Optimize Array.partition by changing slice to truncateLength

Author: mikaelbr

jscomp/others/belt_Array.ml

@@ -425,8 +425,8 @@ let partitionU a f =
       incr j
     )
   done;
-  let a1 = slice a1 ~offset:0 ~len:!i in
-  let a2 = slice a2 ~offset:0 ~len:!j in
+  truncateToLengthUnsafe a1 !i;
+  truncateToLengthUnsafe a2 !j;
   (a1, a2)
 
 let partition a f = partitionU a (fun [@bs] x -> f x)

lib/js/belt_Array.js

@@ -557,11 +557,11 @@ function partitionU(a, f) {
       j = j + 1 | 0;
     }
   }
-  var a1$1 = slice(a1, 0, i);
-  var a2$1 = slice(a2, 0, j);
+  a1.length = i;
+  a2.length = j;
   return /* tuple */[
-          a1$1,
-          a2$1
+          a1,
+          a2
         ];
 }