Inline List.iter/List.iteri for performance (dotnet#8176)

src/fsharp/FSharp.Core/list.fs

@@ -100,7 +100,7 @@ namespace Microsoft.FSharp.Collections
                 loop ([], state) (rev list)
 
         [<CompiledName("Iterate")>]
-        let iter action list = Microsoft.FSharp.Primitives.Basics.List.iter action list
+        let inline iter action (list:'T list) = for x in list do action x
 
         [<CompiledName("Distinct")>]
         let distinct (list:'T list) = Microsoft.FSharp.Primitives.Basics.List.distinctWithComparer HashIdentity.Structural<'T> list
@@ -164,7 +164,9 @@ namespace Microsoft.FSharp.Collections
         let takeWhile predicate (list: 'T list) = Microsoft.FSharp.Primitives.Basics.List.takeWhile predicate list
 
         [<CompiledName("IterateIndexed")>]
-        let iteri action list = Microsoft.FSharp.Primitives.Basics.List.iteri action list
+        let inline iteri action (list: 'T list) =
+            let mutable n = 0
+            for x in list do action n x; n <- n + 1
 
         [<CompiledName("Initialize")>]
         let init length initializer = Microsoft.FSharp.Primitives.Basics.List.init length initializer

src/fsharp/FSharp.Core/list.fsi

@@ -380,7 +380,7 @@ namespace Microsoft.FSharp.Collections
         /// <param name="action">The function to apply to elements from the input list.</param>
         /// <param name="list">The input list.</param>
         [<CompiledName("Iterate")>]
-        val iter: action:('T -> unit) -> list:'T list -> unit
+        val inline iter: action:('T -> unit) -> list:'T list -> unit
 
         /// <summary>Applies the given function to two collections simultaneously. The
         /// collections must have identical size.</summary>
@@ -395,7 +395,7 @@ namespace Microsoft.FSharp.Collections
         /// <param name="action">The function to apply to the elements of the list along with their index.</param>
         /// <param name="list">The input list.</param>
         [<CompiledName("IterateIndexed")>]
-        val iteri: action:(int -> 'T -> unit) -> list:'T list -> unit
+        val inline iteri: action:(int -> 'T -> unit) -> list:'T list -> unit
 
         /// <summary>Applies the given function to two collections simultaneously. The
         /// collections must have identical size. The integer passed to the

src/fsharp/FSharp.Core/local.fs

@@ -88,8 +88,6 @@ module internal List =
     [<SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")>]
     let nonempty x = match x with [] -> false | _ -> true
 
-    let rec iter f x = match x with [] -> () | h :: t -> f h; iter f t
-
     // optimized mutation-based implementation. This code is only valid in fslib, where mutation of private
     // tail cons cells is permitted in carefully written library code.
     let inline setFreshConsTail cons t = cons.( :: ).1 <- t
@@ -499,15 +497,6 @@ module internal List =
             else
                 filter predicate t
 
-    let iteri action x =
-        let f = OptimizedClosures.FSharpFunc<_, _, _>.Adapt(action)
-        let rec loop n x =
-            match x with
-            | [] -> ()
-            | h :: t -> f.Invoke(n, h); loop (n+1) t
-
-        loop 0 x
-
     // optimized mutation-based implementation. This code is only valid in fslib, where mutation of private
     // tail cons cells is permitted in carefully written library code.
     let rec concatToFreshConsTail cons h1 l =

src/fsharp/FSharp.Core/local.fsi

@@ -32,7 +32,6 @@ module internal List =
     val distinctWithComparer : System.Collections.Generic.IEqualityComparer<'T> -> 'T list -> 'T list
     val distinctByWithComparer : System.Collections.Generic.IEqualityComparer<'Key> -> ('T -> 'Key) -> list:'T list -> 'T list when 'Key : equality
     val init : int -> (int -> 'T) -> 'T list
-    val iter : ('T -> unit) -> 'T list -> unit
     val filter : predicate:('T -> bool) -> 'T list -> 'T list
     val collect : ('T -> 'U list) -> 'T list -> 'U list
     val partition : predicate:('T -> bool) -> 'T list -> 'T list * 'T list
@@ -48,7 +47,6 @@ module internal List =
     val exists : predicate:('T -> bool) -> 'T list -> bool
     val rev: 'T list -> 'T list
     val concat : seq<'T list> -> 'T list
-    val iteri : action:(int -> 'T -> unit) -> 'T list -> unit
     val unfold : ('State -> ('T * 'State) option) -> 'State -> 'T list
     val unzip : ('T1 * 'T2) list -> 'T1 list * 'T2 list
     val unzip3 : ('T1 * 'T2 * 'T3) list -> 'T1 list * 'T2 list * 'T3 list