Better syntax errors for [| and |>

CHANGELOG.md

@@ -28,6 +28,8 @@
 
 #### :nail_care: Polish
 
+- Dedicated error messages for old Reason array literal syntax (`[|` and `|]`), and for the old pipe (`|>`). Primarly intended to help LLMs that might try to use old code patterns. https://github.com/rescript-lang/rescript/pull/8010
+
 #### :house: Internal
 
 - Rename Core to Stdlib in tests/tests. https://github.com/rescript-lang/rescript/pull/8005

compiler/syntax/src/res_diagnostics.ml

@@ -137,12 +137,43 @@ let print_report ?(custom_intro = None) ?(formatter = Format.err_formatter)
     match diagnostics with
     | [] -> ()
     | d :: rest ->
+      (* A few specializations for best-effort error messages for old syntax etc. *)
+      let msg =
+        match d.category with
+        | Unexpected {token = Token.Bar; _} ->
+          let idx_prev = d.start_pos.pos_cnum - 1 in
+          let idx_next = d.end_pos.pos_cnum in
+          if
+            idx_prev >= 0
+            && idx_prev < String.length src
+            && String.get src idx_prev = '['
+          then
+            let base = explain d in
+            base
+            ^ "\n\n\
+              \  Did you mean to write an array literal? Arrays are written \
+               with `[ ... ]` (not `[| ... |]`)."
+            ^ "\n  Quick fix: replace `[|` with `[` and `|]` with `]`."
+            ^ "\n  Example: `[|1, 2, 3|]` -> `[1, 2, 3]`"
+          else if
+            idx_next >= 0
+            && idx_next < String.length src
+            && String.get src idx_next = '>'
+          then
+            let base = explain d in
+            base
+            ^ "\n\n\
+              \  The old data-last pipe `|>` has been removed from the language.\n\
+              \  Refactor to use a data-first `->` pipe instead."
+          else explain d
+        | _ -> explain d
+      in
       Location.report_error ~custom_intro ~src:(Some src) formatter
         Location.
           {
             loc =
               {loc_start = d.start_pos; loc_end = d.end_pos; loc_ghost = false};
-            msg = explain d;
+            msg;
             sub = [];
             if_highlight = "";
           };

tests/syntax_tests/data/parsing/errors/expressions/expected/oldArraySyntax.res.txt

@@ -0,0 +1,16 @@
+
+  Syntax error!
+  syntax_tests/data/parsing/errors/expressions/oldArraySyntax.res:2:10
+
+  1 │ // Old Reason array literal
+  2 │ let s = [|1, 2, 3|]
+  3 │ 
+  4 │ 
+
+  I'm not sure what to parse here when looking at "|".
+
+  Did you mean to write an array literal? Arrays are written with `[ ... ]` (not `[| ... |]`).
+  Quick fix: replace `[|` with `[` and `|]` with `]`.
+  Example: `[|1, 2, 3|]` -> `[1, 2, 3]`
+
+let s = [|1;2;3|]

tests/syntax_tests/data/parsing/errors/expressions/expected/oldDataLastPipe.res.txt

@@ -0,0 +1,16 @@
+
+  Syntax error!
+  syntax_tests/data/parsing/errors/expressions/oldDataLastPipe.res:2:16
+
+  1 │ // Old data-last pipe
+  2 │ let x = f => f |> String.length
+  3 │ 
+  4 │ 
+
+  I'm not sure what to parse here when looking at "|".
+
+  The old data-last pipe `|>` has been removed from the language.
+  Refactor to use a data-first `->` pipe instead.
+
+let x [arity:1]f = f
+;;String.length

tests/syntax_tests/data/parsing/errors/expressions/oldArraySyntax.res

@@ -0,0 +1,3 @@
+// Old Reason array literal
+let s = [|1, 2, 3|]
+

tests/syntax_tests/data/parsing/errors/expressions/oldDataLastPipe.res

@@ -0,0 +1,3 @@
+// Old data-last pipe
+let x = f => f |> String.length
+