Rather dubious struct-based stuff all plumbed through

Raft/EfficientString.fs

@@ -1,8 +1,20 @@
-namespace Raft
+#if NETSTANDARD2_0
+namespace System.Runtime.CompilerServices
 
 open System
 
-type internal EfficientString =
+[<Sealed ; AttributeUsage(AttributeTargets.Struct)>]
+type IsByRefLikeAttribute () =
+    inherit Attribute ()
+#endif
+
+namespace Raft
+
+open System
+open System.Collections
+open System.Runtime.CompilerServices
+
+type EfficientString =
 #if NETSTANDARD2_0
     string
 #else
@@ -10,7 +22,15 @@ type internal EfficientString =
 #endif
 
 [<RequireQualifiedAccess>]
-module internal EfficientString =
+module EfficientString =
+
+    let inline isEmpty (s : EfficientString) : bool =
+#if NETSTANDARD2_0
+        s = ""
+#else
+        s.IsEmpty
+#endif
+
 
     let inline ofString (s : string) : EfficientString =
 #if NETSTANDARD2_0
@@ -19,6 +39,13 @@ module internal EfficientString =
         s.AsSpan ()
 #endif
 
+    let inline toString (s : EfficientString) : string =
+#if NETSTANDARD2_0
+        s
+#else
+        s.ToString ()
+#endif
+
     let inline trimStart (s : EfficientString) : EfficientString = s.TrimStart ()
 
     let inline slice (start : int) (length : int) (s : EfficientString) : EfficientString =
@@ -42,3 +69,39 @@ module internal EfficientString =
             let toRet = slice 0 first s
             s <- slice (first + 1) (s.Length - first - 1) s
             toRet
+
+[<Struct>]
+[<IsByRefLike>]
+type StringSplitEnumerator =
+    internal
+        {
+            Original : EfficientString
+            mutable Remaining : EfficientString
+            mutable InternalCurrent : EfficientString
+            SplitOn : char
+        }
+
+    interface System.IDisposable with
+        member this.Dispose () = ()
+
+    member this.Current : EfficientString = this.InternalCurrent
+
+    member this.MoveNext () =
+        if this.Remaining.Length = 0 then
+            false
+        else
+            this.InternalCurrent <- EfficientString.takeUntil this.SplitOn &this.Remaining
+            true
+
+    member this.GetEnumerator () = this
+
+[<RequireQualifiedAccess>]
+module StringSplitEnumerator =
+
+    let make (splitChar : char) (s : string) : StringSplitEnumerator =
+        {
+            Original = EfficientString.ofString s
+            Remaining = EfficientString.ofString s
+            InternalCurrent = EfficientString.Empty
+            SplitOn = splitChar
+        }