destructure record rest elements by tsnobip · Pull Request #8317 · rescript-lang/rescript

tsnobip · 2026-03-28T09:38:21Z

pkg-pr-new · 2026-03-28T09:58:16Z

rescript

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commit: a87f7f9

tsnobip · 2026-03-31T11:17:44Z

      construct ~loc ~attrs (map_loc sub l) (map_opt (sub.pat sub) p)
    | Ppat_variant (l, p) -> variant ~loc ~attrs l (map_opt (sub.pat sub) p)
-    | Ppat_record (lpl, cf) ->
+    | Ppat_record (lpl, cf, _rest) ->


I'm not sure about what should be done here

good question: it goes to ast0 and then back during ppx, so any use of a ppx would discard this at the moment, and change the code

normally, one tries to encode additional ast information in special annotations, so they can be recovered on the way back (from_0)

cknitt · 2026-03-31T11:29:08Z

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chatgpt-codex-connector · 2026-03-31T11:36:03Z

+  match pat.pat_desc with
+  | Tpat_record (_, _, Some rest) ->


Bind rest variables in nested record patterns

inject_record_rest_binding only handles cases where the top-level pattern is Tpat_record (_, _, Some rest). When a record-rest appears inside another pattern shape (for example a tuple, constructor, or-or pattern), no Precord_spread_new binding is emitted even though typing has already introduced rest as a bound identifier. This causes incorrect code generation for valid nested destructuring forms because the rest variable is referenced without being initialized from the matched record.

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chatgpt-codex-connector · 2026-03-31T11:36:03Z

+  | Ppat_record (pl, _, _rest) ->
    List.iter
      (fun {lid = lbl; x = p} ->
        add bv lbl;


Track dependencies from record-rest annotations

The dependency walker now matches Ppat_record (pl, _, _rest) but never visits _rest. As a result, module/type paths used only in record-rest annotations (e.g. ...M.t as rest) are omitted from dependency collection, so changing those modules may not trigger recompilation or proper dependency invalidation.

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chatgpt-codex-connector · 2026-03-31T11:36:03Z

+        | PatRest rest_pat -> (fields, flag, Some rest_pat)
+        | PatUnderscore -> (fields, flag, rest))


Reject duplicate record-rest clauses during parsing

parse_record_pattern stores rest as Some rest_pat every time it sees PatRest and does not check whether a previous rest was already parsed. This lets patterns with multiple rest clauses be accepted while silently discarding one clause, which is ambiguous and can mask user mistakes instead of producing a syntax error.

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tsnobip · 2026-04-01T13:06:30Z

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chatgpt-codex-connector · 2026-04-01T13:17:06Z

+          let rest_field_names =
+            List.map
+              (fun (l : Types.label_declaration) -> Ident.name l.ld_id)
+              rest_labels


Check rest record field types, not just field names

The new rest-pattern validation only compares field names (rest_field_names) and never unifies the rest type’s field types with the source record’s field types. That lets incompatible rest annotations type-check (for example, source b: string with rest type b: int), so rest.b is typed incorrectly and can break soundness at runtime.

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chatgpt-codex-connector · 2026-04-01T13:17:06Z

          simplify ((p :: patl, bind Alias id arg action) :: rem)
-        | Tpat_record ([], _) -> (omega :: patl, action) :: simplify rem
-        | Tpat_record (lbls, closed) ->
+        | Tpat_record ([], _, _rest) -> (omega :: patl, action) :: simplify rem


Preserve rest binding for empty-field record patterns

The Tpat_record ([], _, _rest) fast path drops the rest binder entirely, so a pattern like {...Sub.t as rest} is simplified to _ without generating the rest binding. If such a pattern is accepted by parsing/type-checking, code that uses rest in the branch can compile to incorrect/unbound behavior.

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tsnobip · 2026-04-01T14:11:53Z

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tsnobip · 2026-04-01T15:25:22Z

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tsnobip · 2026-04-01T16:06:18Z

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tsnobip · 2026-04-01T21:09:53Z

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tsnobip · 2026-04-02T16:18:06Z

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tsnobip · 2026-04-02T16:21:28Z

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tsnobip · 2026-04-07T07:05:22Z

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tsnobip · 2026-04-07T07:51:48Z

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tsnobip · 2026-04-07T08:44:44Z

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codecov · 2026-06-16T16:51:41Z

Codecov Report

❌ Patch coverage is 86.27451% with 91 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.67%. Comparing base (516e650) to head (a87f7f9).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/ml/typecore_record_rest.ml	88.23%	14 Missing ⚠️
compiler/core/js_record_map.ml	36.36%	7 Missing ⚠️
compiler/ml/printast.ml	0.00%	6 Missing ⚠️
compiler/core/js_dump.ml	91.22%	5 Missing ⚠️
compiler/core/js_pass_flatten_and_mark_dead.ml	68.75%	5 Missing ⚠️
compiler/ml/matching.ml	79.16%	5 Missing ⚠️
compiler/core/js_record_iter.ml	33.33%	4 Missing ⚠️
compiler/ml/ast_mapper_from0.ml	60.00%	4 Missing ⚠️
compiler/ml/parmatch.ml	83.33%	4 Missing ⚠️
compiler/core/j.ml	66.66%	3 Missing ⚠️
... and 17 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8317      +/-   ##
==========================================
+ Coverage   74.42%   74.67%   +0.25%     
==========================================
  Files         451      454       +3     
  Lines       61459    61990     +531     
==========================================
+ Hits        45743    46294     +551     
+ Misses      15716    15696      -20

