Restore legacy (. ...) uncurried syntax with deprecation warning

CHANGELOG.md

@@ -40,6 +40,7 @@
 #### :nail_care: Polish
 
 - Allow builds while watchers are running. https://github.com/rescript-lang/rescript/pull/8349
+- Restore parsing of the legacy `(. ...)` uncurried syntax for backwards compatibility with libraries still on older ReScript versions; emit a deprecation warning when it is used. Rewatch also surfaces this specific deprecation when it originates from an external dependency so users can report breakage upstream. https://github.com/rescript-lang/rescript/pull/8383
 - Rewatch: replace wave-based compile scheduler with a work-stealing DAG dispatcher ordered by critical-path priority, avoiding the per-wave stall on the slowest file. https://github.com/rescript-lang/rescript/pull/8374
 
 #### :house: Internal

compiler/syntax/src/res_core.ml

@@ -285,6 +285,19 @@ let tagged_template_literal_attr =
 let spread_attr = (Location.mknoloc "res.spread", Parsetree.PStr [])
 let dict_spread_attr = (Location.mknoloc "res.dictSpread", Parsetree.PStr [])
 
+(* Emit a deprecation warning when the legacy [(. ...)] uncurried syntax is
+   encountered. Uncurried is the default since ReScript v11, so the leading
+   dot is no longer meaningful; we still accept it so dependencies on older
+   libraries keep parsing. *)
+let warn_uncurried_dot_syntax ~loc =
+  Location.prerr_warning loc
+    (Warnings.Deprecated
+       ( "The `(. ...)` uncurried syntax is deprecated. Uncurried is now the \
+          default in ReScript — remove the leading dot.",
+         loc,
+         loc,
+         false ))
+
 type argument = {label: Asttypes.arg_label; expr: Parsetree.expression}
 
 type type_parameter = {
@@ -1857,6 +1870,9 @@ and parse_es6_arrow_expression ?(arrow_attrs = []) ?(arrow_start_pos = None)
   {arrow_expr with pexp_loc = {arrow_expr.pexp_loc with loc_start = start_pos}}
 
 (*
+ * dotted_parameter ::=
+ *   | . parameter                 (* deprecated uncurried syntax *)
+ *
  * parameter ::=
  *   | pattern
  *   | pattern : type
@@ -1874,10 +1890,14 @@ and parse_es6_arrow_expression ?(arrow_attrs = []) ?(arrow_start_pos = None)
  *)
 and parse_parameter p =
   if
-    p.Parser.token = Token.Typ || p.token = Tilde
+    p.Parser.token = Token.Typ || p.token = Tilde || p.token = Dot
     || Grammar.is_pattern_start p.token
-  then
+  then (
     let start_pos = p.Parser.start_pos in
+    if p.Parser.token = Token.Dot then (
+      let dot_loc = mk_loc start_pos p.end_pos in
+      Parser.next p;
+      warn_uncurried_dot_syntax ~loc:dot_loc);
     let attrs = parse_attributes p in
     if p.Parser.token = Typ then (
       Parser.next p;
@@ -1962,7 +1982,7 @@ and parse_parameter p =
       | _ ->
         Some
           (TermParameter
-             {attrs; p_label = lbl; expr = None; pat; p_pos = start_pos})
+             {attrs; p_label = lbl; expr = None; pat; p_pos = start_pos}))
   else None
 
 and parse_parameter_list p =
@@ -1977,6 +1997,7 @@ and parse_parameter_list p =
  *   | _
  *   | lident
  *   | ()
+ *   | (.)                                   (* deprecated uncurried syntax *)
  *   | ( parameter {, parameter} [,] )
  *)
 and parse_parameters p : fundef_type_param option * fundef_term_param list =
@@ -2025,6 +2046,10 @@ and parse_parameters p : fundef_type_param option * fundef_term_param list =
       ] )
   | Lparen ->
     Parser.next p;
+    if p.Parser.token = Token.Dot then (
+      let dot_loc = mk_loc p.start_pos p.end_pos in
+      Parser.next p;
+      warn_uncurried_dot_syntax ~loc:dot_loc);
     let type_params, term_params = parse_parameter_list p in
     let term_params =
       if term_params <> [] then term_params else [unit_term_parameter ()]
@@ -4033,13 +4058,31 @@ and parse_switch_expression p =
  *   | ~ label-name = ? _           (* syntax sugar *)
  *   | ~ label-name = ? expr : type
  *
+ *  dotted_argument ::=
+ *   | . argument                   (* deprecated uncurried syntax *)
  *)
 and parse_argument p : argument option =
   if
     p.Parser.token = Token.Tilde
-    || p.token = Underscore
+    || p.token = Dot || p.token = Underscore
     || Grammar.is_expr_start p.token
-  then parse_argument2 p
+  then
+    match p.Parser.token with
+    | Dot -> (
+      let dot_loc = mk_loc p.start_pos p.end_pos in
+      Parser.next p;
+      warn_uncurried_dot_syntax ~loc:dot_loc;
+      match p.token with
+      (* apply(.) — legacy uncurried unit call *)
+      | Rparen ->
+        let unit_expr =
+          Ast_helper.Exp.construct
+            (Location.mknoloc (Longident.Lident "()"))
+            None
+        in
+        Some {label = Asttypes.Nolabel; expr = unit_expr}
+      | _ -> parse_argument2 p)
+    | _ -> parse_argument2 p
   else None
 
