Fix formatting of external with as attribute and _ placeholder (rescript-lang#6970)

* Handle different arity when arrow has as attribute during printing

* Small cleanup

* Add tests

* Update CHANGELOG

---------

Co-authored-by: Shulhi Sapli <shulhi@mba-13.local>

Author: shulhi

CHANGELOG.md

@@ -29,6 +29,7 @@
 - Fix issue where long layout break added a trailing comma in partial application `...`. https://github.com/rescript-lang/rescript-compiler/pull/6949
 - Fix incorrect format of function under unary operator. https://github.com/rescript-lang/rescript-compiler/pull/6953
 - Fix incorrect incorrect printing of module binding with signature. https://github.com/rescript-lang/rescript-compiler/pull/6963
+- Fix incorrect printing of external with `@as` attribute and `_` placholder (fixed argument). https://github.com/rescript-lang/rescript-compiler/pull/6970
 
 #### :house: Internal
 

jscomp/syntax/src/res_parsetree_viewer.ml

@@ -1,9 +1,12 @@
 open Parsetree
 
 let arrow_type ?(arity = max_int) ct =
+  let has_as_attr attrs =
+    Ext_list.exists attrs (fun (x, _) -> x.Asttypes.txt = "as")
+  in
   let rec process attrs_before acc typ arity =
     match typ with
-    | typ when arity <= 0 -> (attrs_before, List.rev acc, typ)
+    | typ when arity < 0 -> (attrs_before, List.rev acc, typ)
     | {
      ptyp_desc = Ptyp_arrow ((Nolabel as lbl), typ1, typ2);
      ptyp_attributes = [];
@@ -25,8 +28,20 @@ let arrow_type ?(arity = max_int) ct =
      ptyp_desc = Ptyp_arrow (((Labelled _ | Optional _) as lbl), typ1, typ2);
      ptyp_attributes = attrs;
     } ->
+      (* Res_core.parse_es6_arrow_type has a workaround that removed an extra arity for the function if the
+         argument is a Ptyp_any with @as attribute i.e. ~x: @as(`{prop: value}`) _.
+
+         When this case is encountered we add that missing arity so the arrow is printed properly.
+      *)
+      let arity =
+        match typ1 with
+        | {ptyp_desc = Ptyp_any; ptyp_attributes = attrs1}
+          when has_as_attr attrs1 ->
+          arity
+        | _ -> arity - 1
+      in
       let arg = (attrs, lbl, typ1) in
-      process attrs_before (arg :: acc) typ2 (arity - 1)
+      process attrs_before (arg :: acc) typ2 arity
     | typ -> (attrs_before, List.rev acc, typ)
   in
   match ct with

jscomp/syntax/tests/printer/other/attributes.res

@@ -24,3 +24,30 @@ let x = 1
 )
 let x = 1
 
+@obj external ff: (~x: int, ~g: int, ~h: int) => _ = ""
+
+@obj external ff: (~x: int, ~g: int, ~h: @as(3) _) => _ = ""
+
+@obj external ff: (~x: int) => (~h: @as(3) _) => _ = ""
+
+@obj
+external ff: (
+  ~lo: @as(3) _,
+  ~mid: @as(3) _,
+  ~hi: int,
+) => _ = ""
+
+
+@obj
+external ff: (
+  ~hi: int,
+  ~lo: @as(3) _
+) => _ = ""
+
+@obj
+external ff: (
+  ~hi: int,
+  ~mid: @as(3) _,
+  ~lo: @as(3) _
+) => _ = ""
+

jscomp/syntax/tests/printer/other/expected/attributes.res.txt

@@ -22,3 +22,18 @@ let x = 1
 
 @inlinePrivate(@module("./logo.svg") external logo: string = "default")
 let x = 1
+
+@obj external ff: (~x: int, ~g: int, ~h: int) => _ = ""
+
+@obj external ff: (~x: int, ~g: int, ~h: @as(3) _) => _ = ""
+
+@obj external ff: (~x: int) => (~h: @as(3) _) => _ = ""
+
+@obj
+external ff: (~lo: @as(3) _, ~mid: @as(3) _, ~hi: int) => _ = ""
+
+@obj
+external ff: (~hi: int, ~lo: @as(3) _) => _ = ""
+
+@obj
+external ff: (~hi: int, ~mid: @as(3) _, ~lo: @as(3) _) => _ = ""