Files with missing lines	Coverage Δ
analysis/reanalyze/src/dead_value.ml	`86.11% <100.00%> (ø)`
analysis/src/completion_front_end.ml	`81.53% <100.00%> (ø)`
analysis/src/completion_patterns.ml	`90.38% <100.00%> (ø)`
analysis/src/hint.ml	`73.07% <100.00%> (+1.07%)`	⬆️
analysis/src/process_cmt.ml	`81.81% <100.00%> (+0.32%)`	⬆️
analysis/src/process_extra.ml	`86.74% <100.00%> (+0.21%)`	⬆️
analysis/src/signature_help.ml	`78.66% <100.00%> (ø)`
analysis/src/xform.ml	`88.68% <100.00%> (ø)`
compiler/common/pattern_printer.ml	`87.50% <100.00%> (ø)`
compiler/core/js_exp_make.ml	`64.20% <100.00%> (+0.14%)`	⬆️
... and 45 more

... and 6 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

github-actions · 2026-06-17T10:29:09Z

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8317

tsnobip · 2026-06-17T14:40:44Z

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tsnobip · 2026-06-17T16:52:13Z

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tsnobip · 2026-06-17T17:27:21Z

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tsnobip · 2026-06-18T10:40:47Z

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tsnobip · 2026-06-18T15:04:21Z

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chatgpt-codex-connector · 2026-06-18T15:12:24Z

+  | Tpat_record (fields, _, _rest) ->
    List.concat (List.map (fun (_, _, p, _) -> pattern_variables p) fields)


Include rest binders in let-rec checks

For recursive value bindings that use a record-rest pattern, the rest identifier is a real binding but it is dropped here. In a case like let rec {...config as rest}: config = rest or an and binding that refers to rest, the let-rec safety pass builds ids without the rest binder, so it can treat the recursive reference as non-recursive and accept code that later emits a TDZ/self-reference instead of raising Illegal_letrec_expr.

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chatgpt-codex-connector · 2026-06-18T15:12:24Z

  let pos_from = pat.Typedtree.pat_loc.loc_start in
  (match pat.pat_desc with
-  | Typedtree.Tpat_record (cases, _clodsedFlag) ->
+  | Typedtree.Tpat_record (cases, _clodsedFlag, _rest) ->


Track rest type annotations in dead-type analysis

When analyze_types is enabled, this branch records only the explicit record field labels and ignores the new _rest payload. Since the typed-tree mapper also leaves record_pat_rest untouched, a type referenced only by a rest annotation such as ...SubConfig.t as rest gets no reference recorded and can be reported or removed as dead even though the code needs it.

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tsnobip changed the title ~~destructure record rest elements - fixes #8311~~ destructure record rest elements Mar 28, 2026

tsnobip force-pushed the fix-8311 branch 2 times, most recently from 536855c to 2245765 Compare March 31, 2026 10:50

tsnobip marked this pull request as ready for review March 31, 2026 10:51

tsnobip commented Mar 31, 2026

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tsnobip force-pushed the fix-8311 branch from e366137 to da71513 Compare April 1, 2026 14:08

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tsnobip force-pushed the fix-8311 branch from 4eed0c7 to 96722aa Compare April 7, 2026 09:19

tsnobip force-pushed the fix-8311 branch 2 times, most recently from dbe99a2 to 84b5246 Compare June 16, 2026 16:30

tsnobip force-pushed the fix-8311 branch 3 times, most recently from 5b5fae2 to 9d0c34d Compare June 17, 2026 09:04

improve output (compile to JS destructuring)

bcc6c80

tsnobip force-pushed the fix-8311 branch from 9d0c34d to bcc6c80 Compare June 17, 2026 09:19

update changelog

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tsnobip added 2 commits June 17, 2026 16:35

remove leading _ of used variable

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add more tests

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tsnobip added 4 commits June 17, 2026 18:02

disallow spreading rest to unboxed record

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address #8317 (comment)

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document type_record_pat_rest function

633c6ad

add comment about Tpat_record rest

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forbid destructure to rest of record with mutable fields

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do not spread function params to avoid issues with 'use strict'

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fix comment table for rest spread

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		\| PatRest rest_pat -> (fields, flag, Some rest_pat)
		\| PatUnderscore -> (fields, flag, rest))

		\| Tpat_record (fields, _, _rest) ->
		List.concat (List.map (fun (_, _, p, _) -> pattern_variables p) fields)

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