 and parse_argument2 p : argument option =
@@ -4796,6 +4839,9 @@ and parse_type_alias p typ =
  * note:
  *  | attrs ~ident: type_expr    -> attrs are on the arrow
  *  | attrs type_expr            -> attrs are here part of the type_expr
+ *
+ * dotted_type_parameter ::=
+ *  | . type_parameter           (* deprecated uncurried syntax *)
  *)
 and parse_type_parameter ?current_type_name_path ?inline_types_context
     ?positional_type_name_path p =
@@ -4806,8 +4852,16 @@ and parse_type_parameter ?current_type_name_path ?inline_types_context
       [doc_comment_to_attribute loc s]
     | _ -> []
   in
-  if p.Parser.token = Token.Tilde || Grammar.is_typ_expr_start p.token then
+  if
+    p.Parser.token = Token.Tilde
+    || p.token = Dot
+    || Grammar.is_typ_expr_start p.token
+  then (
     let start_pos = p.Parser.start_pos in
+    if p.Parser.token = Token.Dot then (
+      let dot_loc = mk_loc start_pos p.end_pos in
+      Parser.next p;
+      warn_uncurried_dot_syntax ~loc:dot_loc);
     let attrs = doc_attr @ parse_attributes p in
     match p.Parser.token with
     | Tilde -> (
@@ -4879,7 +4933,7 @@ and parse_type_parameter ?current_type_name_path ?inline_types_context
       let typ_with_attributes =
         {typ with ptyp_attributes = List.concat [attrs; typ.ptyp_attributes]}
       in
-      Some {attrs = []; label = Nolabel; typ = typ_with_attributes; start_pos}
+      Some {attrs = []; label = Nolabel; typ = typ_with_attributes; start_pos})
   else None
 
 (* (int, ~x:string, float) *)

compiler/syntax/src/res_grammar.ml

@@ -178,7 +178,7 @@ let is_pattern_start = function
   | _ -> false
 
 let is_parameter_start = function
-  | Token.Typ | Tilde -> true
+  | Token.Typ | Tilde | Dot -> true
   | token when is_pattern_start token -> true
   | _ -> false
 
@@ -206,7 +206,7 @@ let is_typ_expr_start = function
   | _ -> false
 
 let is_type_parameter_start = function
-  | Token.Tilde -> true
+  | Token.Tilde | Dot -> true
   | token when is_typ_expr_start token -> true
   | _ -> false
 
@@ -239,7 +239,7 @@ let is_record_row_string_key_start = function
   | _ -> false
 
 let is_argument_start = function
-  | Token.Tilde | Underscore -> true
+  | Token.Tilde | Dot | Underscore -> true
   | t when is_expr_start t -> true
   | _ -> false
 

rewatch/src/build/compile.rs

@@ -1095,10 +1095,13 @@ fn compile_file(
 
             if helpers::contains_ascii_characters(&err) {
                 if package.is_local_dep {
-                    // suppress warnings of external deps
                     Ok(Some(err))
                 } else {
-                    Ok(None)
+                    // Warnings from external deps are suppressed by default —
+                    // users can't act on them. A small allow-list of critical
+                    // deprecations still gets through so breakage signals are
+                    // visible (and can be reported upstream).
+                    Ok(retain_critical_external_warnings(&err))
                 }
             } else {
                 Ok(None)
@@ -1107,6 +1110,35 @@ fn compile_file(
     }
 }
 
