Remove the unused res.iflet encoding of if let

CHANGELOG.md

@@ -23,6 +23,7 @@
 
 #### :bug: Bug fix
 
+- Keep `@warning("-4")` on `if` expressions when formatting; the formatter dropped it. https://github.com/rescript-lang/rescript/pull/8741
 - Fix the formatter dropping a `@res.ternary` attribute written in source, e.g. `let b = @res.ternary x`, and print `@JSX` and `@res.ternary` with the same parens as other attributes, e.g. `(@JSX x) + 1`. https://github.com/rescript-lang/rescript/pull/8740
 - Respect `@warning` attributes on the module in `module type of`, e.g. `module type of @warning("-3") DeprecatedModule`, which still reported the warning. https://github.com/rescript-lang/rescript/pull/8738
 - Fix the formatter dropping or moving attributes and `await` on module expressions, such as `module M = @attr F(X)`, and printing invalid code for module constraints and functors in some positions, such as `module M: T = (X: S)`. https://github.com/rescript-lang/rescript/pull/8735
@@ -46,6 +47,7 @@
 
 #### :house: Internal
 
+- Remove the unused `res.iflet` encoding of `if let`, which the parser always rejects with a syntax error suggesting a `switch`. The error is unchanged. https://github.com/rescript-lang/rescript/pull/8741
 - Remove the obsolete `res.ternary` and `JSX` attributes from the formatter's internal attribute filters and merge the filters into one. https://github.com/rescript-lang/rescript/pull/8740
 - Represent `await` on module expressions with a dedicated `Pmod_await` parsetree node instead of a `res.await` attribute, which remains only in the PPX wire representation. https://github.com/rescript-lang/rescript/pull/8738
 - Remove dead compiler code: warning 24, the `J.String_index` JS IR node, the `Pisint` and `Parraysets` Lambda primitives with their unused `%` primitive names (`%obj_is_int`, `%obj_dup`, `%obj_field`, `%array_safe_set`, `%null_to_opt`, `%nullable_to_opt`), the no-op `-pp` bsc flag and unread `Clflags` fields (`-unsafe` stays accepted as an explicit no-op), and the unused `unit` slot of `Pexp_coerce`. https://github.com/rescript-lang/rescript/pull/8731

compiler/syntax/src/res_comments_table.ml

@@ -1381,56 +1381,6 @@ and walk_expression expr t comments =
     attach t.leading mod_expr.pmod_loc before;
     walk_module_expr mod_expr t inside;
     attach t.trailing mod_expr.pmod_loc after
-  | Pexp_match (expr1, [case; else_branch])
-    when Res_parsetree_viewer.has_if_let_attribute expr.pexp_attributes ->
-    let before, inside, after =
-      partition_by_loc comments case.pc_lhs.ppat_loc
-    in
-    attach t.leading case.pc_lhs.ppat_loc before;
-    walk_pattern case.pc_lhs t inside;
-    let after_pat, rest =
-      partition_adjacent_trailing case.pc_lhs.ppat_loc after
-    in
-    attach t.trailing case.pc_lhs.ppat_loc after_pat;
-    let before, inside, after = partition_by_loc rest expr1.pexp_loc in
-    attach t.leading expr1.pexp_loc before;
-    walk_expression expr1 t inside;
-    let after_expr, rest = partition_adjacent_trailing expr1.pexp_loc after in
-    attach t.trailing expr1.pexp_loc after_expr;
-    let before, inside, after = partition_by_loc rest case.pc_rhs.pexp_loc in
-    let after =
-      if is_block_expr case.pc_rhs then (
-        let after_expr, rest =
-          partition_adjacent_trailing case.pc_rhs.pexp_loc after
-        in
-        walk_expression case.pc_rhs t (List.concat [before; inside; after_expr]);
-        rest)
-      else (
-        attach t.leading case.pc_rhs.pexp_loc before;
-        walk_expression case.pc_rhs t inside;
-        after)
-    in
-    let after_expr, rest =
-      partition_adjacent_trailing case.pc_rhs.pexp_loc after
-    in
-    attach t.trailing case.pc_rhs.pexp_loc after_expr;
-    let before, inside, after =
-      partition_by_loc rest else_branch.pc_rhs.pexp_loc
-    in
-    let after =
-      if is_block_expr else_branch.pc_rhs then (
-        let after_expr, rest =
-          partition_adjacent_trailing else_branch.pc_rhs.pexp_loc after
-        in
-        walk_expression else_branch.pc_rhs t
-          (List.concat [before; inside; after_expr]);
-        rest)
-      else (
-        attach t.leading else_branch.pc_rhs.pexp_loc before;
-        walk_expression else_branch.pc_rhs t inside;
-        after)
-    in
-    attach t.trailing else_branch.pc_rhs.pexp_loc after
   | Pexp_match (expr, cases) | Pexp_try (expr, cases) ->
     let before, inside, after = partition_by_loc comments expr.pexp_loc in
     let after =

compiler/syntax/src/res_core.ml

@@ -174,8 +174,7 @@ module Error_messages = struct
     "A polymorphic variant (e.g. #id) must start with an alphabetical letter \
      or be a number (e.g. #742)"
 
