Improve array pattern spread error message

CHANGELOG.md

@@ -52,6 +52,7 @@
 - Improve doc comment formatting to match the style of multiline comments. https://github.com/rescript-lang/rescript/pull/7529
 - Improve error messages around type mismatches for try/catch, if, for, while, and optional record fields + optional function arguments. https://github.com/rescript-lang/rescript/pull/7522
 - sync Reanalyze with the new APIs around exception. https://github.com/rescript-lang/rescript/pull/7536
+- Improve array pattern spread error message. https://github.com/rescript-lang/rescript/pull/7549
 
 #### :house: Internal
 

compiler/syntax/src/res_core.ml

@@ -61,7 +61,7 @@ module ErrorMessages = struct
      matching currently guarantees to never create new intermediate data."
 
   let record_pattern_spread =
-    "Record's `...` spread is not supported in pattern matches.\n\
+    "Record spread (`...`) is not supported in pattern matches.\n\
      Explanation: you can't collect a subset of a record's field into its own \
      record, since a record needs an explicit declaration and that subset \
      wouldn't have one.\n\
@@ -70,13 +70,14 @@ module ErrorMessages = struct
   [@@live]
 
   let array_pattern_spread =
-    "Array's `...` spread is not supported in pattern matches.\n\
-     Explanation: such spread would create a subarray; out of performance \
-     concern, our pattern matching currently guarantees to never create new \
-     intermediate data.\n\
-     Solution: if it's to validate the first few elements, use a `when` clause \
-     + Array size check + `get` checks on the current pattern. If it's to \
-     obtain a subarray, use `Array.sub` or `Belt.Array.slice`."
+    "Array spread (`...`) is not supported in pattern matches.\n\n\
+     Explanation: Allowing `...` here would require creating a new subarray at \
+     match time, but for performance reasons pattern matching is guaranteed to \
+     never create intermediate data.\n\n\
+     Possible solutions:\n\
+     - To validate specific elements: Use `if` with length checks and \
+     `Array.get`\n\
+     - To extract a subarray: Use `Array.slice`"
 
   let record_expr_spread =
     "Records can only have one `...` spread, at the beginning.\n\

tests/syntax_tests/data/parsing/errors/other/expected/spread.res.txt

@@ -6,9 +6,13 @@
   2 │ 
   3 │ let record = {...x, ...y}
 
-  Array's `...` spread is not supported in pattern matches.
-Explanation: such spread would create a subarray; out of performance concern, our pattern matching currently guarantees to never create new intermediate data.
-Solution: if it's to validate the first few elements, use a `when` clause + Array size check + `get` checks on the current pattern. If it's to obtain a subarray, use `Array.sub` or `Belt.Array.slice`.
+  Array spread (`...`) is not supported in pattern matches.
+
+Explanation: Allowing `...` here would require creating a new subarray at match time, but for performance reasons pattern matching is guaranteed to never create intermediate data.
+
+Possible solutions:
+- To validate specific elements: Use `if` with length checks and `Array.get`
+- To extract a subarray: Use `Array.slice`
 
 
   Syntax error!
@@ -33,7 +37,7 @@ Explanation: since records have a known, fixed shape, a spread like `{a, ...b}`
   5 │ 
   6 │ let list{...x, ...y} = myList
 
-  Record's `...` spread is not supported in pattern matches.
+  Record spread (`...`) is not supported in pattern matches.
 Explanation: you can't collect a subset of a record's field into its own record, since a record needs an explicit declaration and that subset wouldn't have one.
 Solution: you need to pull out each field you want explicitly.
 

tests/syntax_tests/data/parsing/recovery/expression/expected/list.res.txt

@@ -8,9 +8,13 @@
   7 ┆     | [x, ...rest] => [x, ...loop(rest)]
   8 ┆     | [] => []
 
-  Array's `...` spread is not supported in pattern matches.
-Explanation: such spread would create a subarray; out of performance concern, our pattern matching currently guarantees to never create new intermediate data.
-Solution: if it's to validate the first few elements, use a `when` clause + Array size check + `get` checks on the current pattern. If it's to obtain a subarray, use `Array.sub` or `Belt.Array.slice`.
+  Array spread (`...`) is not supported in pattern matches.
+
+Explanation: Allowing `...` here would require creating a new subarray at match time, but for performance reasons pattern matching is guaranteed to never create intermediate data.
+
+Possible solutions:
+- To validate specific elements: Use `if` with length checks and `Array.get`
+- To extract a subarray: Use `Array.slice`
 
 
   Syntax error!
@@ -22,9 +26,13 @@ Solution: if it's to validate the first few elements, use a `when` clause + Arra
   8 ┆   | [] => []
   9 ┆ }
 
-  Array's `...` spread is not supported in pattern matches.
-Explanation: such spread would create a subarray; out of performance concern, our pattern matching currently guarantees to never create new intermediate data.
-Solution: if it's to validate the first few elements, use a `when` clause + Array size check + `get` checks on the current pattern. If it's to obtain a subarray, use `Array.sub` or `Belt.Array.slice`.
+  Array spread (`...`) is not supported in pattern matches.
+
+Explanation: Allowing `...` here would require creating a new subarray at match time, but for performance reasons pattern matching is guaranteed to never create intermediate data.
+
+Possible solutions:
+- To validate specific elements: Use `if` with length checks and `Array.get`
+- To extract a subarray: Use `Array.slice`
 
 let flags =
   ((if reasonFormat

tests/syntax_tests/data/parsing/recovery/pattern/expected/record.res.txt

@@ -19,7 +19,7 @@
   4 │ | {a, _, b} => ()
   5 │ }
 
-  Record's `...` spread is not supported in pattern matches.
+  Record spread (`...`) is not supported in pattern matches.
 Explanation: you can't collect a subset of a record's field into its own record, since a record needs an explicit declaration and that subset wouldn't have one.
 Solution: you need to pull out each field you want explicitly.