Use ListCollector for DList.toList

Author: gusty

src/FSharpPlus/Data/DList.fs

@@ -3,6 +3,7 @@
 open System
 open System.Collections.Generic
 open System.ComponentModel
+open FSharp.Core.CompilerServices
 open FSharpPlus
 
 // DList from FSharpx.Collections
@@ -183,6 +184,26 @@ type DList<'T> (length: int, data: DListData<'T>) =
             | Join (x, y) -> yield! walk (y::rights) x }               
         (walk [] data).GetEnumerator ()
 
+    member internal this.toList () =
+    #if FABLE_COMPILER
+        DList<'T>.foldBack List.cons this []
+    #else
+        let mutable coll = new ListCollector<_> ()
+        let rec walk rights = function
+            | Nil ->
+                match rights with
+                | []    -> ()
+                | t::ts -> walk ts t
+            | Unit x ->
+                coll.Add x
+                match rights with
+                | []    -> ()
+                | t::ts -> walk ts t
+            | Join (x, y) -> walk (y::rights) x
+        walk [] data
+        coll.Close ()
+    #endif
+
     interface IEquatable<DList<'T>> with
         member this.Equals(y: DList<'T>) =
             if this.Length <> y.Length then false
@@ -251,7 +272,7 @@ module DList =
     let ofSeq s = DList<'T>.ofSeq s
 
     /// O(n). Returns a list of the DList elements.
-    let inline toList l = foldBack List.cons l []
+    let toList (l: DList<'T>) = l.toList ()
 
     /// O(n). Returns a seq of the DList elements.
     let inline toSeq (l: DList<'T>) = l :> seq<'T>