-  let experimental_if_let expr =
-    let switch_expr = {expr with Parsetree.pexp_attributes = []} in
+  let experimental_if_let switch_expr =
     Doc.concat
       [
         Doc.text "If-let is currently highly experimental.";
@@ -265,17 +264,8 @@ module Error_messages = struct
     "Spreading JSX children is no longer supported."
 end
 
-let if_let_attr = (Location.mknoloc "res.iflet", Parsetree.PStr [])
-let suppress_fragile_match_warning_attr =
-  ( Location.mknoloc "warning",
-    Parsetree.PStr
-      [
-        Ast_helper.Str.eval
-          (Ast_helper.Exp.constant (Ast_helper.Const.string "-4"));
-      ] )
 let make_pat_variant_spread_attr =
   (Location.mknoloc "res.patVariantSpread", Parsetree.PStr [])
-
 let spread_attr = (Location.mknoloc "res.spread", Parsetree.PStr [])
 let dict_spread_attr = (Location.mknoloc "res.dictSpread", Parsetree.PStr [])
 
@@ -3610,9 +3600,7 @@ and parse_if_let_expr start_pos p =
         (Location.mkloc [] loc)
   in
   let loc = mk_loc start_pos (Parser.position p) in
-  Ast_helper.Exp.match_
-    ~attrs:[if_let_attr; suppress_fragile_match_warning_attr]
-    ~loc condition_expr
+  Ast_helper.Exp.match_ ~loc condition_expr
     [
       Ast_helper.Exp.case pattern then_expr;
       Ast_helper.Exp.case (Ast_helper.Pat.any ()) else_expr;

compiler/syntax/src/res_parens.ml

@@ -70,8 +70,7 @@ let structure_expr expr =
   | None -> (
     match expr with
     | {pexp_desc = Pexp_jsx_element _} -> Nothing
-    | _ when Parsetree_viewer.has_attributes expr.pexp_attributes ->
-      Parenthesized
+    | _ when expr.pexp_attributes <> [] -> Parenthesized
     | {
      Parsetree.pexp_desc =
        Pexp_constraint ({pexp_desc = Pexp_pack _}, {ptyp_desc = Ptyp_package _});

compiler/syntax/src/res_parsetree_viewer.ml

@@ -223,8 +223,7 @@ let is_parsing_attr (attr : Parsetree.attribute) =
   match attr with
   | ( {
         Location.txt =
-          ( "res.iflet" | "res.patVariantSpread" | "res.dictPattern"
-          | "res.dictSpread" );
+          "res.patVariantSpread" | "res.dictPattern" | "res.dictSpread";
       },
       _ ) ->
     true
@@ -365,39 +364,6 @@ let flattenable_operators parent_operator child_operator =
       && is_equality_operator child_operator)
   else false
 
-let rec has_if_let_attribute attrs =
-  match attrs with
-  | [] -> false
-  | ({Location.txt = "res.iflet"}, _) :: _ -> true
-  | _ :: attrs -> has_if_let_attribute attrs
-
-let is_if_let_expr expr =
-  match expr with
-  | {pexp_attributes = attrs; pexp_desc = Pexp_match _}
-    when has_if_let_attribute attrs ->
-    true
-  | _ -> false
-
-let has_attributes attrs =
-  List.exists
-    (fun attr ->
-      match attr with
-      | {Location.txt = "res.iflet"}, _ -> false
-      (* Remove the fragile pattern warning for iflet expressions *)
-      | ( {Location.txt = "warning"},
-          PStr
-            [
-              {
-                pstr_desc =
-                  Pstr_eval
-                    ({pexp_desc = Pexp_constant (Pconst_string payload)}, _);
-              };
-            ] ) ->
-        String_literal.string_semantic payload <> "-4"
-        || not (has_if_let_attribute attrs)
-      | _ -> true)
-    attrs
-
 let is_array_access expr =
   match expr.pexp_desc with
   | Pexp_apply
@@ -413,43 +379,15 @@ let is_array_access expr =
     true
   | _ -> false
 
