diff --git a/lib/es6/js_exn.js b/lib/es6/js_exn.js
index b377a7d4ec..3e55225bfe 100644
--- a/lib/es6/js_exn.js
+++ b/lib/es6/js_exn.js
@@ -5,9 +5,8 @@ import * as Primitive_option from "./primitive_option.js";
 let $$Error = "JsError";
 
 function asJsExn(exn) {
-  let t = exn;
-  if (t.RE_EXN_ID === $$Error) {
-    return Primitive_option.some(t._1);
+  if (exn.RE_EXN_ID === $$Error) {
+    return Primitive_option.some(exn._1);
   }
   
 }
diff --git a/lib/es6/obj.js b/lib/es6/obj.js
index c436633a10..ae1b9f17e6 100644
--- a/lib/es6/obj.js
+++ b/lib/es6/obj.js
@@ -1,51 +1 @@
-
-
-
-function magic(prim) {
-  return prim;
-}
-
-function repr(prim) {
-  return prim;
-}
-
-function obj(prim) {
-  return prim;
-}
-
-function tag(prim) {
-  return prim.TAG;
-}
-
-function size(prim) {
-  return prim.length | 0;
-}
-
-function field(prim0, prim1) {
-  return prim0[prim1];
-}
-
-function set_field(prim0, prim1, prim2) {
-  prim0[prim1] = prim2;
-}
-
-function dup(prim) {
-  return {...prim};
-}
-
-function update_dummy(prim0, prim1) {
-  Object.assign(prim0, prim1);
-}
-
-export {
-  magic,
-  repr,
-  obj,
-  tag,
-  size,
-  field,
-  set_field,
-  dup,
-  update_dummy,
-}
-/* No side effect */
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/lib/es6/string.js b/lib/es6/string.js
index 9f8621b6ae..66c0818ad4 100644
--- a/lib/es6/string.js
+++ b/lib/es6/string.js
@@ -17,18 +17,6 @@ function concat(sep, xs) {
   return $$Array.of_list(xs).join(sep);
 }
 
-function length(prim) {
-  return prim.length;
-}
-
-function get(prim0, prim1) {
-  return prim0.codePointAt(prim1);
-}
-
-function unsafe_get(prim0, prim1) {
-  return prim0.codePointAt(prim1);
-}
-
 function bos(str) {
   return $$Array.map(str => str.codePointAt(0), Array.from(str));
 }
@@ -306,8 +294,6 @@ function split_on_char(sep, s) {
 }
 
 export {
-  length,
-  get,
   make,
   init,
   sub,
@@ -333,6 +319,5 @@ export {
   capitalize_ascii,
   uncapitalize_ascii,
   split_on_char,
-  unsafe_get,
 }
 /* No side effect */
diff --git a/lib/js/js_exn.js b/lib/js/js_exn.js
index 07a77f6631..119fd1a8ea 100644
--- a/lib/js/js_exn.js
+++ b/lib/js/js_exn.js
@@ -5,9 +5,8 @@ let Primitive_option = require("./primitive_option.js");
 let $$Error = "JsError";
 
 function asJsExn(exn) {
-  let t = exn;
-  if (t.RE_EXN_ID === $$Error) {
-    return Primitive_option.some(t._1);
+  if (exn.RE_EXN_ID === $$Error) {
+    return Primitive_option.some(exn._1);
   }
   
