "" x : 'U #)
    
    let inline private asTaskSeq< ^a, ^e, ^awaitable, ^awaiter, 'T, 'U
        when ^a:(member GetAsyncEnumerator: unit -> ^e)
        and ^e:(member MoveNextAsync: unit -> ^awaitable)
        and ^e:(member Current : 'T)
        and ^awaitable:(member GetAwaiter: unit -> ^awaiter)
        and ^awaiter:(member GetResult: unit -> 'U)
        and ^awaiter:(member get_IsCompleted: unit -> bool)
        and ^awaiter :> ICriticalNotifyCompletion > (source : ^a)= taskSeq {
        let enumerator = source.GetAsyncEnumerator()
        let! tmpMoveNext = task { return! enumerator.MoveNextAsync() }
        let mutable moveNext = retype tmpMoveNext
        while moveNext do
            yield enumerator.Current
            let! tmpMoveNext = task { return! enumerator.MoveNextAsync() }
            moveNext <- retype tmpMoveNext
        ()
    }
    let withCancellation predicate (ct: CancellationToken) (source: IAsyncEnumerable<'T>) = taskSeq {
        for item in (asTaskSeq (source.WithCancellation(ct))) do
            yield predicate item
    }
    
let results = 
    File.ReadLinesAsync("path/to/file.txt")
    |> AsyncEnum.withCancellation Console.WriteLine CancellationToken.None
    |> TaskSeq.toArray