Fix @val shadowing (rewrite using globalThis) (#8098)

* Fix shadowing @Val bindings (rewrite using globalThis)

* Add example with @new

* CHANGELOG

Author: cknitt

CHANGELOG.md

@@ -24,6 +24,7 @@
 
 - Reanalyze: make optional args analysis liveness-aware, preventing false positives when functions are only called from dead code. https://github.com/rescript-lang/rescript/pull/8082
 - Fix: do not warn for "editor" field in `rescript.json`. https://github.com/rescript-lang/rescript/pull/8084
+- Fix `@val` shadowing (rewrite using `globalThis`). https://github.com/rescript-lang/rescript/pull/8098
 
 #### :memo: Documentation
 

compiler/core/js_pass_external_shadow.ml

@@ -0,0 +1,73 @@
+(* Copyright (C) 2025 - Authors of ReScript
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * In addition to the permissions granted to you by the LGPL, you may combine
+ * or link a "work that uses the Library" with a publicly distributed version
+ * of this file to produce a combined library or application, then distribute
+ * that combined work under the terms of your choosing, with no requirement
+ * to comply with the obligations normally placed on you by section 4 of the
+ * LGPL version 3 (or the corresponding section of a later version of the LGPL
+ * should you choose to use a later version).
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. *)
+
+module E = Js_exp_make
+
+let global_this = E.js_global "globalThis"
+
+(* Only rewrite for lexical bindings (let-style). *)
+let is_lexical_binding_kind (property : J.property) =
+  match property with
+  | Strict | StrictOpt | Alias -> true
+  | Variable -> false
+
+(* Skip JS globals/keywords and compiler-reserved JS ids. *)
+let should_rewrite_binding (ident : Ident.t) =
+  (not (Ext_ident.is_js ident))
+  && (not (Js_reserved_map.is_js_keyword ident.name))
+  && not (Js_reserved_map.is_js_global ident.name)
+
+let rewrite_shadowed_global_in_expr ~(name : string) (expr : J.expression) :
+    J.expression =
+  let super = Js_record_map.super in
+  let self =
+    {
+      super with
+      expression =
+        (fun self expr ->
+          match expr.expression_desc with
+          | Var (Id id) when Ext_ident.is_js id && String.equal id.name name ->
+            E.dot global_this name
+          | _ -> super.expression self expr);
+    }
+  in
+  self.expression self expr
+
+let program (js : J.program) : J.program =
+  let super = Js_record_map.super in
+  let self =
+    {
+      super with
+      variable_declaration =
+        (fun self (vd : J.variable_declaration) ->
+          match vd with
+          | {ident; value = Some expr; property}
+            when is_lexical_binding_kind property
+                 && should_rewrite_binding ident ->
+            let expr = rewrite_shadowed_global_in_expr ~name:ident.name expr in
+            {vd with value = Some expr}
+          | _ -> super.variable_declaration self vd);
+    }
+  in
+  self.program self js

compiler/core/js_pass_external_shadow.mli

@@ -0,0 +1,26 @@
+(* Copyright (C) 2025 - Authors of ReScript
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * In addition to the permissions granted to you by the LGPL, you may combine
+ * or link a "work that uses the Library" with a publicly distributed version
+ * of this file to produce a combined library or application, then distribute
+ * that combined work under the terms of your choosing, with no requirement
+ * to comply with the obligations normally placed on you by section 4 of the
+ * LGPL version 3 (or the corresponding section of a later version of the LGPL
+ * should you choose to use a later version).
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. *)
+
+(* Rewrites @val/@new shadowing to avoid reading a let before it is set. *)
+val program : J.program -> J.program

compiler/core/lam_compile_main.ml

@@ -242,7 +242,9 @@ in
 js 
 |> _j "initial"
 |> Js_pass_flatten.program
-|> _j "flattern"
+|> _j "flatten"
+|> Js_pass_external_shadow.program
+|> _j "external_shadow"
 |> Js_pass_tailcall_inline.tailcall_inline
 |> _j "inline_and_shake"
 |> Js_pass_flatten_and_mark_dead.program

tests/tests/src/ExternalShadow.mjs

@@ -0,0 +1,29 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+
+let X = {};
+
+let process = globalThis.process;
+
+let proc = process;
+
+let New = {};
+
+function url(prim) {
+  return new URL(prim);
+}
+
+let Global = {};
+
+let $$parseInt = parseInt;
+
+export {
+  X,
+  process,
+  proc,
+  New,
+  url,
+  Global,
+  $$parseInt,
+}
+/* process Not a pure module */

tests/tests/src/ExternalShadow.res

@@ -0,0 +1,23 @@
+/* Avoid @val shadowing: `let process = X.process` must not read itself.
+   The initializer should compile to `globalThis.process`, while other
+   uses of the external stay as plain `process`. */
+module X = {
+  @val external process: unknown = "process"
+}
+
+let process = X.process /* expect `globalThis.process` */
+let proc = X.process /* expect plain `process` */
+
+/* @new does not have the same shadowing issue because it uses `new URL(...)`. */
+module New = {
+  @new external url: string => unknown = "URL"
+}
+
+let url = New.url /* expect `new URL(...)` in a wrapper function */
+
+/* Reserved JS globals should not be rewritten to globalThis. */
+module Global = {
+  @val external parseInt: unknown = "parseInt"
+}
+
+let parseInt = Global.parseInt /* expect plain `parseInt` */