F# TryGetNonEnumeratedCount helper

Seq.fs

module Seq =
    open System.Linq

    /// Attempts to determine the number of elements in a sequence without forcing an enumeration.
    let inline tryLength (xs : seq<_>) =
        match Enumerable.TryGetNonEnumeratedCount xs with
        | true, count -> ValueSome count
        | _ ->
            match xs with
            // F# list implements IReadOnlyCollection/IReadOnlyList
            // but TryGetNonEnumeratedCount doesn't support it
            // see https://github.com/dotnet/runtime/issues/42254
            | :? List<_> as l -> ValueSome (List.length l)
            | _ -> ValueNone