 }
diff --git a/lib/js/obj.js b/lib/js/obj.js
index fffba72d0c..ae1b9f17e6 100644
--- a/lib/js/obj.js
+++ b/lib/js/obj.js
@@ -1,49 +1 @@
-'use strict';
-
-
-function magic(prim) {
-  return prim;
-}
-
-function repr(prim) {
-  return prim;
-}
-
-function obj(prim) {
-  return prim;
-}
-
-function tag(prim) {
-  return prim.TAG;
-}
-
-function size(prim) {
-  return prim.length | 0;
-}
-
-function field(prim0, prim1) {
-  return prim0[prim1];
-}
-
-function set_field(prim0, prim1, prim2) {
-  prim0[prim1] = prim2;
-}
-
-function dup(prim) {
-  return {...prim};
-}
-
-function update_dummy(prim0, prim1) {
-  Object.assign(prim0, prim1);
-}
-
-exports.magic = magic;
-exports.repr = repr;
-exports.obj = obj;
-exports.tag = tag;
-exports.size = size;
-exports.field = field;
-exports.set_field = set_field;
-exports.dup = dup;
-exports.update_dummy = update_dummy;
-/* No side effect */
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/lib/js/string.js b/lib/js/string.js
index b9e2c62b0e..1ac454fdd7 100644
--- a/lib/js/string.js
+++ b/lib/js/string.js
@@ -17,18 +17,6 @@ function concat(sep, xs) {
   return $$Array.of_list(xs).join(sep);
 }
 
-function length(prim) {
-  return prim.length;
-}
-
-function get(prim0, prim1) {
-  return prim0.codePointAt(prim1);
-}
-
-function unsafe_get(prim0, prim1) {
-  return prim0.codePointAt(prim1);
-}
-
 function bos(str) {
   return $$Array.map(str => str.codePointAt(0), Array.from(str));
 }
@@ -305,8 +293,6 @@ function split_on_char(sep, s) {
   };
 }
 
-exports.length = length;
-exports.get = get;
 exports.make = make;
 exports.init = init;
 exports.sub = sub;
@@ -332,5 +318,4 @@ exports.lowercase_ascii = lowercase_ascii;
 exports.capitalize_ascii = capitalize_ascii;
 exports.uncapitalize_ascii = uncapitalize_ascii;
 exports.split_on_char = split_on_char;
-exports.unsafe_get = unsafe_get;
 /* No side effect */
diff --git a/runtime/char.res b/runtime/char.res
index a80fc11663..a3f1b05ecc 100644
--- a/runtime/char.res
+++ b/runtime/char.res
@@ -14,7 +14,7 @@ let chr = unsafe_chr
 
 external bytes_create: int => array<char> = "Array"
 
-let bytes_unsafe_set = Primitive_array_extern.setUnsafe
+external bytes_unsafe_set: (array<'a>, int, 'a) => unit = "%array_unsafe_set"
 
 @scope("String") @variadic
 external unsafe_to_string: array<char> => string = "fromCodePoint"
diff --git a/runtime/obj.res b/runtime/obj.res
index d794096fad..eacdb95a6e 100644
--- a/runtime/obj.res
+++ b/runtime/obj.res
@@ -4,28 +4,28 @@
 
 type t = Primitive_object_extern.t
 
-let magic = Primitive_object_extern.magic
+external magic: 'a => 'b = "%identity"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let repr = Primitive_object_extern.repr
+external repr: 'a => t = "%identity"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let obj = Primitive_object_extern.obj
+external obj: t => 'a = "%identity"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let tag = Primitive_object_extern.tag
+external tag: t => int = "%obj_tag"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let size = Primitive_object_extern.size
+external size: t => int = "%obj_size"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let field = Primitive_object_extern.getField
+external getField: (t, 'a) => t = "%obj_get_field"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let set_field = Primitive_object_extern.setField
+external setField: (t, 'a, t) => unit = "%obj_set_field"
 
 @deprecated("Do not use directly. This will be removed in v13")
-let dup = Primitive_object_extern.dup
+external dup: t => t = "%obj_dup"
 
