Don't convert attributes to array and then to list (#3159)

* Don't convert attributes to array and then to list

* Don't convert attributes to array and then to list

src/absil/il.fs

@@ -1588,7 +1588,7 @@ and [<Sealed>] ILTypeDefs(f : unit -> (string list * string * ILAttributes * Laz
             t)
 
     member x.AsArray = [| for (_,_,_,ltd) in array.Value -> ltd.Force() |]
-    member x.AsList = x.AsArray |> Array.toList
+    member x.AsList = [ for (_,_,_,ltd) in array.Value -> ltd.Force() ]
 
     interface IEnumerable with 
         member x.GetEnumerator() = ((x :> IEnumerable<ILTypeDef>).GetEnumerator() :> IEnumerator)

src/absil/il.fsi

@@ -854,6 +854,7 @@ type ILAttribute =
 
 [<NoEquality; NoComparison; Sealed>]
 type ILAttributes =
+    member AsArray : ILAttribute []
     member AsList : ILAttribute list
 
 /// Method parameters and return values.

src/fsharp/TastOps.fs

@@ -2718,18 +2718,17 @@ let isILAttrib (tref:ILTypeRef) (attr: ILAttribute) =
 // These linear iterations cost us a fair bit when there are lots of attributes
 // on imported types. However this is fairly rare and can also be solved by caching the
 // results of attribute lookups in the TAST
-let HasILAttribute tref (attrs: ILAttributes) = List.exists (isILAttrib tref) attrs.AsList
+let HasILAttribute tref (attrs: ILAttributes) = Array.exists (isILAttrib tref) attrs.AsArray
 
-let HasILAttributeByName tname (attrs: ILAttributes) = List.exists (isILAttribByName ([],tname)) attrs.AsList
+let HasILAttributeByName tname (attrs: ILAttributes) = Array.exists (isILAttribByName ([],tname)) attrs.AsArray
 
 let TryDecodeILAttribute (g:TcGlobals) tref (attrs: ILAttributes) = 
-    attrs.AsList |> List.tryPick(fun x -> if isILAttrib tref x then Some(decodeILAttribData g.ilg x)  else None)
+    attrs.AsArray |> Array.tryPick (fun x -> if isILAttrib tref x then Some(decodeILAttribData g.ilg x)  else None)
 
 // This one is done by name to ensure the compiler doesn't take a dependency on dereferencing a type that only exists in .NET 3.5
 let ILThingHasExtensionAttribute (attrs : ILAttributes) = 
-    attrs.AsList 
-    |> List.exists (fun attr -> 
-        attr.Method.EnclosingType.TypeSpec.Name = "System.Runtime.CompilerServices.ExtensionAttribute")
+    attrs.AsArray
+    |> Array.exists (fun attr -> attr.Method.EnclosingType.TypeSpec.Name = "System.Runtime.CompilerServices.ExtensionAttribute")
     
 // F# view of attributes (these get converted to AbsIL attributes in ilxgen) 
 let IsMatchingFSharpAttribute g (AttribInfo(_,tcref)) (Attrib(tcref2,_,_,_,_,_,_)) = tyconRefEq g tcref  tcref2