+/// Filter a bsc stderr capture to the warning blocks the user needs to see
+/// even when they originate in an external dependency.
+///
+/// Currently preserved:
+/// - Warning 3 deprecations mentioning the legacy `(. ...)` uncurried syntax.
+///   These indicate source that parses today but is scheduled for removal, so
+///   consumers need to hear about them even when the code isn't theirs.
+pub(super) fn retain_critical_external_warnings(stderr: &str) -> Option<String> {
+    const UNCURRIED_DOT_MARKER: &str = "`(. ...)` uncurried syntax";
+    if !stderr.contains(UNCURRIED_DOT_MARKER) {
+        return None;
+    }
+    // bsc prints each warning as its own block separated by a blank-line pair
+    // (three consecutive newlines). On Windows the same stream uses CRLF, so
+    // normalize before splitting to keep the block boundary recognizable —
+    // otherwise the whole stderr would be treated as a single block and
+    // unrelated warnings would leak through alongside the critical one.
+    let normalized = stderr.replace("\r\n", "\n");
+    let kept: Vec<&str> = normalized
+        .split("\n\n\n")
+        .filter(|block| block.contains(UNCURRIED_DOT_MARKER))
+        .collect();
+    if kept.is_empty() {
+        None
+    } else {
+        Some(kept.join("\n\n\n"))
+    }
+}
+
 pub fn mark_modules_with_deleted_deps_dirty(build_state: &mut BuildState) {
     build_state.modules.iter_mut().for_each(|(_, module)| {
         if !module.deps.is_disjoint(&build_state.deleted_modules) {
@@ -1203,3 +1235,42 @@ pub fn mark_modules_with_expired_deps_dirty(build_state: &mut BuildCommandState)
         }
     });
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn retain_critical_external_warnings_returns_none_without_marker() {
+        let input = "\n  Warning number 26\n  foo.res:1:1\n\n  unused variable x.\n";
+        assert_eq!(retain_critical_external_warnings(input), None);
+    }
+
+    #[test]
+    fn retain_critical_external_warnings_keeps_uncurried_dot_block() {
+        let input = concat!(
+            "\n  Warning number 26\n  foo.res:1:1\n\n  unused variable x.\n",
+            "\n\n\n  Warning number 3\n  bar.res:5:10\n\n  ",
+            "deprecated: The `(. ...)` uncurried syntax is deprecated.\n",
+        );
+        let kept = retain_critical_external_warnings(input).expect("uncurried-dot warning should survive");
+        assert!(kept.contains("`(. ...)` uncurried syntax"));
+        assert!(!kept.contains("unused variable"));
+    }
+
+    #[test]
+    fn retain_critical_external_warnings_handles_crlf_line_endings() {
+        // Windows stderr from bsc uses CRLF. Without normalization the "\n\n\n"
+        // splitter would find no boundary and return the entire stream, which
+        // would effectively disable suppression for external deps on Windows.
+        let input = concat!(
+            "\r\n  Warning number 26\r\n  foo.res:1:1\r\n\r\n  unused variable x.\r\n",
+            "\r\n\r\n\r\n  Warning number 3\r\n  bar.res:5:10\r\n\r\n  ",
+            "deprecated: The `(. ...)` uncurried syntax is deprecated.\r\n",
+        );
+        let kept = retain_critical_external_warnings(input)
+            .expect("uncurried-dot warning should survive on Windows too");
+        assert!(kept.contains("`(. ...)` uncurried syntax"));
+        assert!(!kept.contains("unused variable"));
+    }
+}

rewatch/src/build/parse.rs

@@ -1,4 +1,5 @@
 use super::build_types::*;
+use super::compile::retain_critical_external_warnings;
 use super::logs;
 use super::namespaces;
 use crate::build::packages::Package;
@@ -139,7 +140,18 @@ pub fn generate_asts(
                             logs::append(package, &stderr_warnings);
                             stderr.push_str(&stderr_warnings);
                         }
-                        Ok((_path, Some(_))) | Ok((_path, None)) => {
+                        Ok((_path, Some(stderr_warnings))) => {
+                            source_file.implementation.parse_state = ParseState::Success;
+                            source_file.implementation.parse_dirty = false;
+                            // External dep: surface only the critical warnings
+                            // (e.g. legacy `(. ...)` uncurried syntax) so
+                            // downstream users can report breakage upstream.
+                            if let Some(kept) = retain_critical_external_warnings(&stderr_warnings) {
+                                logs::append(package, &kept);
+                                stderr.push_str(&kept);
+                            }
+                        }
+                        Ok((_path, None)) => {
                             source_file.implementation.parse_state = ParseState::Success;
                             source_file.implementation.parse_dirty = false;
                         }
@@ -167,7 +179,17 @@ pub fn generate_asts(
                             logs::append(package, &stderr_warnings);
                             stderr.push_str(&stderr_warnings);
                         }
-                        Ok(Some((_, None))) | Ok(Some((_, Some(_)))) => {
+                        Ok(Some((_, Some(stderr_warnings)))) => {
+                            if let Some(interface) = source_file.interface.as_mut() {
+                                interface.parse_state = ParseState::Success;
+                                interface.parse_dirty = false;
+                            }
+                            if let Some(kept) = retain_critical_external_warnings(&stderr_warnings) {
+                                logs::append(package, &kept);
+                                stderr.push_str(&kept);
+                            }
+                        }
+                        Ok(Some((_, None))) => {
                             if let Some(interface) = source_file.interface.as_mut() {
                                 interface.parse_state = ParseState::Success;
                                 interface.parse_dirty = false;