-@deprecated("Do not use directly. This will be removed in v13")
-let update_dummy = Primitive_object_extern.updateDummy
+@deprecated("Do not use directly. This will be removed in v13") @scope("Object")
+external updateDummy: (t, t) => unit = "assign"
diff --git a/runtime/primitive_object.res b/runtime/primitive_object.res
index 3324543da4..ae655a20d5 100644
--- a/runtime/primitive_object.res
+++ b/runtime/primitive_object.res
@@ -27,6 +27,8 @@ module Js = Primitive_js_extern
 
 type t = Primitive_object_extern.t
 
+// Note: this only works as intended as long as the runtime is compiled
+// with -bs-cross-module-opt.
 let repr = Primitive_object_extern.repr
 let magic = Primitive_object_extern.magic
 let tag = Primitive_object_extern.tag
diff --git a/runtime/string.res b/runtime/string.res
index 11fa4877f1..3613f1dde4 100644
--- a/runtime/string.res
+++ b/runtime/string.res
@@ -30,11 +30,11 @@ module B = {
 
 let concat = (sep: string, xs: list<string>) => xs->Array.of_list->join(sep)
 
-let length = Primitive_string_extern.length
+external length: string => int = "%string_length"
 
-let get = Primitive_string_extern.getCharUnsafe
+@send external get: (string, int) => char = "codePointAt"
 
-let unsafe_get = Primitive_string_extern.getCharUnsafe
+@send external unsafe_get: (string, int) => char = "codePointAt"
 
 @scope("Array") external bos: string => array<string> = "from"
 let bos = str => B.map(str => str->unsafe_get(0), str->bos)
diff --git a/runtime/string.resi b/runtime/string.resi
index 8ef4fbbc7d..a5bc660235 100644
--- a/runtime/string.resi
+++ b/runtime/string.resi
@@ -6,14 +6,15 @@
 
 /** Return the length (number of characters) of the given string. */
 @deprecated("Use Core instead. This will be removed in v13")
-let length: string => int
+external length: string => int = "%string_length"
 
 /** [String.get s n] returns the character at index [n] in string [s].
    You can also write [s.[n]] instead of [String.get s n].
 
    Raise [Invalid_argument] if [n] not a valid index in [s]. */
 @deprecated("Use Core instead. This will be removed in v13")
-let get: (string, int) => char
+@send
+external get: (string, int) => char = "codePointAt"
 
 /** [String.make n c] returns a fresh string of length [n],
    filled with the character [c]. */
@@ -245,5 +246,5 @@ external equal: (t, t) => bool = "%equal"
 let split_on_char: (char, string) => list<string>
 
 /* The following is for system use only. Do not call directly. */
-@deprecated("Use Core instead. This will be removed in v13")
-let unsafe_get: (string, int) => char
+@deprecated("Use Core instead. This will be removed in v13") @send
+external unsafe_get: (string, int) => char = "codePointAt"
diff --git a/tests/gentype_tests/typescript-react-example/src/Hooks.res.js b/tests/gentype_tests/typescript-react-example/src/Hooks.res.js
index d4e7549a27..669725e3fa 100644
--- a/tests/gentype_tests/typescript-react-example/src/Hooks.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/Hooks.res.js
@@ -1,6 +1,5 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Obj from "rescript/lib/es6/obj.js";
 import * as React from "react";
 import * as ImportHooks from "./ImportHooks.res.js";
 import * as Primitive_option from "rescript/lib/es6/primitive_option.js";
@@ -149,7 +148,7 @@ let testForwardRef = React.forwardRef((x, y) => makeWithRef(x)(y));
 
 let input = React.forwardRef((r, ref) => JsxRuntime.jsx("div", {
   children: r.x,
-  ref: Primitive_option.some(Obj.magic(ref))
+  ref: Primitive_option.some(ref)
 }));
 
 let ForwardRef = {
diff --git a/tests/tests/src/module_missing_conversion.js b/tests/tests/src/module_missing_conversion.js
index bc223525cb..6ccd36f3f2 100644
--- a/tests/tests/src/module_missing_conversion.js
+++ b/tests/tests/src/module_missing_conversion.js
@@ -3,7 +3,6 @@
 
 let $$String = require("rescript/lib/js/string.js");
 let Belt_Array = require("rescript/lib/js/belt_Array.js");
-let Primitive_array = require("rescript/lib/js/primitive_array.js");
 
 function f(x) {
   return x;
@@ -87,11 +86,9 @@ let XX = {
 
 let u = [$$String];
 
