Fix dynamic Import issue with function-defined externals

CHANGELOG.md

@@ -16,6 +16,7 @@
 
 - Use FORCE_COLOR environmental variable to force colorized output https://github.com/rescript-lang/rescript-compiler/pull/7033
 - Allow spreads of variants in patterns (`| ...someVariant as v => `) when the variant spread is a subtype of the variant matched on. https://github.com/rescript-lang/rescript-compiler/pull/6721
+- Fix the issue where dynamic imports are not working for function-defined externals. https://github.com/rescript-lang/rescript-compiler/pull/7060 
 
 #### :bug: Bug fix
 

jscomp/core/lam_compile.ml

@@ -1625,6 +1625,60 @@ and compile_prim (prim_info : Lam.prim_info)
       Js_output.output_of_block_and_expression lambda_cxt.continuation
         (Ext_list.concat_append args_block block)
         exp
+  | { primitive = Pimport; args = [] | _ :: _ :: _; loc } -> 
+    Location.raise_errorf ~loc
+      "Missing argument: Dynamic import requires a module or \
+      module value that is a file as argument."
+  | { primitive = Pimport as primitive; args = [ mod_ ]; loc} ->
+      (match mod_ with
+      | Lglobal_module _ | Lvar _
+      | Lprim { primitive = Pfield _ | Pjs_call _ ; _ } ->
+        let args_block, args_expr =
+          let new_cxt =
+            { lambda_cxt with continuation = NeedValue Not_tail }
+          in
+          match compile_lambda new_cxt mod_ with
+          | { block; value = Some b; _ } -> ([ block ], b)
+          | { value = None; _ } -> assert false
+        in
+        let args_code : J.block = List.concat args_block in
+        let exp =
+          Lam_compile_primitive.translate output_prefix loc lambda_cxt primitive [args_expr]
+        in
+        Js_output.output_of_block_and_expression lambda_cxt.continuation args_code
+        exp
+      | Lfunction {
+          body =
+            ( (Lprim _ as body)
+            | Lsequence ((Lprim _ as body), Lconst Const_js_undefined _) );
+          _;
+        } ->
+        let body = match body with
+          | Lprim ({ primitive = Pjs_call prim_info; args; loc }) -> 
+            Lam.prim 
+              ~primitive:(Lam_primitive.Pjs_call { prim_info with dynamic_import = true }) 
+              ~args
+              loc
+          | _ -> body
+        in
+        let args_block, args_expr =
+          let new_cxt =
+            { lambda_cxt with continuation = NeedValue Not_tail }
+          in
+          match compile_lambda new_cxt body with
+          | { block; value = Some b; _ } -> ([ block ], b)
+          | { value = None; _ } -> assert false
+        in
+        let args_code : J.block = List.concat args_block in
+        let exp =
+          Lam_compile_primitive.translate output_prefix loc lambda_cxt primitive [args_expr]
+        in
+        Js_output.output_of_block_and_expression lambda_cxt.continuation args_code
+          exp
+      | _ ->
+        Location.raise_errorf ~loc
+          "Invalid argument: unsupported argument to dynamic import. If \
+            you believe this should be supported, please open an issue.")
   | { primitive; args; loc } ->
       let args_block, args_expr =
         if args = [] then ([], [])

jscomp/core/lam_compile_primitive.ml

@@ -38,6 +38,7 @@ let ensure_value_unit (st : Lam_compile_context.continuation) e : E.t =
 
 let module_of_expression = function
   | J.Var (J.Qualified (module_id, value)) -> [ (module_id, value) ]
+  | J.Call ({expression_desc = (J.Var (J.Qualified (module_id, value)))}, _, _) -> [ (module_id, value) ]
   | _ -> []
 
 let get_module_system () =

jscomp/test/import_external.res

@@ -2,3 +2,8 @@
 external makeA: string = "default"
 
 let f8 = Js.import(makeA)
+
+@module("b")
+external makeB: string => unit = "default"
+
+let f9 = Js.import(makeB)