Declare exports left unbound by a toplevel throw

CHANGELOG.md

@@ -21,6 +21,7 @@
 #### :bug: Bug fix
 
 - Make rewatch compile independent modules after an unrelated failure and recompile blocked dependents when a changed interface survives a failed implementation, including across full watcher rebuilds. https://github.com/rescript-lang/rescript/pull/8667
+- Fix invalid JavaScript that exported names left unbound when a module's toplevel always throws. https://github.com/rescript-lang/rescript/pull/8692
 - Make module inclusion error messages independent of the length of the source file path. https://github.com/rescript-lang/rescript/pull/8691
 
 #### :memo: Documentation

compiler/core/lam_compile_main.ml

@@ -63,6 +63,24 @@ let compile_group output_prefix (meta : Lam_stats.t) (x : Lam_group.t) :
       }
       lam
 
+(* When a toplevel group always throws, [Js_output.concat] drops every later
+   group, and a [let] whose right-hand side throws compiles to the [throw]
+   alone. The identifiers those groups bind are then undeclared, and an export
+   list naming them is a JavaScript syntax error. This appends a declaration
+   for each such exported identifier after the [throw]. *)
+let declare_undeclared_exports (exports : Ident.t list) (block : J.block) :
+    J.block =
+  let declared =
+    Ext_list.fold_left block Set_ident.empty (fun acc (stmt : J.statement) ->
+        match stmt.statement_desc with
+        | Variable {ident} -> Set_ident.add acc ident
+        | _ -> acc)
+  in
+  block
+  @ Ext_list.filter_map exports (fun id ->
+      if Set_ident.mem declared id then None
+      else Some (Js_stmt_make.declare_variable ~kind:Strict id))
+
 (** Also need analyze its depenency is pure or not *)
 let no_side_effects (rest : Lam_group.t list) : string option =
   Ext_list.find_opt rest (fun x ->
@@ -355,8 +373,14 @@ let compile (output_prefix : string) export_idents hoisted (lam : Lambda.t) =
         (Sys.time () *. 1000.)
   in
   let body =
-    Ext_list.map groups (fun group -> compile_group output_prefix meta group)
-    |> Js_output.concat |> Js_output.output_as_block
+    let output =
+      Ext_list.map groups (fun group -> compile_group output_prefix meta group)
+      |> Js_output.concat
+    in
+    let block = Js_output.output_as_block output in
+    if output.output_finished = True then
+      declare_undeclared_exports meta.exports block
+    else block
   in
   let () =
     if debug_ir then

tests/tests/src/conditional/cond_a.mjs

@@ -11,6 +11,8 @@ throw {
   Error: new Error()
 };
 
+let u;
+
 export {
   u,
 }

tests/tests/src/conditional/cond_a_load_test.mjs

@@ -0,0 +1,44 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+import * as Mocha from "mocha";
+import * as Pervasives from "@rescript/runtime/lib/es6/Pervasives.mjs";
+import * as Primitive_exceptions from "@rescript/runtime/lib/es6/Primitive_exceptions.mjs";
+
+async function loadCondANone() {
+  let M = await import("./cond_a_none.mjs");
+  return M.A.u;
+}
+
+async function raisesAssertFailure(load) {
+  try {
+    await load();
+    return false;
+  } catch (raw_exn) {
+    let exn = Primitive_exceptions.internalToException(raw_exn);
+    if (exn.RE_EXN_ID === "Assert_failure") {
+      return true;
+    }
+    throw exn;
+  }
+}
+
+Mocha.describe("Cond_a_load_test", () => {
+  Mocha.test("cond_a loads as a module whose body throws Assert_failure", async () => {
+    let ok = await raisesAssertFailure(() => import("./cond_a.mjs").then(m => m.u));
+    if (!ok) {
+      return Pervasives.failwith("cond_a did not throw Assert_failure on load");
+    }
+  });
+  Mocha.test("cond_a_none loads as a module whose body throws Assert_failure", async () => {
+    let ok = await raisesAssertFailure(loadCondANone);
+    if (!ok) {
+      return Pervasives.failwith("cond_a_none did not throw Assert_failure on load");
+    }
+  });
+});
+
+export {
+  loadCondANone,
+  raisesAssertFailure,
+}
+/*  Not a pure module */

tests/tests/src/conditional/cond_a_load_test.res

@@ -0,0 +1,30 @@
+open Mocha
+
+@module("mocha")
+external testAsync: (string, unit => promise<unit>) => unit = "test"
+
+let loadCondANone = async () => {
+  module M = await Cond_a_none
+  M.A.u
+}
+
+let raisesAssertFailure = async load =>
+  switch await load() {
+  | _ => false
+  | exception Assert_failure(_) => true
+  }
+
+describe(__MODULE__, () => {
+  testAsync("cond_a loads as a module whose body throws Assert_failure", async () => {
+    let ok = await raisesAssertFailure(() => import(Cond_a.u))
+    if !ok {
+      failwith("cond_a did not throw Assert_failure on load")
+    }
+  })
+  testAsync("cond_a_none loads as a module whose body throws Assert_failure", async () => {
+    let ok = await raisesAssertFailure(loadCondANone)
+    if !ok {
+      failwith("cond_a_none did not throw Assert_failure on load")
+    }
+  })
+})

tests/tests/src/conditional/cond_a_none.mjs

@@ -11,6 +11,8 @@ throw {
   Error: new Error()
 };
 
+let A;
+
 export {
   A,
 }