-let $$String$1 = Primitive_array.get(u, 0);
-
-let hh = $$String$1.length("x");
+let hh = "x".length;
 
 exports.XX = XX;
 exports.u = u;
 exports.hh = hh;
-/* hh Not a pure module */
+/* No side effect */
diff --git a/tests/tests/src/module_parameter_test.js b/tests/tests/src/module_parameter_test.js
index 0d923d5e38..ecfda775fd 100644
--- a/tests/tests/src/module_parameter_test.js
+++ b/tests/tests/src/module_parameter_test.js
@@ -14,10 +14,10 @@ let N = {
   s: s
 };
 
-let v0 = s.length("x");
+let v0 = "x".length;
 
 function v(x) {
-  return s.length(x);
+  return x.length;
 }
 
 let suites_0 = [
@@ -53,4 +53,4 @@ exports.N = N;
 exports.v0 = v0;
 exports.v = v;
 exports.suites = suites;
-/* v0 Not a pure module */
+/*  Not a pure module */
diff --git a/tests/tests/src/obj_magic_test.js b/tests/tests/src/obj_magic_test.js
index 3485e5f0f1..7e6071f1f2 100644
--- a/tests/tests/src/obj_magic_test.js
+++ b/tests/tests/src/obj_magic_test.js
@@ -12,7 +12,7 @@ let suites_0 = [
   param => ({
     TAG: "Eq",
     _0: false,
-    _1: is_block(3)
+    _1: "number" !== "number"
   })
 ];
 
@@ -22,10 +22,10 @@ let suites_1 = {
     param => ({
       TAG: "Eq",
       _0: true,
-      _1: is_block({
+      _1: typeof ({
         hd: 3,
         tl: /* [] */0
-      })
+      }) !== "number"
     })
   ],
   tl: {
@@ -34,7 +34,7 @@ let suites_1 = {
       param => ({
         TAG: "Eq",
         _0: true,
-        _1: is_block("x")
+        _1: "string" !== "number"
       })
     ],
     tl: {
@@ -43,7 +43,7 @@ let suites_1 = {
         param => ({
           TAG: "Eq",
           _0: false,
-          _1: is_block(3.0)
+          _1: "number" !== "number"
         })
       ],
       tl: /* [] */0
diff --git a/tests/tests/src/reasonReact.js b/tests/tests/src/reasonReact.js
index daf33e32c8..afbce1d035 100644
--- a/tests/tests/src/reasonReact.js
+++ b/tests/tests/src/reasonReact.js
@@ -84,8 +84,7 @@ function element(keyOpt, refOpt, component) {
 }
 
 function wrapReasonForJs(component, jsPropsToReason) {
-  let jsPropsToReason$1 = jsPropsToReason;
-  let uncurriedJsPropsToReason = jsProps => jsPropsToReason$1(jsProps);
+  let uncurriedJsPropsToReason = jsProps => jsPropsToReason(jsProps);
   component.reactClassInternal.prototype.jsPropsToReason = uncurriedJsPropsToReason;
   return component.reactClassInternal;
 }
diff --git a/tests/tests/src/rec_module_opt.js b/tests/tests/src/rec_module_opt.js
index b5c4a5bfd7..51401e97a4 100644
--- a/tests/tests/src/rec_module_opt.js
+++ b/tests/tests/src/rec_module_opt.js
@@ -1,7 +1,6 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 'use strict';
 
-let Obj = require("rescript/lib/js/obj.js");
 let Belt_Id = require("rescript/lib/js/belt_Id.js");
 let Primitive_module = require("rescript/lib/js/primitive_module.js");
 let Primitive_string = require("rescript/lib/js/primitive_string.js");
@@ -30,7 +29,7 @@ function cmp(t1, t2) {
   } else if (t2.TAG === "Leaf") {
     return -1;
   } else {
-    return Obj.magic(AComparable.cmp)(t1._0, t2._0);
+    return AComparable.cmp(t1._0, t2._0);
   }
 }