-type if_condition_kind =
-  | If of Parsetree.expression
-  | IfLet of Parsetree.pattern * Parsetree.expression
-
 let collect_if_expressions expr =
   let rec collect acc expr =
     let expr_loc = expr.pexp_loc in
     match expr.pexp_desc with
     | Pexp_ifthenelse (if_expr, then_expr, Some else_expr) ->
-      collect ((expr_loc, If if_expr, then_expr) :: acc) else_expr
+      collect ((expr_loc, if_expr, then_expr) :: acc) else_expr
     | Pexp_ifthenelse (if_expr, then_expr, (None as else_expr)) ->
-      let ifs = List.rev ((expr_loc, If if_expr, then_expr) :: acc) in
+      let ifs = List.rev ((expr_loc, if_expr, then_expr) :: acc) in
       (ifs, else_expr)
-    | Pexp_match
-        ( condition,
-          [
-            {pc_lhs = pattern; pc_guard = None; pc_rhs = then_expr};
-            {
-              pc_rhs =
-                {pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, _)};
-            };
-          ] )
-      when is_if_let_expr expr ->
-      let ifs =
-        List.rev ((expr_loc, IfLet (pattern, condition), then_expr) :: acc)
-      in
-      (ifs, None)
-    | Pexp_match
-        ( condition,
-          [
-            {pc_lhs = pattern; pc_guard = None; pc_rhs = then_expr};
-            {pc_rhs = else_expr};
-          ] )
-      when is_if_let_expr expr ->
-      collect
-        ((expr_loc, IfLet (pattern, condition), then_expr) :: acc)
-        else_expr
     | _ -> (List.rev acc, Some expr)
   in
   collect [] expr
@@ -475,23 +413,6 @@ let parameters_should_hug parameters =
     true
   | _ -> false
 
-let filter_fragile_match_attributes attrs =
-  List.filter
-    (fun attr ->
-      match attr with
-      | ( {Location.txt = "warning"},
-          PStr
-            [
-              {
-                pstr_desc =
-                  Pstr_eval
-                    ({pexp_desc = Pexp_constant (Pconst_string payload)}, _);
-              };
-            ] ) ->
-        String_literal.string_semantic payload <> "-4"
-      | _ -> true)
-    attrs
-
 let should_indent_binary_expr expr =
   let same_precedence_sub_expression operator sub_expression =
     match sub_expression with

compiler/syntax/src/res_parsetree_viewer.mli

@@ -23,16 +23,12 @@ type dict_expr_part =
   | DictExprRows of Parsetree.expression
   | DictExprSpread of Parsetree.expression
 
-type if_condition_kind =
-  | If of Parsetree.expression
-  | IfLet of Parsetree.pattern * Parsetree.expression
-
 (* if ... else if ... else ... is represented as nested expressions: if ... else { if ... }
  * The purpose of this function is to flatten nested ifs into one sequence.
  * Basically compute: ([if, else if, else if, else if], else) *)
 val collect_if_expressions :
   Parsetree.expression ->
-  (Location.t * if_condition_kind * Parsetree.expression) list
+  (Location.t * Parsetree.expression * Parsetree.expression) list
   * Parsetree.expression option
 
 val collect_array_expressions :
@@ -77,21 +73,15 @@ val is_equality_operator : string -> bool
 
 val flattenable_operators : string -> string -> bool
 
-val has_attributes : Parsetree.attributes -> bool
-
 val is_array_access : Parsetree.expression -> bool
 val is_ternary_expr : Parsetree.expression -> bool
-val is_if_let_expr : Parsetree.expression -> bool
 
 val collect_ternary_parts :
   Parsetree.expression ->
   (Parsetree.expression * Parsetree.expression) list * Parsetree.expression
 
 val parameters_should_hug : fun_param_kind list -> bool
 
-val filter_fragile_match_attributes :
-  Parsetree.attributes -> Parsetree.attributes
-
 val should_indent_binary_expr : Parsetree.expression -> bool
 val should_inline_rhs_binary_expr : Parsetree.expression -> bool
 val has_printable_attributes : Parsetree.attributes -> bool
@@ -159,8 +149,6 @@ val rewrite_underscore_apply_in_pipe :
 (* (__x) => f(a, __x, c) -----> f(a, _, c)  *)
 val is_underscore_apply_sugar : Parsetree.expression -> bool
 
-val has_if_let_attribute : Parsetree.attributes -> bool
-
 val is_rewritten_underscore_apply_sugar : Parsetree.expression -> bool
 
 val is_fun_expr : Parsetree.expression -> bool